| /* |
| * Written by Doug Lea with assistance from members of JCP JSR-166 |
| * Expert Group and released to the public domain, as explained at |
| * http://creativecommons.org/publicdomain/zero/1.0/ |
| */ |
| |
| package java.util.concurrent; |
| |
| import java.util.ArrayList; |
| import java.util.Arrays; |
| import java.util.Collection; |
| import java.util.Collections; |
| import java.util.List; |
| import java.util.Random; |
| import java.util.concurrent.AbstractExecutorService; |
| import java.util.concurrent.Callable; |
| import java.util.concurrent.ExecutorService; |
| import java.util.concurrent.Future; |
| import java.util.concurrent.RejectedExecutionException; |
| import java.util.concurrent.RunnableFuture; |
| import java.util.concurrent.TimeUnit; |
| import java.util.concurrent.atomic.AtomicInteger; |
| import java.util.concurrent.locks.LockSupport; |
| import java.util.concurrent.locks.ReentrantLock; |
| import java.util.concurrent.locks.Condition; |
| import libcore.util.SneakyThrow; |
| |
| // BEGIN android-note |
| // removed security manager docs |
| // END android-note |
| |
| /** |
| * An {@link ExecutorService} for running {@link ForkJoinTask}s. |
| * A {@code ForkJoinPool} provides the entry point for submissions |
| * from non-{@code ForkJoinTask} clients, as well as management and |
| * monitoring operations. |
| * |
| * <p>A {@code ForkJoinPool} differs from other kinds of {@link |
| * ExecutorService} mainly by virtue of employing |
| * <em>work-stealing</em>: all threads in the pool attempt to find and |
| * execute subtasks created by other active tasks (eventually blocking |
| * waiting for work if none exist). This enables efficient processing |
| * when most tasks spawn other subtasks (as do most {@code |
| * ForkJoinTask}s). When setting <em>asyncMode</em> to true in |
| * constructors, {@code ForkJoinPool}s may also be appropriate for use |
| * with event-style tasks that are never joined. |
| * |
| * <p>A {@code ForkJoinPool} is constructed with a given target |
| * parallelism level; by default, equal to the number of available |
| * processors. The pool attempts to maintain enough active (or |
| * available) threads by dynamically adding, suspending, or resuming |
| * internal worker threads, even if some tasks are stalled waiting to |
| * join others. However, no such adjustments are guaranteed in the |
| * face of blocked IO or other unmanaged synchronization. The nested |
| * {@link ManagedBlocker} interface enables extension of the kinds of |
| * synchronization accommodated. |
| * |
| * <p>In addition to execution and lifecycle control methods, this |
| * class provides status check methods (for example |
| * {@link #getStealCount}) that are intended to aid in developing, |
| * tuning, and monitoring fork/join applications. Also, method |
| * {@link #toString} returns indications of pool state in a |
| * convenient form for informal monitoring. |
| * |
| * <p> As is the case with other ExecutorServices, there are three |
| * main task execution methods summarized in the following |
| * table. These are designed to be used by clients not already engaged |
| * in fork/join computations in the current pool. The main forms of |
| * these methods accept instances of {@code ForkJoinTask}, but |
| * overloaded forms also allow mixed execution of plain {@code |
| * Runnable}- or {@code Callable}- based activities as well. However, |
| * tasks that are already executing in a pool should normally |
| * <em>NOT</em> use these pool execution methods, but instead use the |
| * within-computation forms listed in the table. |
| * |
| * <table BORDER CELLPADDING=3 CELLSPACING=1> |
| * <tr> |
| * <td></td> |
| * <td ALIGN=CENTER> <b>Call from non-fork/join clients</b></td> |
| * <td ALIGN=CENTER> <b>Call from within fork/join computations</b></td> |
| * </tr> |
| * <tr> |
| * <td> <b>Arrange async execution</td> |
| * <td> {@link #execute(ForkJoinTask)}</td> |
| * <td> {@link ForkJoinTask#fork}</td> |
| * </tr> |
| * <tr> |
| * <td> <b>Await and obtain result</td> |
| * <td> {@link #invoke(ForkJoinTask)}</td> |
| * <td> {@link ForkJoinTask#invoke}</td> |
| * </tr> |
| * <tr> |
| * <td> <b>Arrange exec and obtain Future</td> |
| * <td> {@link #submit(ForkJoinTask)}</td> |
| * <td> {@link ForkJoinTask#fork} (ForkJoinTasks <em>are</em> Futures)</td> |
| * </tr> |
| * </table> |
| * |
| * <p><b>Sample Usage.</b> Normally a single {@code ForkJoinPool} is |
| * used for all parallel task execution in a program or subsystem. |
| * Otherwise, use would not usually outweigh the construction and |
| * bookkeeping overhead of creating a large set of threads. For |
| * example, a common pool could be used for the {@code SortTasks} |
| * illustrated in {@link RecursiveAction}. Because {@code |
| * ForkJoinPool} uses threads in {@linkplain java.lang.Thread#isDaemon |
| * daemon} mode, there is typically no need to explicitly {@link |
| * #shutdown} such a pool upon program exit. |
| * |
| * <pre> {@code |
| * static final ForkJoinPool mainPool = new ForkJoinPool(); |
| * ... |
| * public void sort(long[] array) { |
| * mainPool.invoke(new SortTask(array, 0, array.length)); |
| * }}</pre> |
| * |
| * <p><b>Implementation notes</b>: This implementation restricts the |
| * maximum number of running threads to 32767. Attempts to create |
| * pools with greater than the maximum number result in |
| * {@code IllegalArgumentException}. |
| * |
| * <p>This implementation rejects submitted tasks (that is, by throwing |
| * {@link RejectedExecutionException}) only when the pool is shut down |
| * or internal resources have been exhausted. |
| * |
| * @since 1.7 |
| * @hide |
| * @author Doug Lea |
| */ |
| public class ForkJoinPool extends AbstractExecutorService { |
| |
| /* |
| * Implementation Overview |
| * |
| * This class provides the central bookkeeping and control for a |
| * set of worker threads: Submissions from non-FJ threads enter |
| * into a submission queue. Workers take these tasks and typically |
| * split them into subtasks that may be stolen by other workers. |
| * Preference rules give first priority to processing tasks from |
| * their own queues (LIFO or FIFO, depending on mode), then to |
| * randomized FIFO steals of tasks in other worker queues, and |
| * lastly to new submissions. |
| * |
| * The main throughput advantages of work-stealing stem from |
| * decentralized control -- workers mostly take tasks from |
| * themselves or each other. We cannot negate this in the |
| * implementation of other management responsibilities. The main |
| * tactic for avoiding bottlenecks is packing nearly all |
| * essentially atomic control state into a single 64bit volatile |
| * variable ("ctl"). This variable is read on the order of 10-100 |
| * times as often as it is modified (always via CAS). (There is |
| * some additional control state, for example variable "shutdown" |
| * for which we can cope with uncoordinated updates.) This |
| * streamlines synchronization and control at the expense of messy |
| * constructions needed to repack status bits upon updates. |
| * Updates tend not to contend with each other except during |
| * bursts while submitted tasks begin or end. In some cases when |
| * they do contend, threads can instead do something else |
| * (usually, scan for tasks) until contention subsides. |
| * |
| * To enable packing, we restrict maximum parallelism to (1<<15)-1 |
| * (which is far in excess of normal operating range) to allow |
| * ids, counts, and their negations (used for thresholding) to fit |
| * into 16bit fields. |
| * |
| * Recording Workers. Workers are recorded in the "workers" array |
| * that is created upon pool construction and expanded if (rarely) |
| * necessary. This is an array as opposed to some other data |
| * structure to support index-based random steals by workers. |
| * Updates to the array recording new workers and unrecording |
| * terminated ones are protected from each other by a seqLock |
| * (scanGuard) but the array is otherwise concurrently readable, |
| * and accessed directly by workers. To simplify index-based |
| * operations, the array size is always a power of two, and all |
| * readers must tolerate null slots. To avoid flailing during |
| * start-up, the array is presized to hold twice #parallelism |
| * workers (which is unlikely to need further resizing during |
| * execution). But to avoid dealing with so many null slots, |
| * variable scanGuard includes a mask for the nearest power of two |
| * that contains all current workers. All worker thread creation |
| * is on-demand, triggered by task submissions, replacement of |
| * terminated workers, and/or compensation for blocked |
| * workers. However, all other support code is set up to work with |
| * other policies. To ensure that we do not hold on to worker |
| * references that would prevent GC, ALL accesses to workers are |
| * via indices into the workers array (which is one source of some |
| * of the messy code constructions here). In essence, the workers |
| * array serves as a weak reference mechanism. Thus for example |
| * the wait queue field of ctl stores worker indices, not worker |
| * references. Access to the workers in associated methods (for |
| * example signalWork) must both index-check and null-check the |
| * IDs. All such accesses ignore bad IDs by returning out early |
| * from what they are doing, since this can only be associated |
| * with termination, in which case it is OK to give up. |
| * |
| * All uses of the workers array, as well as queue arrays, check |
| * that the array is non-null (even if previously non-null). This |
| * allows nulling during termination, which is currently not |
| * necessary, but remains an option for resource-revocation-based |
| * shutdown schemes. |
| * |
| * Wait Queuing. Unlike HPC work-stealing frameworks, we cannot |
| * let workers spin indefinitely scanning for tasks when none can |
| * be found immediately, and we cannot start/resume workers unless |
| * there appear to be tasks available. On the other hand, we must |
| * quickly prod them into action when new tasks are submitted or |
| * generated. We park/unpark workers after placing in an event |
| * wait queue when they cannot find work. This "queue" is actually |
| * a simple Treiber stack, headed by the "id" field of ctl, plus a |
| * 15bit counter value to both wake up waiters (by advancing their |
| * count) and avoid ABA effects. Successors are held in worker |
| * field "nextWait". Queuing deals with several intrinsic races, |
| * mainly that a task-producing thread can miss seeing (and |
| * signalling) another thread that gave up looking for work but |
| * has not yet entered the wait queue. We solve this by requiring |
| * a full sweep of all workers both before (in scan()) and after |
| * (in tryAwaitWork()) a newly waiting worker is added to the wait |
| * queue. During a rescan, the worker might release some other |
| * queued worker rather than itself, which has the same net |
| * effect. Because enqueued workers may actually be rescanning |
| * rather than waiting, we set and clear the "parked" field of |
| * ForkJoinWorkerThread to reduce unnecessary calls to unpark. |
| * (Use of the parked field requires a secondary recheck to avoid |
| * missed signals.) |
| * |
| * Signalling. We create or wake up workers only when there |
| * appears to be at least one task they might be able to find and |
| * execute. When a submission is added or another worker adds a |
| * task to a queue that previously had two or fewer tasks, they |
| * signal waiting workers (or trigger creation of new ones if |
| * fewer than the given parallelism level -- see signalWork). |
| * These primary signals are buttressed by signals during rescans |
| * as well as those performed when a worker steals a task and |
| * notices that there are more tasks too; together these cover the |
| * signals needed in cases when more than two tasks are pushed |
| * but untaken. |
| * |
| * Trimming workers. To release resources after periods of lack of |
| * use, a worker starting to wait when the pool is quiescent will |
| * time out and terminate if the pool has remained quiescent for |
| * SHRINK_RATE nanosecs. This will slowly propagate, eventually |
| * terminating all workers after long periods of non-use. |
| * |
| * Submissions. External submissions are maintained in an |
| * array-based queue that is structured identically to |
| * ForkJoinWorkerThread queues except for the use of |
| * submissionLock in method addSubmission. Unlike the case for |
| * worker queues, multiple external threads can add new |
| * submissions, so adding requires a lock. |
| * |
| * Compensation. Beyond work-stealing support and lifecycle |
| * control, the main responsibility of this framework is to take |
| * actions when one worker is waiting to join a task stolen (or |
| * always held by) another. Because we are multiplexing many |
| * tasks on to a pool of workers, we can't just let them block (as |
| * in Thread.join). We also cannot just reassign the joiner's |
| * run-time stack with another and replace it later, which would |
| * be a form of "continuation", that even if possible is not |
| * necessarily a good idea since we sometimes need both an |
| * unblocked task and its continuation to progress. Instead we |
| * combine two tactics: |
| * |
| * Helping: Arranging for the joiner to execute some task that it |
| * would be running if the steal had not occurred. Method |
| * ForkJoinWorkerThread.joinTask tracks joining->stealing |
| * links to try to find such a task. |
| * |
| * Compensating: Unless there are already enough live threads, |
| * method tryPreBlock() may create or re-activate a spare |
| * thread to compensate for blocked joiners until they |
| * unblock. |
| * |
| * The ManagedBlocker extension API can't use helping so relies |
| * only on compensation in method awaitBlocker. |
| * |
| * It is impossible to keep exactly the target parallelism number |
| * of threads running at any given time. Determining the |
| * existence of conservatively safe helping targets, the |
| * availability of already-created spares, and the apparent need |
| * to create new spares are all racy and require heuristic |
| * guidance, so we rely on multiple retries of each. Currently, |
| * in keeping with on-demand signalling policy, we compensate only |
| * if blocking would leave less than one active (non-waiting, |
| * non-blocked) worker. Additionally, to avoid some false alarms |
| * due to GC, lagging counters, system activity, etc, compensated |
| * blocking for joins is only attempted after rechecks stabilize |
| * (retries are interspersed with Thread.yield, for good |
| * citizenship). The variable blockedCount, incremented before |
| * blocking and decremented after, is sometimes needed to |
| * distinguish cases of waiting for work vs blocking on joins or |
| * other managed sync. Both cases are equivalent for most pool |
| * control, so we can update non-atomically. (Additionally, |
| * contention on blockedCount alleviates some contention on ctl). |
| * |
| * Shutdown and Termination. A call to shutdownNow atomically sets |
| * the ctl stop bit and then (non-atomically) sets each workers |
| * "terminate" status, cancels all unprocessed tasks, and wakes up |
| * all waiting workers. Detecting whether termination should |
| * commence after a non-abrupt shutdown() call requires more work |
| * and bookkeeping. We need consensus about quiescence (i.e., that |
| * there is no more work) which is reflected in active counts so |
| * long as there are no current blockers, as well as possible |
| * re-evaluations during independent changes in blocking or |
| * quiescing workers. |
| * |
| * Style notes: There is a lot of representation-level coupling |
| * among classes ForkJoinPool, ForkJoinWorkerThread, and |
| * ForkJoinTask. Most fields of ForkJoinWorkerThread maintain |
| * data structures managed by ForkJoinPool, so are directly |
| * accessed. Conversely we allow access to "workers" array by |
| * workers, and direct access to ForkJoinTask.status by both |
| * ForkJoinPool and ForkJoinWorkerThread. There is little point |
| * trying to reduce this, since any associated future changes in |
| * representations will need to be accompanied by algorithmic |
| * changes anyway. All together, these low-level implementation |
| * choices produce as much as a factor of 4 performance |
| * improvement compared to naive implementations, and enable the |
| * processing of billions of tasks per second, at the expense of |
| * some ugliness. |
| * |
| * Methods signalWork() and scan() are the main bottlenecks so are |
| * especially heavily micro-optimized/mangled. There are lots of |
| * inline assignments (of form "while ((local = field) != 0)") |
| * which are usually the simplest way to ensure the required read |
| * orderings (which are sometimes critical). This leads to a |
| * "C"-like style of listing declarations of these locals at the |
| * heads of methods or blocks. There are several occurrences of |
| * the unusual "do {} while (!cas...)" which is the simplest way |
| * to force an update of a CAS'ed variable. There are also other |
| * coding oddities that help some methods perform reasonably even |
| * when interpreted (not compiled). |
| * |
| * The order of declarations in this file is: (1) declarations of |
| * statics (2) fields (along with constants used when unpacking |
| * some of them), listed in an order that tends to reduce |
| * contention among them a bit under most JVMs. (3) internal |
| * control methods (4) callbacks and other support for |
| * ForkJoinTask and ForkJoinWorkerThread classes, (5) exported |
| * methods (plus a few little helpers). (6) static block |
| * initializing all statics in a minimally dependent order. |
| */ |
| |
| /** |
| * Factory for creating new {@link ForkJoinWorkerThread}s. |
| * A {@code ForkJoinWorkerThreadFactory} must be defined and used |
| * for {@code ForkJoinWorkerThread} subclasses that extend base |
| * functionality or initialize threads with different contexts. |
| */ |
| public static interface ForkJoinWorkerThreadFactory { |
| /** |
| * Returns a new worker thread operating in the given pool. |
| * |
| * @param pool the pool this thread works in |
| * @throws NullPointerException if the pool is null |
| */ |
| public ForkJoinWorkerThread newThread(ForkJoinPool pool); |
| } |
| |
| /** |
| * Default ForkJoinWorkerThreadFactory implementation; creates a |
| * new ForkJoinWorkerThread. |
| */ |
| static class DefaultForkJoinWorkerThreadFactory |
| implements ForkJoinWorkerThreadFactory { |
| public ForkJoinWorkerThread newThread(ForkJoinPool pool) { |
| return new ForkJoinWorkerThread(pool); |
| } |
| } |
| |
| /** |
| * Creates a new ForkJoinWorkerThread. This factory is used unless |
| * overridden in ForkJoinPool constructors. |
| */ |
| public static final ForkJoinWorkerThreadFactory |
| defaultForkJoinWorkerThreadFactory; |
| |
| /** |
| * Permission required for callers of methods that may start or |
| * kill threads. |
| */ |
| private static final RuntimePermission modifyThreadPermission; |
| |
| /** |
| * If there is a security manager, makes sure caller has |
| * permission to modify threads. |
| */ |
| private static void checkPermission() { |
| SecurityManager security = System.getSecurityManager(); |
| if (security != null) |
| security.checkPermission(modifyThreadPermission); |
| } |
| |
| /** |
| * Generator for assigning sequence numbers as pool names. |
| */ |
| private static final AtomicInteger poolNumberGenerator; |
| |
| /** |
| * Generator for initial random seeds for worker victim |
| * selection. This is used only to create initial seeds. Random |
| * steals use a cheaper xorshift generator per steal attempt. We |
| * don't expect much contention on seedGenerator, so just use a |
| * plain Random. |
| */ |
| static final Random workerSeedGenerator; |
| |
| /** |
| * Array holding all worker threads in the pool. Initialized upon |
| * construction. Array size must be a power of two. Updates and |
| * replacements are protected by scanGuard, but the array is |
| * always kept in a consistent enough state to be randomly |
| * accessed without locking by workers performing work-stealing, |
| * as well as other traversal-based methods in this class, so long |
| * as reads memory-acquire by first reading ctl. All readers must |
| * tolerate that some array slots may be null. |
| */ |
| ForkJoinWorkerThread[] workers; |
| |
| /** |
| * Initial size for submission queue array. Must be a power of |
| * two. In many applications, these always stay small so we use a |
| * small initial cap. |
| */ |
| private static final int INITIAL_QUEUE_CAPACITY = 8; |
| |
| /** |
| * Maximum size for submission queue array. Must be a power of two |
| * less than or equal to 1 << (31 - width of array entry) to |
| * ensure lack of index wraparound, but is capped at a lower |
| * value to help users trap runaway computations. |
| */ |
| private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 24; // 16M |
| |
| /** |
| * Array serving as submission queue. Initialized upon construction. |
| */ |
| private ForkJoinTask<?>[] submissionQueue; |
| |
| /** |
| * Lock protecting submissions array for addSubmission |
| */ |
| private final ReentrantLock submissionLock; |
| |
| /** |
| * Condition for awaitTermination, using submissionLock for |
| * convenience. |
| */ |
| private final Condition termination; |
| |
| /** |
| * Creation factory for worker threads. |
| */ |
| private final ForkJoinWorkerThreadFactory factory; |
| |
| /** |
| * The uncaught exception handler used when any worker abruptly |
| * terminates. |
| */ |
| final Thread.UncaughtExceptionHandler ueh; |
| |
| /** |
| * Prefix for assigning names to worker threads |
| */ |
| private final String workerNamePrefix; |
| |
| /** |
| * Sum of per-thread steal counts, updated only when threads are |
| * idle or terminating. |
| */ |
| private volatile long stealCount; |
| |
| /** |
| * Main pool control -- a long packed with: |
| * AC: Number of active running workers minus target parallelism (16 bits) |
| * TC: Number of total workers minus target parallelism (16 bits) |
| * ST: true if pool is terminating (1 bit) |
| * EC: the wait count of top waiting thread (15 bits) |
| * ID: ~poolIndex of top of Treiber stack of waiting threads (16 bits) |
| * |
| * When convenient, we can extract the upper 32 bits of counts and |
| * the lower 32 bits of queue state, u = (int)(ctl >>> 32) and e = |
| * (int)ctl. The ec field is never accessed alone, but always |
| * together with id and st. The offsets of counts by the target |
| * parallelism and the positionings of fields makes it possible to |
| * perform the most common checks via sign tests of fields: When |
| * ac is negative, there are not enough active workers, when tc is |
| * negative, there are not enough total workers, when id is |
| * negative, there is at least one waiting worker, and when e is |
| * negative, the pool is terminating. To deal with these possibly |
| * negative fields, we use casts in and out of "short" and/or |
| * signed shifts to maintain signedness. |
| */ |
| volatile long ctl; |
| |
| // bit positions/shifts for fields |
| private static final int AC_SHIFT = 48; |
| private static final int TC_SHIFT = 32; |
| private static final int ST_SHIFT = 31; |
| private static final int EC_SHIFT = 16; |
| |
| // bounds |
| private static final int MAX_ID = 0x7fff; // max poolIndex |
| private static final int SMASK = 0xffff; // mask short bits |
| private static final int SHORT_SIGN = 1 << 15; |
| private static final int INT_SIGN = 1 << 31; |
| |
| // masks |
| private static final long STOP_BIT = 0x0001L << ST_SHIFT; |
| private static final long AC_MASK = ((long)SMASK) << AC_SHIFT; |
| private static final long TC_MASK = ((long)SMASK) << TC_SHIFT; |
| |
| // units for incrementing and decrementing |
| private static final long TC_UNIT = 1L << TC_SHIFT; |
| private static final long AC_UNIT = 1L << AC_SHIFT; |
| |
| // masks and units for dealing with u = (int)(ctl >>> 32) |
| private static final int UAC_SHIFT = AC_SHIFT - 32; |
| private static final int UTC_SHIFT = TC_SHIFT - 32; |
| private static final int UAC_MASK = SMASK << UAC_SHIFT; |
| private static final int UTC_MASK = SMASK << UTC_SHIFT; |
| private static final int UAC_UNIT = 1 << UAC_SHIFT; |
| private static final int UTC_UNIT = 1 << UTC_SHIFT; |
| |
| // masks and units for dealing with e = (int)ctl |
| private static final int E_MASK = 0x7fffffff; // no STOP_BIT |
| private static final int EC_UNIT = 1 << EC_SHIFT; |
| |
| /** |
| * The target parallelism level. |
| */ |
| final int parallelism; |
| |
| /** |
| * Index (mod submission queue length) of next element to take |
| * from submission queue. Usage is identical to that for |
| * per-worker queues -- see ForkJoinWorkerThread internal |
| * documentation. |
| */ |
| volatile int queueBase; |
| |
| /** |
| * Index (mod submission queue length) of next element to add |
| * in submission queue. Usage is identical to that for |
| * per-worker queues -- see ForkJoinWorkerThread internal |
| * documentation. |
| */ |
| int queueTop; |
| |
| /** |
| * True when shutdown() has been called. |
| */ |
| volatile boolean shutdown; |
| |
| /** |
| * True if use local fifo, not default lifo, for local polling. |
| * Read by, and replicated by ForkJoinWorkerThreads. |
| */ |
| final boolean locallyFifo; |
| |
| /** |
| * The number of threads in ForkJoinWorkerThreads.helpQuiescePool. |
| * When non-zero, suppresses automatic shutdown when active |
| * counts become zero. |
| */ |
| volatile int quiescerCount; |
| |
| /** |
| * The number of threads blocked in join. |
| */ |
| volatile int blockedCount; |
| |
| /** |
| * Counter for worker Thread names (unrelated to their poolIndex) |
| */ |
| private volatile int nextWorkerNumber; |
| |
| /** |
| * The index for the next created worker. Accessed under scanGuard. |
| */ |
| private int nextWorkerIndex; |
| |
| /** |
| * SeqLock and index masking for updates to workers array. Locked |
| * when SG_UNIT is set. Unlocking clears bit by adding |
| * SG_UNIT. Staleness of read-only operations can be checked by |
| * comparing scanGuard to value before the reads. The low 16 bits |
| * (i.e, anding with SMASK) hold (the smallest power of two |
| * covering all worker indices, minus one, and is used to avoid |
| * dealing with large numbers of null slots when the workers array |
| * is overallocated. |
| */ |
| volatile int scanGuard; |
| |
| private static final int SG_UNIT = 1 << 16; |
| |
| /** |
| * The wakeup interval (in nanoseconds) for a worker waiting for a |
| * task when the pool is quiescent to instead try to shrink the |
| * number of workers. The exact value does not matter too |
| * much. It must be short enough to release resources during |
| * sustained periods of idleness, but not so short that threads |
| * are continually re-created. |
| */ |
| private static final long SHRINK_RATE = |
| 4L * 1000L * 1000L * 1000L; // 4 seconds |
| |
| /** |
| * Top-level loop for worker threads: On each step: if the |
| * previous step swept through all queues and found no tasks, or |
| * there are excess threads, then possibly blocks. Otherwise, |
| * scans for and, if found, executes a task. Returns when pool |
| * and/or worker terminate. |
| * |
| * @param w the worker |
| */ |
| final void work(ForkJoinWorkerThread w) { |
| boolean swept = false; // true on empty scans |
| long c; |
| while (!w.terminate && (int)(c = ctl) >= 0) { |
| int a; // active count |
| if (!swept && (a = (int)(c >> AC_SHIFT)) <= 0) |
| swept = scan(w, a); |
| else if (tryAwaitWork(w, c)) |
| swept = false; |
| } |
| } |
| |
| // Signalling |
| |
| /** |
| * Wakes up or creates a worker. |
| */ |
| final void signalWork() { |
| /* |
| * The while condition is true if: (there is are too few total |
| * workers OR there is at least one waiter) AND (there are too |
| * few active workers OR the pool is terminating). The value |
| * of e distinguishes the remaining cases: zero (no waiters) |
| * for create, negative if terminating (in which case do |
| * nothing), else release a waiter. The secondary checks for |
| * release (non-null array etc) can fail if the pool begins |
| * terminating after the test, and don't impose any added cost |
| * because JVMs must perform null and bounds checks anyway. |
| */ |
| long c; int e, u; |
| while ((((e = (int)(c = ctl)) | (u = (int)(c >>> 32))) & |
| (INT_SIGN|SHORT_SIGN)) == (INT_SIGN|SHORT_SIGN) && e >= 0) { |
| if (e > 0) { // release a waiting worker |
| int i; ForkJoinWorkerThread w; ForkJoinWorkerThread[] ws; |
| if ((ws = workers) == null || |
| (i = ~e & SMASK) >= ws.length || |
| (w = ws[i]) == null) |
| break; |
| long nc = (((long)(w.nextWait & E_MASK)) | |
| ((long)(u + UAC_UNIT) << 32)); |
| if (w.eventCount == e && |
| UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) { |
| w.eventCount = (e + EC_UNIT) & E_MASK; |
| if (w.parked) |
| UNSAFE.unpark(w); |
| break; |
| } |
| } |
| else if (UNSAFE.compareAndSwapLong |
| (this, ctlOffset, c, |
| (long)(((u + UTC_UNIT) & UTC_MASK) | |
| ((u + UAC_UNIT) & UAC_MASK)) << 32)) { |
| addWorker(); |
| break; |
| } |
| } |
| } |
| |
| /** |
| * Variant of signalWork to help release waiters on rescans. |
| * Tries once to release a waiter if active count < 0. |
| * |
| * @return false if failed due to contention, else true |
| */ |
| private boolean tryReleaseWaiter() { |
| long c; int e, i; ForkJoinWorkerThread w; ForkJoinWorkerThread[] ws; |
| if ((e = (int)(c = ctl)) > 0 && |
| (int)(c >> AC_SHIFT) < 0 && |
| (ws = workers) != null && |
| (i = ~e & SMASK) < ws.length && |
| (w = ws[i]) != null) { |
| long nc = ((long)(w.nextWait & E_MASK) | |
| ((c + AC_UNIT) & (AC_MASK|TC_MASK))); |
| if (w.eventCount != e || |
| !UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) |
| return false; |
| w.eventCount = (e + EC_UNIT) & E_MASK; |
| if (w.parked) |
| UNSAFE.unpark(w); |
| } |
| return true; |
| } |
| |
| // Scanning for tasks |
| |
| /** |
| * Scans for and, if found, executes one task. Scans start at a |
| * random index of workers array, and randomly select the first |
| * (2*#workers)-1 probes, and then, if all empty, resort to 2 |
| * circular sweeps, which is necessary to check quiescence. and |
| * taking a submission only if no stealable tasks were found. The |
| * steal code inside the loop is a specialized form of |
| * ForkJoinWorkerThread.deqTask, followed bookkeeping to support |
| * helpJoinTask and signal propagation. The code for submission |
| * queues is almost identical. On each steal, the worker completes |
| * not only the task, but also all local tasks that this task may |
| * have generated. On detecting staleness or contention when |
| * trying to take a task, this method returns without finishing |
| * sweep, which allows global state rechecks before retry. |
| * |
| * @param w the worker |
| * @param a the number of active workers |
| * @return true if swept all queues without finding a task |
| */ |
| private boolean scan(ForkJoinWorkerThread w, int a) { |
| int g = scanGuard; // mask 0 avoids useless scans if only one active |
| int m = (parallelism == 1 - a && blockedCount == 0) ? 0 : g & SMASK; |
| ForkJoinWorkerThread[] ws = workers; |
| if (ws == null || ws.length <= m) // staleness check |
| return false; |
| for (int r = w.seed, k = r, j = -(m + m); j <= m + m; ++j) { |
| ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i; |
| ForkJoinWorkerThread v = ws[k & m]; |
| if (v != null && (b = v.queueBase) != v.queueTop && |
| (q = v.queue) != null && (i = (q.length - 1) & b) >= 0) { |
| long u = (i << ASHIFT) + ABASE; |
| if ((t = q[i]) != null && v.queueBase == b && |
| UNSAFE.compareAndSwapObject(q, u, t, null)) { |
| int d = (v.queueBase = b + 1) - v.queueTop; |
| v.stealHint = w.poolIndex; |
| if (d != 0) |
| signalWork(); // propagate if nonempty |
| w.execTask(t); |
| } |
| r ^= r << 13; r ^= r >>> 17; w.seed = r ^ (r << 5); |
| return false; // store next seed |
| } |
| else if (j < 0) { // xorshift |
| r ^= r << 13; r ^= r >>> 17; k = r ^= r << 5; |
| } |
| else |
| ++k; |
| } |
| if (scanGuard != g) // staleness check |
| return false; |
| else { // try to take submission |
| ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i; |
| if ((b = queueBase) != queueTop && |
| (q = submissionQueue) != null && |
| (i = (q.length - 1) & b) >= 0) { |
| long u = (i << ASHIFT) + ABASE; |
| if ((t = q[i]) != null && queueBase == b && |
| UNSAFE.compareAndSwapObject(q, u, t, null)) { |
| queueBase = b + 1; |
| w.execTask(t); |
| } |
| return false; |
| } |
| return true; // all queues empty |
| } |
| } |
| |
| /** |
| * Tries to enqueue worker w in wait queue and await change in |
| * worker's eventCount. If the pool is quiescent and there is |
| * more than one worker, possibly terminates worker upon exit. |
| * Otherwise, before blocking, rescans queues to avoid missed |
| * signals. Upon finding work, releases at least one worker |
| * (which may be the current worker). Rescans restart upon |
| * detected staleness or failure to release due to |
| * contention. Note the unusual conventions about Thread.interrupt |
| * here and elsewhere: Because interrupts are used solely to alert |
| * threads to check termination, which is checked here anyway, we |
| * clear status (using Thread.interrupted) before any call to |
| * park, so that park does not immediately return due to status |
| * being set via some other unrelated call to interrupt in user |
| * code. |
| * |
| * @param w the calling worker |
| * @param c the ctl value on entry |
| * @return true if waited or another thread was released upon enq |
| */ |
| private boolean tryAwaitWork(ForkJoinWorkerThread w, long c) { |
| int v = w.eventCount; |
| w.nextWait = (int)c; // w's successor record |
| long nc = (long)(v & E_MASK) | ((c - AC_UNIT) & (AC_MASK|TC_MASK)); |
| if (ctl != c || !UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) { |
| long d = ctl; // return true if lost to a deq, to force scan |
| return (int)d != (int)c && (d & AC_MASK) >= (c & AC_MASK); |
| } |
| for (int sc = w.stealCount; sc != 0;) { // accumulate stealCount |
| long s = stealCount; |
| if (UNSAFE.compareAndSwapLong(this, stealCountOffset, s, s + sc)) |
| sc = w.stealCount = 0; |
| else if (w.eventCount != v) |
| return true; // update next time |
| } |
| if ((!shutdown || !tryTerminate(false)) && |
| (int)c != 0 && parallelism + (int)(nc >> AC_SHIFT) == 0 && |
| blockedCount == 0 && quiescerCount == 0) |
| idleAwaitWork(w, nc, c, v); // quiescent |
| for (boolean rescanned = false;;) { |
| if (w.eventCount != v) |
| return true; |
| if (!rescanned) { |
| int g = scanGuard, m = g & SMASK; |
| ForkJoinWorkerThread[] ws = workers; |
| if (ws != null && m < ws.length) { |
| rescanned = true; |
| for (int i = 0; i <= m; ++i) { |
| ForkJoinWorkerThread u = ws[i]; |
| if (u != null) { |
| if (u.queueBase != u.queueTop && |
| !tryReleaseWaiter()) |
| rescanned = false; // contended |
| if (w.eventCount != v) |
| return true; |
| } |
| } |
| } |
| if (scanGuard != g || // stale |
| (queueBase != queueTop && !tryReleaseWaiter())) |
| rescanned = false; |
| if (!rescanned) |
| Thread.yield(); // reduce contention |
| else |
| Thread.interrupted(); // clear before park |
| } |
| else { |
| w.parked = true; // must recheck |
| if (w.eventCount != v) { |
| w.parked = false; |
| return true; |
| } |
| LockSupport.park(this); |
| rescanned = w.parked = false; |
| } |
| } |
| } |
| |
| /** |
| * If inactivating worker w has caused pool to become |
| * quiescent, check for pool termination, and wait for event |
| * for up to SHRINK_RATE nanosecs (rescans are unnecessary in |
| * this case because quiescence reflects consensus about lack |
| * of work). On timeout, if ctl has not changed, terminate the |
| * worker. Upon its termination (see deregisterWorker), it may |
| * wake up another worker to possibly repeat this process. |
| * |
| * @param w the calling worker |
| * @param currentCtl the ctl value after enqueuing w |
| * @param prevCtl the ctl value if w terminated |
| * @param v the eventCount w awaits change |
| */ |
| private void idleAwaitWork(ForkJoinWorkerThread w, long currentCtl, |
| long prevCtl, int v) { |
| if (w.eventCount == v) { |
| if (shutdown) |
| tryTerminate(false); |
| ForkJoinTask.helpExpungeStaleExceptions(); // help clean weak refs |
| while (ctl == currentCtl) { |
| long startTime = System.nanoTime(); |
| w.parked = true; |
| if (w.eventCount == v) // must recheck |
| LockSupport.parkNanos(this, SHRINK_RATE); |
| w.parked = false; |
| if (w.eventCount != v) |
| break; |
| else if (System.nanoTime() - startTime < |
| SHRINK_RATE - (SHRINK_RATE / 10)) // timing slop |
| Thread.interrupted(); // spurious wakeup |
| else if (UNSAFE.compareAndSwapLong(this, ctlOffset, |
| currentCtl, prevCtl)) { |
| w.terminate = true; // restore previous |
| w.eventCount = ((int)currentCtl + EC_UNIT) & E_MASK; |
| break; |
| } |
| } |
| } |
| } |
| |
| // Submissions |
| |
| /** |
| * Enqueues the given task in the submissionQueue. Same idea as |
| * ForkJoinWorkerThread.pushTask except for use of submissionLock. |
| * |
| * @param t the task |
| */ |
| private void addSubmission(ForkJoinTask<?> t) { |
| final ReentrantLock lock = this.submissionLock; |
| lock.lock(); |
| try { |
| ForkJoinTask<?>[] q; int s, m; |
| if ((q = submissionQueue) != null) { // ignore if queue removed |
| long u = (((s = queueTop) & (m = q.length-1)) << ASHIFT)+ABASE; |
| UNSAFE.putOrderedObject(q, u, t); |
| queueTop = s + 1; |
| if (s - queueBase == m) |
| growSubmissionQueue(); |
| } |
| } finally { |
| lock.unlock(); |
| } |
| signalWork(); |
| } |
| |
| // (pollSubmission is defined below with exported methods) |
| |
| /** |
| * Creates or doubles submissionQueue array. |
| * Basically identical to ForkJoinWorkerThread version. |
| */ |
| private void growSubmissionQueue() { |
| ForkJoinTask<?>[] oldQ = submissionQueue; |
| int size = oldQ != null ? oldQ.length << 1 : INITIAL_QUEUE_CAPACITY; |
| if (size > MAXIMUM_QUEUE_CAPACITY) |
| throw new RejectedExecutionException("Queue capacity exceeded"); |
| if (size < INITIAL_QUEUE_CAPACITY) |
| size = INITIAL_QUEUE_CAPACITY; |
| ForkJoinTask<?>[] q = submissionQueue = new ForkJoinTask<?>[size]; |
| int mask = size - 1; |
| int top = queueTop; |
| int oldMask; |
| if (oldQ != null && (oldMask = oldQ.length - 1) >= 0) { |
| for (int b = queueBase; b != top; ++b) { |
| long u = ((b & oldMask) << ASHIFT) + ABASE; |
| Object x = UNSAFE.getObjectVolatile(oldQ, u); |
| if (x != null && UNSAFE.compareAndSwapObject(oldQ, u, x, null)) |
| UNSAFE.putObjectVolatile |
| (q, ((b & mask) << ASHIFT) + ABASE, x); |
| } |
| } |
| } |
| |
| // Blocking support |
| |
| /** |
| * Tries to increment blockedCount, decrement active count |
| * (sometimes implicitly) and possibly release or create a |
| * compensating worker in preparation for blocking. Fails |
| * on contention or termination. |
| * |
| * @return true if the caller can block, else should recheck and retry |
| */ |
| private boolean tryPreBlock() { |
| int b = blockedCount; |
| if (UNSAFE.compareAndSwapInt(this, blockedCountOffset, b, b + 1)) { |
| int pc = parallelism; |
| do { |
| ForkJoinWorkerThread[] ws; ForkJoinWorkerThread w; |
| int e, ac, tc, i; |
| long c = ctl; |
| int u = (int)(c >>> 32); |
| if ((e = (int)c) < 0) { |
| // skip -- terminating |
| } |
| else if ((ac = (u >> UAC_SHIFT)) <= 0 && e != 0 && |
| (ws = workers) != null && |
| (i = ~e & SMASK) < ws.length && |
| (w = ws[i]) != null) { |
| long nc = ((long)(w.nextWait & E_MASK) | |
| (c & (AC_MASK|TC_MASK))); |
| if (w.eventCount == e && |
| UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) { |
| w.eventCount = (e + EC_UNIT) & E_MASK; |
| if (w.parked) |
| UNSAFE.unpark(w); |
| return true; // release an idle worker |
| } |
| } |
| else if ((tc = (short)(u >>> UTC_SHIFT)) >= 0 && ac + pc > 1) { |
| long nc = ((c - AC_UNIT) & AC_MASK) | (c & ~AC_MASK); |
| if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) |
| return true; // no compensation needed |
| } |
| else if (tc + pc < MAX_ID) { |
| long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK); |
| if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) { |
| addWorker(); |
| return true; // create a replacement |
| } |
| } |
| // try to back out on any failure and let caller retry |
| } while (!UNSAFE.compareAndSwapInt(this, blockedCountOffset, |
| b = blockedCount, b - 1)); |
| } |
| return false; |
| } |
| |
| /** |
| * Decrements blockedCount and increments active count. |
| */ |
| private void postBlock() { |
| long c; |
| do {} while (!UNSAFE.compareAndSwapLong(this, ctlOffset, // no mask |
| c = ctl, c + AC_UNIT)); |
| int b; |
| do {} while (!UNSAFE.compareAndSwapInt(this, blockedCountOffset, |
| b = blockedCount, b - 1)); |
| } |
| |
| /** |
| * Possibly blocks waiting for the given task to complete, or |
| * cancels the task if terminating. Fails to wait if contended. |
| * |
| * @param joinMe the task |
| */ |
| final void tryAwaitJoin(ForkJoinTask<?> joinMe) { |
| Thread.interrupted(); // clear interrupts before checking termination |
| if (joinMe.status >= 0) { |
| if (tryPreBlock()) { |
| joinMe.tryAwaitDone(0L); |
| postBlock(); |
| } |
| else if ((ctl & STOP_BIT) != 0L) |
| joinMe.cancelIgnoringExceptions(); |
| } |
| } |
| |
| /** |
| * Possibly blocks the given worker waiting for joinMe to |
| * complete or timeout. |
| * |
| * @param joinMe the task |
| * @param nanos the wait time for underlying Object.wait |
| */ |
| final void timedAwaitJoin(ForkJoinTask<?> joinMe, long nanos) { |
| while (joinMe.status >= 0) { |
| Thread.interrupted(); |
| if ((ctl & STOP_BIT) != 0L) { |
| joinMe.cancelIgnoringExceptions(); |
| break; |
| } |
| if (tryPreBlock()) { |
| long last = System.nanoTime(); |
| while (joinMe.status >= 0) { |
| long millis = TimeUnit.NANOSECONDS.toMillis(nanos); |
| if (millis <= 0) |
| break; |
| joinMe.tryAwaitDone(millis); |
| if (joinMe.status < 0) |
| break; |
| if ((ctl & STOP_BIT) != 0L) { |
| joinMe.cancelIgnoringExceptions(); |
| break; |
| } |
| long now = System.nanoTime(); |
| nanos -= now - last; |
| last = now; |
| } |
| postBlock(); |
| break; |
| } |
| } |
| } |
| |
| /** |
| * If necessary, compensates for blocker, and blocks. |
| */ |
| private void awaitBlocker(ManagedBlocker blocker) |
| throws InterruptedException { |
| while (!blocker.isReleasable()) { |
| if (tryPreBlock()) { |
| try { |
| do {} while (!blocker.isReleasable() && !blocker.block()); |
| } finally { |
| postBlock(); |
| } |
| break; |
| } |
| } |
| } |
| |
| // Creating, registering and deregistring workers |
| |
| /** |
| * Tries to create and start a worker; minimally rolls back counts |
| * on failure. |
| */ |
| private void addWorker() { |
| Throwable ex = null; |
| ForkJoinWorkerThread t = null; |
| try { |
| t = factory.newThread(this); |
| } catch (Throwable e) { |
| ex = e; |
| } |
| if (t == null) { // null or exceptional factory return |
| long c; // adjust counts |
| do {} while (!UNSAFE.compareAndSwapLong |
| (this, ctlOffset, c = ctl, |
| (((c - AC_UNIT) & AC_MASK) | |
| ((c - TC_UNIT) & TC_MASK) | |
| (c & ~(AC_MASK|TC_MASK))))); |
| // Propagate exception if originating from an external caller |
| if (!tryTerminate(false) && ex != null && |
| !(Thread.currentThread() instanceof ForkJoinWorkerThread)) |
| SneakyThrow.sneakyThrow(ex); // android-changed |
| } |
| else |
| t.start(); |
| } |
| |
| /** |
| * Callback from ForkJoinWorkerThread constructor to assign a |
| * public name |
| */ |
| final String nextWorkerName() { |
| for (int n;;) { |
| if (UNSAFE.compareAndSwapInt(this, nextWorkerNumberOffset, |
| n = nextWorkerNumber, ++n)) |
| return workerNamePrefix + n; |
| } |
| } |
| |
| /** |
| * Callback from ForkJoinWorkerThread constructor to |
| * determine its poolIndex and record in workers array. |
| * |
| * @param w the worker |
| * @return the worker's pool index |
| */ |
| final int registerWorker(ForkJoinWorkerThread w) { |
| /* |
| * In the typical case, a new worker acquires the lock, uses |
| * next available index and returns quickly. Since we should |
| * not block callers (ultimately from signalWork or |
| * tryPreBlock) waiting for the lock needed to do this, we |
| * instead help release other workers while waiting for the |
| * lock. |
| */ |
| for (int g;;) { |
| ForkJoinWorkerThread[] ws; |
| if (((g = scanGuard) & SG_UNIT) == 0 && |
| UNSAFE.compareAndSwapInt(this, scanGuardOffset, |
| g, g | SG_UNIT)) { |
| int k = nextWorkerIndex; |
| try { |
| if ((ws = workers) != null) { // ignore on shutdown |
| int n = ws.length; |
| if (k < 0 || k >= n || ws[k] != null) { |
| for (k = 0; k < n && ws[k] != null; ++k) |
| ; |
| if (k == n) |
| ws = workers = Arrays.copyOf(ws, n << 1); |
| } |
| ws[k] = w; |
| nextWorkerIndex = k + 1; |
| int m = g & SMASK; |
| g = (k > m) ? ((m << 1) + 1) & SMASK : g + (SG_UNIT<<1); |
| } |
| } finally { |
| scanGuard = g; |
| } |
| return k; |
| } |
| else if ((ws = workers) != null) { // help release others |
| for (ForkJoinWorkerThread u : ws) { |
| if (u != null && u.queueBase != u.queueTop) { |
| if (tryReleaseWaiter()) |
| break; |
| } |
| } |
| } |
| } |
| } |
| |
| /** |
| * Final callback from terminating worker. Removes record of |
| * worker from array, and adjusts counts. If pool is shutting |
| * down, tries to complete termination. |
| * |
| * @param w the worker |
| */ |
| final void deregisterWorker(ForkJoinWorkerThread w, Throwable ex) { |
| int idx = w.poolIndex; |
| int sc = w.stealCount; |
| int steps = 0; |
| // Remove from array, adjust worker counts and collect steal count. |
| // We can intermix failed removes or adjusts with steal updates |
| do { |
| long s, c; |
| int g; |
| if (steps == 0 && ((g = scanGuard) & SG_UNIT) == 0 && |
| UNSAFE.compareAndSwapInt(this, scanGuardOffset, |
| g, g |= SG_UNIT)) { |
| ForkJoinWorkerThread[] ws = workers; |
| if (ws != null && idx >= 0 && |
| idx < ws.length && ws[idx] == w) |
| ws[idx] = null; // verify |
| nextWorkerIndex = idx; |
| scanGuard = g + SG_UNIT; |
| steps = 1; |
| } |
| if (steps == 1 && |
| UNSAFE.compareAndSwapLong(this, ctlOffset, c = ctl, |
| (((c - AC_UNIT) & AC_MASK) | |
| ((c - TC_UNIT) & TC_MASK) | |
| (c & ~(AC_MASK|TC_MASK))))) |
| steps = 2; |
| if (sc != 0 && |
| UNSAFE.compareAndSwapLong(this, stealCountOffset, |
| s = stealCount, s + sc)) |
| sc = 0; |
| } while (steps != 2 || sc != 0); |
| if (!tryTerminate(false)) { |
| if (ex != null) // possibly replace if died abnormally |
| signalWork(); |
| else |
| tryReleaseWaiter(); |
| } |
| } |
| |
| // Shutdown and termination |
| |
| /** |
| * Possibly initiates and/or completes termination. |
| * |
| * @param now if true, unconditionally terminate, else only |
| * if shutdown and empty queue and no active workers |
| * @return true if now terminating or terminated |
| */ |
| private boolean tryTerminate(boolean now) { |
| long c; |
| while (((c = ctl) & STOP_BIT) == 0) { |
| if (!now) { |
| if ((int)(c >> AC_SHIFT) != -parallelism) |
| return false; |
| if (!shutdown || blockedCount != 0 || quiescerCount != 0 || |
| queueBase != queueTop) { |
| if (ctl == c) // staleness check |
| return false; |
| continue; |
| } |
| } |
| if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, c | STOP_BIT)) |
| startTerminating(); |
| } |
| if ((short)(c >>> TC_SHIFT) == -parallelism) { // signal when 0 workers |
| final ReentrantLock lock = this.submissionLock; |
| lock.lock(); |
| try { |
| termination.signalAll(); |
| } finally { |
| lock.unlock(); |
| } |
| } |
| return true; |
| } |
| |
| /** |
| * Runs up to three passes through workers: (0) Setting |
| * termination status for each worker, followed by wakeups up to |
| * queued workers; (1) helping cancel tasks; (2) interrupting |
| * lagging threads (likely in external tasks, but possibly also |
| * blocked in joins). Each pass repeats previous steps because of |
| * potential lagging thread creation. |
| */ |
| private void startTerminating() { |
| cancelSubmissions(); |
| for (int pass = 0; pass < 3; ++pass) { |
| ForkJoinWorkerThread[] ws = workers; |
| if (ws != null) { |
| for (ForkJoinWorkerThread w : ws) { |
| if (w != null) { |
| w.terminate = true; |
| if (pass > 0) { |
| w.cancelTasks(); |
| if (pass > 1 && !w.isInterrupted()) { |
| try { |
| w.interrupt(); |
| } catch (SecurityException ignore) { |
| } |
| } |
| } |
| } |
| } |
| terminateWaiters(); |
| } |
| } |
| } |
| |
| /** |
| * Polls and cancels all submissions. Called only during termination. |
| */ |
| private void cancelSubmissions() { |
| while (queueBase != queueTop) { |
| ForkJoinTask<?> task = pollSubmission(); |
| if (task != null) { |
| try { |
| task.cancel(false); |
| } catch (Throwable ignore) { |
| } |
| } |
| } |
| } |
| |
| /** |
| * Tries to set the termination status of waiting workers, and |
| * then wakes them up (after which they will terminate). |
| */ |
| private void terminateWaiters() { |
| ForkJoinWorkerThread[] ws = workers; |
| if (ws != null) { |
| ForkJoinWorkerThread w; long c; int i, e; |
| int n = ws.length; |
| while ((i = ~(e = (int)(c = ctl)) & SMASK) < n && |
| (w = ws[i]) != null && w.eventCount == (e & E_MASK)) { |
| if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, |
| (long)(w.nextWait & E_MASK) | |
| ((c + AC_UNIT) & AC_MASK) | |
| (c & (TC_MASK|STOP_BIT)))) { |
| w.terminate = true; |
| w.eventCount = e + EC_UNIT; |
| if (w.parked) |
| UNSAFE.unpark(w); |
| } |
| } |
| } |
| } |
| |
| // misc ForkJoinWorkerThread support |
| |
| /** |
| * Increments or decrements quiescerCount. Needed only to prevent |
| * triggering shutdown if a worker is transiently inactive while |
| * checking quiescence. |
| * |
| * @param delta 1 for increment, -1 for decrement |
| */ |
| final void addQuiescerCount(int delta) { |
| int c; |
| do {} while (!UNSAFE.compareAndSwapInt(this, quiescerCountOffset, |
| c = quiescerCount, c + delta)); |
| } |
| |
| /** |
| * Directly increments or decrements active count without queuing. |
| * This method is used to transiently assert inactivation while |
| * checking quiescence. |
| * |
| * @param delta 1 for increment, -1 for decrement |
| */ |
| final void addActiveCount(int delta) { |
| long d = (long)delta << AC_SHIFT; |
| long c; |
| do {} while (!UNSAFE.compareAndSwapLong(this, ctlOffset, |
| c = ctl, c + d)); |
| } |
| |
| /** |
| * Returns the approximate (non-atomic) number of idle threads per |
| * active thread. |
| */ |
| final int idlePerActive() { |
| // Approximate at powers of two for small values, saturate past 4 |
| int p = parallelism; |
| int a = p + (int)(ctl >> AC_SHIFT); |
| return (a > (p >>>= 1) ? 0 : |
| a > (p >>>= 1) ? 1 : |
| a > (p >>>= 1) ? 2 : |
| a > (p >>>= 1) ? 4 : |
| 8); |
| } |
| |
| // Exported methods |
| |
| // Constructors |
| |
| /** |
| * Creates a {@code ForkJoinPool} with parallelism equal to {@link |
| * java.lang.Runtime#availableProcessors}, using the {@linkplain |
| * #defaultForkJoinWorkerThreadFactory default thread factory}, |
| * no UncaughtExceptionHandler, and non-async LIFO processing mode. |
| */ |
| public ForkJoinPool() { |
| this(Runtime.getRuntime().availableProcessors(), |
| defaultForkJoinWorkerThreadFactory, null, false); |
| } |
| |
| /** |
| * Creates a {@code ForkJoinPool} with the indicated parallelism |
| * level, the {@linkplain |
| * #defaultForkJoinWorkerThreadFactory default thread factory}, |
| * no UncaughtExceptionHandler, and non-async LIFO processing mode. |
| * |
| * @param parallelism the parallelism level |
| * @throws IllegalArgumentException if parallelism less than or |
| * equal to zero, or greater than implementation limit |
| */ |
| public ForkJoinPool(int parallelism) { |
| this(parallelism, defaultForkJoinWorkerThreadFactory, null, false); |
| } |
| |
| /** |
| * Creates a {@code ForkJoinPool} with the given parameters. |
| * |
| * @param parallelism the parallelism level. For default value, |
| * use {@link java.lang.Runtime#availableProcessors}. |
| * @param factory the factory for creating new threads. For default value, |
| * use {@link #defaultForkJoinWorkerThreadFactory}. |
| * @param handler the handler for internal worker threads that |
| * terminate due to unrecoverable errors encountered while executing |
| * tasks. For default value, use {@code null}. |
| * @param asyncMode if true, |
| * establishes local first-in-first-out scheduling mode for forked |
| * tasks that are never joined. This mode may be more appropriate |
| * than default locally stack-based mode in applications in which |
| * worker threads only process event-style asynchronous tasks. |
| * For default value, use {@code false}. |
| * @throws IllegalArgumentException if parallelism less than or |
| * equal to zero, or greater than implementation limit |
| * @throws NullPointerException if the factory is null |
| */ |
| public ForkJoinPool(int parallelism, |
| ForkJoinWorkerThreadFactory factory, |
| Thread.UncaughtExceptionHandler handler, |
| boolean asyncMode) { |
| checkPermission(); |
| if (factory == null) |
| throw new NullPointerException(); |
| if (parallelism <= 0 || parallelism > MAX_ID) |
| throw new IllegalArgumentException(); |
| this.parallelism = parallelism; |
| this.factory = factory; |
| this.ueh = handler; |
| this.locallyFifo = asyncMode; |
| long np = (long)(-parallelism); // offset ctl counts |
| this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK); |
| this.submissionQueue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY]; |
| // initialize workers array with room for 2*parallelism if possible |
| int n = parallelism << 1; |
| if (n >= MAX_ID) |
| n = MAX_ID; |
| else { // See Hackers Delight, sec 3.2, where n < (1 << 16) |
| n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; n |= n >>> 8; |
| } |
| workers = new ForkJoinWorkerThread[n + 1]; |
| this.submissionLock = new ReentrantLock(); |
| this.termination = submissionLock.newCondition(); |
| StringBuilder sb = new StringBuilder("ForkJoinPool-"); |
| sb.append(poolNumberGenerator.incrementAndGet()); |
| sb.append("-worker-"); |
| this.workerNamePrefix = sb.toString(); |
| } |
| |
| // Execution methods |
| |
| /** |
| * Performs the given task, returning its result upon completion. |
| * If the computation encounters an unchecked Exception or Error, |
| * it is rethrown as the outcome of this invocation. Rethrown |
| * exceptions behave in the same way as regular exceptions, but, |
| * when possible, contain stack traces (as displayed for example |
| * using {@code ex.printStackTrace()}) of both the current thread |
| * as well as the thread actually encountering the exception; |
| * minimally only the latter. |
| * |
| * @param task the task |
| * @return the task's result |
| * @throws NullPointerException if the task is null |
| * @throws RejectedExecutionException if the task cannot be |
| * scheduled for execution |
| */ |
| public <T> T invoke(ForkJoinTask<T> task) { |
| Thread t = Thread.currentThread(); |
| if (task == null) |
| throw new NullPointerException(); |
| if (shutdown) |
| throw new RejectedExecutionException(); |
| if ((t instanceof ForkJoinWorkerThread) && |
| ((ForkJoinWorkerThread)t).pool == this) |
| return task.invoke(); // bypass submit if in same pool |
| else { |
| addSubmission(task); |
| return task.join(); |
| } |
| } |
| |
| /** |
| * Unless terminating, forks task if within an ongoing FJ |
| * computation in the current pool, else submits as external task. |
| */ |
| private <T> void forkOrSubmit(ForkJoinTask<T> task) { |
| ForkJoinWorkerThread w; |
| Thread t = Thread.currentThread(); |
| if (shutdown) |
| throw new RejectedExecutionException(); |
| if ((t instanceof ForkJoinWorkerThread) && |
| (w = (ForkJoinWorkerThread)t).pool == this) |
| w.pushTask(task); |
| else |
| addSubmission(task); |
| } |
| |
| /** |
| * Arranges for (asynchronous) execution of the given task. |
| * |
| * @param task the task |
| * @throws NullPointerException if the task is null |
| * @throws RejectedExecutionException if the task cannot be |
| * scheduled for execution |
| */ |
| public void execute(ForkJoinTask<?> task) { |
| if (task == null) |
| throw new NullPointerException(); |
| forkOrSubmit(task); |
| } |
| |
| // AbstractExecutorService methods |
| |
| /** |
| * @throws NullPointerException if the task is null |
| * @throws RejectedExecutionException if the task cannot be |
| * scheduled for execution |
| */ |
| public void execute(Runnable task) { |
| if (task == null) |
| throw new NullPointerException(); |
| ForkJoinTask<?> job; |
| if (task instanceof ForkJoinTask<?>) // avoid re-wrap |
| job = (ForkJoinTask<?>) task; |
| else |
| job = ForkJoinTask.adapt(task, null); |
| forkOrSubmit(job); |
| } |
| |
| /** |
| * Submits a ForkJoinTask for execution. |
| * |
| * @param task the task to submit |
| * @return the task |
| * @throws NullPointerException if the task is null |
| * @throws RejectedExecutionException if the task cannot be |
| * scheduled for execution |
| */ |
| public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) { |
| if (task == null) |
| throw new NullPointerException(); |
| forkOrSubmit(task); |
| return task; |
| } |
| |
| /** |
| * @throws NullPointerException if the task is null |
| * @throws RejectedExecutionException if the task cannot be |
| * scheduled for execution |
| */ |
| public <T> ForkJoinTask<T> submit(Callable<T> task) { |
| if (task == null) |
| throw new NullPointerException(); |
| ForkJoinTask<T> job = ForkJoinTask.adapt(task); |
| forkOrSubmit(job); |
| return job; |
| } |
| |
| /** |
| * @throws NullPointerException if the task is null |
| * @throws RejectedExecutionException if the task cannot be |
| * scheduled for execution |
| */ |
| public <T> ForkJoinTask<T> submit(Runnable task, T result) { |
| if (task == null) |
| throw new NullPointerException(); |
| ForkJoinTask<T> job = ForkJoinTask.adapt(task, result); |
| forkOrSubmit(job); |
| return job; |
| } |
| |
| /** |
| * @throws NullPointerException if the task is null |
| * @throws RejectedExecutionException if the task cannot be |
| * scheduled for execution |
| */ |
| public ForkJoinTask<?> submit(Runnable task) { |
| if (task == null) |
| throw new NullPointerException(); |
| ForkJoinTask<?> job; |
| if (task instanceof ForkJoinTask<?>) // avoid re-wrap |
| job = (ForkJoinTask<?>) task; |
| else |
| job = ForkJoinTask.adapt(task, null); |
| forkOrSubmit(job); |
| return job; |
| } |
| |
| /** |
| * @throws NullPointerException {@inheritDoc} |
| * @throws RejectedExecutionException {@inheritDoc} |
| */ |
| public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) { |
| ArrayList<ForkJoinTask<T>> forkJoinTasks = |
| new ArrayList<ForkJoinTask<T>>(tasks.size()); |
| for (Callable<T> task : tasks) |
| forkJoinTasks.add(ForkJoinTask.adapt(task)); |
| invoke(new InvokeAll<T>(forkJoinTasks)); |
| |
| @SuppressWarnings({"unchecked", "rawtypes"}) |
| List<Future<T>> futures = (List<Future<T>>) (List) forkJoinTasks; |
| return futures; |
| } |
| |
| static final class InvokeAll<T> extends RecursiveAction { |
| final ArrayList<ForkJoinTask<T>> tasks; |
| InvokeAll(ArrayList<ForkJoinTask<T>> tasks) { this.tasks = tasks; } |
| public void compute() { |
| try { invokeAll(tasks); } |
| catch (Exception ignore) {} |
| } |
| private static final long serialVersionUID = -7914297376763021607L; |
| } |
| |
| /** |
| * Returns the factory used for constructing new workers. |
| * |
| * @return the factory used for constructing new workers |
| */ |
| public ForkJoinWorkerThreadFactory getFactory() { |
| return factory; |
| } |
| |
| /** |
| * Returns the handler for internal worker threads that terminate |
| * due to unrecoverable errors encountered while executing tasks. |
| * |
| * @return the handler, or {@code null} if none |
| */ |
| public Thread.UncaughtExceptionHandler getUncaughtExceptionHandler() { |
| return ueh; |
| } |
| |
| /** |
| * Returns the targeted parallelism level of this pool. |
| * |
| * @return the targeted parallelism level of this pool |
| */ |
| public int getParallelism() { |
| return parallelism; |
| } |
| |
| /** |
| * Returns the number of worker threads that have started but not |
| * yet terminated. The result returned by this method may differ |
| * from {@link #getParallelism} when threads are created to |
| * maintain parallelism when others are cooperatively blocked. |
| * |
| * @return the number of worker threads |
| */ |
| public int getPoolSize() { |
| return parallelism + (short)(ctl >>> TC_SHIFT); |
| } |
| |
| /** |
| * Returns {@code true} if this pool uses local first-in-first-out |
| * scheduling mode for forked tasks that are never joined. |
| * |
| * @return {@code true} if this pool uses async mode |
| */ |
| public boolean getAsyncMode() { |
| return locallyFifo; |
| } |
| |
| /** |
| * Returns an estimate of the number of worker threads that are |
| * not blocked waiting to join tasks or for other managed |
| * synchronization. This method may overestimate the |
| * number of running threads. |
| * |
| * @return the number of worker threads |
| */ |
| public int getRunningThreadCount() { |
| int r = parallelism + (int)(ctl >> AC_SHIFT); |
| return (r <= 0) ? 0 : r; // suppress momentarily negative values |
| } |
| |
| /** |
| * Returns an estimate of the number of threads that are currently |
| * stealing or executing tasks. This method may overestimate the |
| * number of active threads. |
| * |
| * @return the number of active threads |
| */ |
| public int getActiveThreadCount() { |
| int r = parallelism + (int)(ctl >> AC_SHIFT) + blockedCount; |
| return (r <= 0) ? 0 : r; // suppress momentarily negative values |
| } |
| |
| /** |
| * Returns {@code true} if all worker threads are currently idle. |
| * An idle worker is one that cannot obtain a task to execute |
| * because none are available to steal from other threads, and |
| * there are no pending submissions to the pool. This method is |
| * conservative; it might not return {@code true} immediately upon |
| * idleness of all threads, but will eventually become true if |
| * threads remain inactive. |
| * |
| * @return {@code true} if all threads are currently idle |
| */ |
| public boolean isQuiescent() { |
| return parallelism + (int)(ctl >> AC_SHIFT) + blockedCount == 0; |
| } |
| |
| /** |
| * Returns an estimate of the total number of tasks stolen from |
| * one thread's work queue by another. The reported value |
| * underestimates the actual total number of steals when the pool |
| * is not quiescent. This value may be useful for monitoring and |
| * tuning fork/join programs: in general, steal counts should be |
| * high enough to keep threads busy, but low enough to avoid |
| * overhead and contention across threads. |
| * |
| * @return the number of steals |
| */ |
| public long getStealCount() { |
| return stealCount; |
| } |
| |
| /** |
| * Returns an estimate of the total number of tasks currently held |
| * in queues by worker threads (but not including tasks submitted |
| * to the pool that have not begun executing). This value is only |
| * an approximation, obtained by iterating across all threads in |
| * the pool. This method may be useful for tuning task |
| * granularities. |
| * |
| * @return the number of queued tasks |
| */ |
| public long getQueuedTaskCount() { |
| long count = 0; |
| ForkJoinWorkerThread[] ws; |
| if ((short)(ctl >>> TC_SHIFT) > -parallelism && |
| (ws = workers) != null) { |
| for (ForkJoinWorkerThread w : ws) |
| if (w != null) |
| count -= w.queueBase - w.queueTop; // must read base first |
| } |
| return count; |
| } |
| |
| /** |
| * Returns an estimate of the number of tasks submitted to this |
| * pool that have not yet begun executing. This method may take |
| * time proportional to the number of submissions. |
| * |
| * @return the number of queued submissions |
| */ |
| public int getQueuedSubmissionCount() { |
| return -queueBase + queueTop; |
| } |
| |
| /** |
| * Returns {@code true} if there are any tasks submitted to this |
| * pool that have not yet begun executing. |
| * |
| * @return {@code true} if there are any queued submissions |
| */ |
| public boolean hasQueuedSubmissions() { |
| return queueBase != queueTop; |
| } |
| |
| /** |
| * Removes and returns the next unexecuted submission if one is |
| * available. This method may be useful in extensions to this |
| * class that re-assign work in systems with multiple pools. |
| * |
| * @return the next submission, or {@code null} if none |
| */ |
| protected ForkJoinTask<?> pollSubmission() { |
| ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i; |
| while ((b = queueBase) != queueTop && |
| (q = submissionQueue) != null && |
| (i = (q.length - 1) & b) >= 0) { |
| long u = (i << ASHIFT) + ABASE; |
| if ((t = q[i]) != null && |
| queueBase == b && |
| UNSAFE.compareAndSwapObject(q, u, t, null)) { |
| queueBase = b + 1; |
| return t; |
| } |
| } |
| return null; |
| } |
| |
| /** |
| * Removes all available unexecuted submitted and forked tasks |
| * from scheduling queues and adds them to the given collection, |
| * without altering their execution status. These may include |
| * artificially generated or wrapped tasks. This method is |
| * designed to be invoked only when the pool is known to be |
| * quiescent. Invocations at other times may not remove all |
| * tasks. A failure encountered while attempting to add elements |
| * to collection {@code c} may result in elements being in |
| * neither, either or both collections when the associated |
| * exception is thrown. The behavior of this operation is |
| * undefined if the specified collection is modified while the |
| * operation is in progress. |
| * |
| * @param c the collection to transfer elements into |
| * @return the number of elements transferred |
| */ |
| protected int drainTasksTo(Collection<? super ForkJoinTask<?>> c) { |
| int count = 0; |
| while (queueBase != queueTop) { |
| ForkJoinTask<?> t = pollSubmission(); |
| if (t != null) { |
| c.add(t); |
| ++count; |
| } |
| } |
| ForkJoinWorkerThread[] ws; |
| if ((short)(ctl >>> TC_SHIFT) > -parallelism && |
| (ws = workers) != null) { |
| for (ForkJoinWorkerThread w : ws) |
| if (w != null) |
| count += w.drainTasksTo(c); |
| } |
| return count; |
| } |
| |
| /** |
| * Returns a string identifying this pool, as well as its state, |
| * including indications of run state, parallelism level, and |
| * worker and task counts. |
| * |
| * @return a string identifying this pool, as well as its state |
| */ |
| public String toString() { |
| long st = getStealCount(); |
| long qt = getQueuedTaskCount(); |
| long qs = getQueuedSubmissionCount(); |
| int pc = parallelism; |
| long c = ctl; |
| int tc = pc + (short)(c >>> TC_SHIFT); |
| int rc = pc + (int)(c >> AC_SHIFT); |
| if (rc < 0) // ignore transient negative |
| rc = 0; |
| int ac = rc + blockedCount; |
| String level; |
| if ((c & STOP_BIT) != 0) |
| level = (tc == 0) ? "Terminated" : "Terminating"; |
| else |
| level = shutdown ? "Shutting down" : "Running"; |
| return super.toString() + |
| "[" + level + |
| ", parallelism = " + pc + |
| ", size = " + tc + |
| ", active = " + ac + |
| ", running = " + rc + |
| ", steals = " + st + |
| ", tasks = " + qt + |
| ", submissions = " + qs + |
| "]"; |
| } |
| |
| /** |
| * Initiates an orderly shutdown in which previously submitted |
| * tasks are executed, but no new tasks will be accepted. |
| * Invocation has no additional effect if already shut down. |
| * Tasks that are in the process of being submitted concurrently |
| * during the course of this method may or may not be rejected. |
| */ |
| public void shutdown() { |
| checkPermission(); |
| shutdown = true; |
| tryTerminate(false); |
| } |
| |
| /** |
| * Attempts to cancel and/or stop all tasks, and reject all |
| * subsequently submitted tasks. Tasks that are in the process of |
| * being submitted or executed concurrently during the course of |
| * this method may or may not be rejected. This method cancels |
| * both existing and unexecuted tasks, in order to permit |
| * termination in the presence of task dependencies. So the method |
| * always returns an empty list (unlike the case for some other |
| * Executors). |
| * |
| * @return an empty list |
| */ |
| public List<Runnable> shutdownNow() { |
| checkPermission(); |
| shutdown = true; |
| tryTerminate(true); |
| return Collections.emptyList(); |
| } |
| |
| /** |
| * Returns {@code true} if all tasks have completed following shut down. |
| * |
| * @return {@code true} if all tasks have completed following shut down |
| */ |
| public boolean isTerminated() { |
| long c = ctl; |
| return ((c & STOP_BIT) != 0L && |
| (short)(c >>> TC_SHIFT) == -parallelism); |
| } |
| |
| /** |
| * Returns {@code true} if the process of termination has |
| * commenced but not yet completed. This method may be useful for |
| * debugging. A return of {@code true} reported a sufficient |
| * period after shutdown may indicate that submitted tasks have |
| * ignored or suppressed interruption, or are waiting for IO, |
| * causing this executor not to properly terminate. (See the |
| * advisory notes for class {@link ForkJoinTask} stating that |
| * tasks should not normally entail blocking operations. But if |
| * they do, they must abort them on interrupt.) |
| * |
| * @return {@code true} if terminating but not yet terminated |
| */ |
| public boolean isTerminating() { |
| long c = ctl; |
| return ((c & STOP_BIT) != 0L && |
| (short)(c >>> TC_SHIFT) != -parallelism); |
| } |
| |
| /** |
| * Returns true if terminating or terminated. Used by ForkJoinWorkerThread. |
| */ |
| final boolean isAtLeastTerminating() { |
| return (ctl & STOP_BIT) != 0L; |
| } |
| |
| /** |
| * Returns {@code true} if this pool has been shut down. |
| * |
| * @return {@code true} if this pool has been shut down |
| */ |
| public boolean isShutdown() { |
| return shutdown; |
| } |
| |
| /** |
| * Blocks until all tasks have completed execution after a shutdown |
| * request, or the timeout occurs, or the current thread is |
| * interrupted, whichever happens first. |
| * |
| * @param timeout the maximum time to wait |
| * @param unit the time unit of the timeout argument |
| * @return {@code true} if this executor terminated and |
| * {@code false} if the timeout elapsed before termination |
| * @throws InterruptedException if interrupted while waiting |
| */ |
| public boolean awaitTermination(long timeout, TimeUnit unit) |
| throws InterruptedException { |
| long nanos = unit.toNanos(timeout); |
| final ReentrantLock lock = this.submissionLock; |
| lock.lock(); |
| try { |
| for (;;) { |
| if (isTerminated()) |
| return true; |
| if (nanos <= 0) |
| return false; |
| nanos = termination.awaitNanos(nanos); |
| } |
| } finally { |
| lock.unlock(); |
| } |
| } |
| |
| /** |
| * Interface for extending managed parallelism for tasks running |
| * in {@link ForkJoinPool}s. |
| * |
| * <p>A {@code ManagedBlocker} provides two methods. Method |
| * {@code isReleasable} must return {@code true} if blocking is |
| * not necessary. Method {@code block} blocks the current thread |
| * if necessary (perhaps internally invoking {@code isReleasable} |
| * before actually blocking). These actions are performed by any |
| * thread invoking {@link ForkJoinPool#managedBlock}. The |
| * unusual methods in this API accommodate synchronizers that may, |
| * but don't usually, block for long periods. Similarly, they |
| * allow more efficient internal handling of cases in which |
| * additional workers may be, but usually are not, needed to |
| * ensure sufficient parallelism. Toward this end, |
| * implementations of method {@code isReleasable} must be amenable |
| * to repeated invocation. |
| * |
| * <p>For example, here is a ManagedBlocker based on a |
| * ReentrantLock: |
| * <pre> {@code |
| * class ManagedLocker implements ManagedBlocker { |
| * final ReentrantLock lock; |
| * boolean hasLock = false; |
| * ManagedLocker(ReentrantLock lock) { this.lock = lock; } |
| * public boolean block() { |
| * if (!hasLock) |
| * lock.lock(); |
| * return true; |
| * } |
| * public boolean isReleasable() { |
| * return hasLock || (hasLock = lock.tryLock()); |
| * } |
| * }}</pre> |
| * |
| * <p>Here is a class that possibly blocks waiting for an |
| * item on a given queue: |
| * <pre> {@code |
| * class QueueTaker<E> implements ManagedBlocker { |
| * final BlockingQueue<E> queue; |
| * volatile E item = null; |
| * QueueTaker(BlockingQueue<E> q) { this.queue = q; } |
| * public boolean block() throws InterruptedException { |
| * if (item == null) |
| * item = queue.take(); |
| * return true; |
| * } |
| * public boolean isReleasable() { |
| * return item != null || (item = queue.poll()) != null; |
| * } |
| * public E getItem() { // call after pool.managedBlock completes |
| * return item; |
| * } |
| * }}</pre> |
| */ |
| public static interface ManagedBlocker { |
| /** |
| * Possibly blocks the current thread, for example waiting for |
| * a lock or condition. |
| * |
| * @return {@code true} if no additional blocking is necessary |
| * (i.e., if isReleasable would return true) |
| * @throws InterruptedException if interrupted while waiting |
| * (the method is not required to do so, but is allowed to) |
| */ |
| boolean block() throws InterruptedException; |
| |
| /** |
| * Returns {@code true} if blocking is unnecessary. |
| */ |
| boolean isReleasable(); |
| } |
| |
| /** |
| * Blocks in accord with the given blocker. If the current thread |
| * is a {@link ForkJoinWorkerThread}, this method possibly |
| * arranges for a spare thread to be activated if necessary to |
| * ensure sufficient parallelism while the current thread is blocked. |
| * |
| * <p>If the caller is not a {@link ForkJoinTask}, this method is |
| * behaviorally equivalent to |
| * <pre> {@code |
| * while (!blocker.isReleasable()) |
| * if (blocker.block()) |
| * return; |
| * }</pre> |
| * |
| * If the caller is a {@code ForkJoinTask}, then the pool may |
| * first be expanded to ensure parallelism, and later adjusted. |
| * |
| * @param blocker the blocker |
| * @throws InterruptedException if blocker.block did so |
| */ |
| public static void managedBlock(ManagedBlocker blocker) |
| throws InterruptedException { |
| Thread t = Thread.currentThread(); |
| if (t instanceof ForkJoinWorkerThread) { |
| ForkJoinWorkerThread w = (ForkJoinWorkerThread) t; |
| w.pool.awaitBlocker(blocker); |
| } |
| else { |
| do {} while (!blocker.isReleasable() && !blocker.block()); |
| } |
| } |
| |
| // AbstractExecutorService overrides. These rely on undocumented |
| // fact that ForkJoinTask.adapt returns ForkJoinTasks that also |
| // implement RunnableFuture. |
| |
| protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) { |
| return (RunnableFuture<T>) ForkJoinTask.adapt(runnable, value); |
| } |
| |
| protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) { |
| return (RunnableFuture<T>) ForkJoinTask.adapt(callable); |
| } |
| |
| // Unsafe mechanics |
| private static final sun.misc.Unsafe UNSAFE; |
| private static final long ctlOffset; |
| private static final long stealCountOffset; |
| private static final long blockedCountOffset; |
| private static final long quiescerCountOffset; |
| private static final long scanGuardOffset; |
| private static final long nextWorkerNumberOffset; |
| private static final long ABASE; |
| private static final int ASHIFT; |
| |
| static { |
| poolNumberGenerator = new AtomicInteger(); |
| workerSeedGenerator = new Random(); |
| modifyThreadPermission = new RuntimePermission("modifyThread"); |
| defaultForkJoinWorkerThreadFactory = |
| new DefaultForkJoinWorkerThreadFactory(); |
| try { |
| UNSAFE = sun.misc.Unsafe.getUnsafe(); |
| Class<?> k = ForkJoinPool.class; |
| ctlOffset = UNSAFE.objectFieldOffset |
| (k.getDeclaredField("ctl")); |
| stealCountOffset = UNSAFE.objectFieldOffset |
| (k.getDeclaredField("stealCount")); |
| blockedCountOffset = UNSAFE.objectFieldOffset |
| (k.getDeclaredField("blockedCount")); |
| quiescerCountOffset = UNSAFE.objectFieldOffset |
| (k.getDeclaredField("quiescerCount")); |
| scanGuardOffset = UNSAFE.objectFieldOffset |
| (k.getDeclaredField("scanGuard")); |
| nextWorkerNumberOffset = UNSAFE.objectFieldOffset |
| (k.getDeclaredField("nextWorkerNumber")); |
| } catch (Exception e) { |
| throw new Error(e); |
| } |
| Class<?> a = ForkJoinTask[].class; |
| ABASE = UNSAFE.arrayBaseOffset(a); |
| int s = UNSAFE.arrayIndexScale(a); |
| if ((s & (s-1)) != 0) |
| throw new Error("data type scale not a power of two"); |
| ASHIFT = 31 - Integer.numberOfLeadingZeros(s); |
| } |
| |
| } |