| /* |
| * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved. |
| * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
| * |
| * This code is free software; you can redistribute it and/or modify it |
| * under the terms of the GNU General Public License version 2 only, as |
| * published by the Free Software Foundation. |
| * |
| * This code is distributed in the hope that it will be useful, but WITHOUT |
| * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
| * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
| * version 2 for more details (a copy is included in the LICENSE file that |
| * accompanied this code). |
| * |
| * You should have received a copy of the GNU General Public License version |
| * 2 along with this work; if not, write to the Free Software Foundation, |
| * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
| * |
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| */ |
| |
| #ifndef SHARE_VM_RUNTIME_BIASEDLOCKING_HPP |
| #define SHARE_VM_RUNTIME_BIASEDLOCKING_HPP |
| |
| #include "runtime/handles.hpp" |
| #include "utilities/growableArray.hpp" |
| |
| // This class describes operations to implement Store-Free Biased |
| // Locking. The high-level properties of the scheme are similar to |
| // IBM's lock reservation, Dice-Moir-Scherer QR locks, and other biased |
| // locking mechanisms. The principal difference is in the handling of |
| // recursive locking which is how this technique achieves a more |
| // efficient fast path than these other schemes. |
| // |
| // The basic observation is that in HotSpot's current fast locking |
| // scheme, recursive locking (in the fast path) causes no update to |
| // the object header. The recursion is described simply by stack |
| // records containing a specific value (NULL). Only the last unlock by |
| // a given thread causes an update to the object header. |
| // |
| // This observation, coupled with the fact that HotSpot only compiles |
| // methods for which monitor matching is obeyed (and which therefore |
| // can not throw IllegalMonitorStateException), implies that we can |
| // completely eliminate modifications to the object header for |
| // recursive locking in compiled code, and perform similar recursion |
| // checks and throwing of IllegalMonitorStateException in the |
| // interpreter with little or no impact on the performance of the fast |
| // path. |
| // |
| // The basic algorithm is as follows (note, see below for more details |
| // and information). A pattern in the low three bits is reserved in |
| // the object header to indicate whether biasing of a given object's |
| // lock is currently being done or is allowed at all. If the bias |
| // pattern is present, the contents of the rest of the header are |
| // either the JavaThread* of the thread to which the lock is biased, |
| // or NULL, indicating that the lock is "anonymously biased". The |
| // first thread which locks an anonymously biased object biases the |
| // lock toward that thread. If another thread subsequently attempts to |
| // lock the same object, the bias is revoked. |
| // |
| // Because there are no updates to the object header at all during |
| // recursive locking while the lock is biased, the biased lock entry |
| // code is simply a test of the object header's value. If this test |
| // succeeds, the lock has been acquired by the thread. If this test |
| // fails, a bit test is done to see whether the bias bit is still |
| // set. If not, we fall back to HotSpot's original CAS-based locking |
| // scheme. If it is set, we attempt to CAS in a bias toward this |
| // thread. The latter operation is expected to be the rarest operation |
| // performed on these locks. We optimistically expect the biased lock |
| // entry to hit most of the time, and want the CAS-based fallthrough |
| // to occur quickly in the situations where the bias has been revoked. |
| // |
| // Revocation of the lock's bias is fairly straightforward. We want to |
| // restore the object's header and stack-based BasicObjectLocks and |
| // BasicLocks to the state they would have been in had the object been |
| // locked by HotSpot's usual fast locking scheme. To do this, we bring |
| // the system to a safepoint and walk the stack of the thread toward |
| // which the lock is biased. We find all of the lock records on the |
| // stack corresponding to this object, in particular the first / |
| // "highest" record. We fill in the highest lock record with the |
| // object's displaced header (which is a well-known value given that |
| // we don't maintain an identity hash nor age bits for the object |
| // while it's in the biased state) and all other lock records with 0, |
| // the value for recursive locks. When the safepoint is released, the |
| // formerly-biased thread and all other threads revert back to |
| // HotSpot's CAS-based locking. |
| // |
| // This scheme can not handle transfers of biases of single objects |
| // from thread to thread efficiently, but it can handle bulk transfers |
| // of such biases, which is a usage pattern showing up in some |
| // applications and benchmarks. We implement "bulk rebias" and "bulk |
| // revoke" operations using a "bias epoch" on a per-data-type basis. |
| // If too many bias revocations are occurring for a particular data |
| // type, the bias epoch for the data type is incremented at a |
| // safepoint, effectively meaning that all previous biases are |
| // invalid. The fast path locking case checks for an invalid epoch in |
| // the object header and attempts to rebias the object with a CAS if |
| // found, avoiding safepoints or bulk heap sweeps (the latter which |
| // was used in a prior version of this algorithm and did not scale |
| // well). If too many bias revocations persist, biasing is completely |
| // disabled for the data type by resetting the prototype header to the |
| // unbiased markOop. The fast-path locking code checks to see whether |
| // the instance's bias pattern differs from the prototype header's and |
| // causes the bias to be revoked without reaching a safepoint or, |
| // again, a bulk heap sweep. |
| |
| // Biased locking counters |
| class BiasedLockingCounters VALUE_OBJ_CLASS_SPEC { |
| private: |
| int _total_entry_count; |
| int _biased_lock_entry_count; |
| int _anonymously_biased_lock_entry_count; |
| int _rebiased_lock_entry_count; |
| int _revoked_lock_entry_count; |
| int _fast_path_entry_count; |
| int _slow_path_entry_count; |
| |
| public: |
| BiasedLockingCounters() : |
| _total_entry_count(0), |
| _biased_lock_entry_count(0), |
| _anonymously_biased_lock_entry_count(0), |
| _rebiased_lock_entry_count(0), |
| _revoked_lock_entry_count(0), |
| _fast_path_entry_count(0), |
| _slow_path_entry_count(0) {} |
| |
| int slow_path_entry_count(); // Compute this field if necessary |
| |
| int* total_entry_count_addr() { return &_total_entry_count; } |
| int* biased_lock_entry_count_addr() { return &_biased_lock_entry_count; } |
| int* anonymously_biased_lock_entry_count_addr() { return &_anonymously_biased_lock_entry_count; } |
| int* rebiased_lock_entry_count_addr() { return &_rebiased_lock_entry_count; } |
| int* revoked_lock_entry_count_addr() { return &_revoked_lock_entry_count; } |
| int* fast_path_entry_count_addr() { return &_fast_path_entry_count; } |
| int* slow_path_entry_count_addr() { return &_slow_path_entry_count; } |
| |
| bool nonzero() { return _total_entry_count > 0; } |
| |
| void print_on(outputStream* st); |
| void print() { print_on(tty); } |
| }; |
| |
| |
| class BiasedLocking : AllStatic { |
| private: |
| static BiasedLockingCounters _counters; |
| |
| public: |
| static int* total_entry_count_addr(); |
| static int* biased_lock_entry_count_addr(); |
| static int* anonymously_biased_lock_entry_count_addr(); |
| static int* rebiased_lock_entry_count_addr(); |
| static int* revoked_lock_entry_count_addr(); |
| static int* fast_path_entry_count_addr(); |
| static int* slow_path_entry_count_addr(); |
| |
| enum Condition { |
| NOT_BIASED = 1, |
| BIAS_REVOKED = 2, |
| BIAS_REVOKED_AND_REBIASED = 3 |
| }; |
| |
| // This initialization routine should only be called once and |
| // schedules a PeriodicTask to turn on biased locking a few seconds |
| // into the VM run to avoid startup time regressions |
| static void init(); |
| |
| // This provides a global switch for leaving biased locking disabled |
| // for the first part of a run and enabling it later |
| static bool enabled(); |
| |
| // This should be called by JavaThreads to revoke the bias of an object |
| static Condition revoke_and_rebias(Handle obj, bool attempt_rebias, TRAPS); |
| |
| // These do not allow rebiasing; they are used by deoptimization to |
| // ensure that monitors on the stack can be migrated |
| static void revoke(GrowableArray<Handle>* objs); |
| static void revoke_at_safepoint(Handle obj); |
| static void revoke_at_safepoint(GrowableArray<Handle>* objs); |
| |
| static void print_counters() { _counters.print(); } |
| static BiasedLockingCounters* counters() { return &_counters; } |
| |
| // These routines are GC-related and should not be called by end |
| // users. GCs which do not do preservation of mark words do not need |
| // to call these routines. |
| static void preserve_marks(); |
| static void restore_marks(); |
| }; |
| |
| #endif // SHARE_VM_RUNTIME_BIASEDLOCKING_HPP |