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* Copyright (C) 2014 The Android Open Source Project
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* See the License for the specific language governing permissions and
* limitations under the License.
#include "monitor.h"
#include "base/allocator.h"
#ifdef __LP64__
#include <stdint.h>
#include "base/atomic.h"
#include "runtime.h"
#include "base/stl_util.h" // STLDeleteElements
namespace art {
// Abstraction to keep monitors small enough to fit in a lock word (32bits). On 32bit systems the
// monitor id loses the alignment bits of the Monitor*.
class MonitorPool {
static MonitorPool* Create() {
#ifndef __LP64__
return nullptr;
return new MonitorPool();
static Monitor* CreateMonitor(Thread* self, Thread* owner, mirror::Object* obj, int32_t hash_code)
REQUIRES_SHARED(Locks::mutator_lock_) {
#ifndef __LP64__
Monitor* mon = new Monitor(self, owner, obj, hash_code);
DCHECK_ALIGNED(mon, LockWord::kMonitorIdAlignment);
return mon;
return GetMonitorPool()->CreateMonitorInPool(self, owner, obj, hash_code);
static void ReleaseMonitor(Thread* self, Monitor* monitor) {
#ifndef __LP64__
delete monitor;
GetMonitorPool()->ReleaseMonitorToPool(self, monitor);
static void ReleaseMonitors(Thread* self, MonitorList::Monitors* monitors) {
#ifndef __LP64__
GetMonitorPool()->ReleaseMonitorsToPool(self, monitors);
static Monitor* MonitorFromMonitorId(MonitorId mon_id) {
#ifndef __LP64__
return reinterpret_cast<Monitor*>(mon_id << LockWord::kMonitorIdAlignmentShift);
return GetMonitorPool()->LookupMonitor(mon_id);
static MonitorId MonitorIdFromMonitor(Monitor* mon) {
#ifndef __LP64__
return reinterpret_cast<MonitorId>(mon) >> LockWord::kMonitorIdAlignmentShift;
return mon->GetMonitorId();
static MonitorId ComputeMonitorId(Monitor* mon, Thread* self) {
#ifndef __LP64__
return MonitorIdFromMonitor(mon);
return GetMonitorPool()->ComputeMonitorIdInPool(mon, self);
static MonitorPool* GetMonitorPool() {
#ifndef __LP64__
return nullptr;
return Runtime::Current()->GetMonitorPool();
~MonitorPool() {
#ifdef __LP64__
#ifdef __LP64__
// When we create a monitor pool, threads have not been initialized, yet, so ignore thread-safety
// analysis.
void AllocateChunk() REQUIRES(Locks::allocated_monitor_ids_lock_);
// Release all chunks and metadata. This is done on shutdown, where threads have been destroyed,
// so ignore thead-safety analysis.
Monitor* CreateMonitorInPool(Thread* self, Thread* owner, mirror::Object* obj, int32_t hash_code)
void ReleaseMonitorToPool(Thread* self, Monitor* monitor);
void ReleaseMonitorsToPool(Thread* self, MonitorList::Monitors* monitors);
// Note: This is safe as we do not ever move chunks. All needed entries in the monitor_chunks_
// data structure are read-only once we get here. Updates happen-before this call because
// the lock word was stored with release semantics and we read it with acquire semantics to
// retrieve the id.
Monitor* LookupMonitor(MonitorId mon_id) {
size_t offset = MonitorIdToOffset(mon_id);
size_t index = offset / kChunkSize;
size_t top_index = index / kMaxListSize;
size_t list_index = index % kMaxListSize;
size_t offset_in_chunk = offset % kChunkSize;
uintptr_t base = monitor_chunks_[top_index][list_index];
return reinterpret_cast<Monitor*>(base + offset_in_chunk);
static bool IsInChunk(uintptr_t base_addr, Monitor* mon) {
uintptr_t mon_ptr = reinterpret_cast<uintptr_t>(mon);
return base_addr <= mon_ptr && (mon_ptr - base_addr < kChunkSize);
MonitorId ComputeMonitorIdInPool(Monitor* mon, Thread* self) {
MutexLock mu(self, *Locks::allocated_monitor_ids_lock_);
for (size_t i = 0; i <= current_chunk_list_index_; ++i) {
for (size_t j = 0; j < ChunkListCapacity(i); ++j) {
if (j >= num_chunks_ && i == current_chunk_list_index_) {
uintptr_t chunk_addr = monitor_chunks_[i][j];
if (IsInChunk(chunk_addr, mon)) {
return OffsetToMonitorId(
reinterpret_cast<uintptr_t>(mon) - chunk_addr
+ i * (kMaxListSize * kChunkSize) + j * kChunkSize);
LOG(FATAL) << "Did not find chunk that contains monitor.";
return 0;
static constexpr size_t MonitorIdToOffset(MonitorId id) {
return id << 3;
static constexpr MonitorId OffsetToMonitorId(size_t offset) {
return static_cast<MonitorId>(offset >> 3);
static constexpr size_t ChunkListCapacity(size_t index) {
return kInitialChunkStorage << index;
// TODO: There are assumptions in the code that monitor addresses are 8B aligned (>>3).
static constexpr size_t kMonitorAlignment = 8;
// Size of a monitor, rounded up to a multiple of alignment.
static constexpr size_t kAlignedMonitorSize = (sizeof(Monitor) + kMonitorAlignment - 1) &
// As close to a page as we can get seems a good start.
static constexpr size_t kChunkCapacity = kPageSize / kAlignedMonitorSize;
// Chunk size that is referenced in the id. We can collapse this to the actually used storage
// in a chunk, i.e., kChunkCapacity * kAlignedMonitorSize, but this will mean proper divisions.
static constexpr size_t kChunkSize = kPageSize;
static_assert(IsPowerOfTwo(kChunkSize), "kChunkSize must be power of 2");
// The number of chunks of storage that can be referenced by the initial chunk list.
// The total number of usable monitor chunks is typically 255 times this number, so it
// should be large enough that we don't run out. We run out of address bits if it's > 512.
// Currently we set it a bit smaller, to save half a page per process. We make it tiny in
// debug builds to catch growth errors. The only value we really expect to tune.
static constexpr size_t kInitialChunkStorage = kIsDebugBuild ? 1U : 256U;
static_assert(IsPowerOfTwo(kInitialChunkStorage), "kInitialChunkStorage must be power of 2");
// The number of lists, each containing pointers to storage chunks.
static constexpr size_t kMaxChunkLists = 8; // Dictated by 3 bit index. Don't increase above 8.
static_assert(IsPowerOfTwo(kMaxChunkLists), "kMaxChunkLists must be power of 2");
static constexpr size_t kMaxListSize = kInitialChunkStorage << (kMaxChunkLists - 1);
// We lose 3 bits in monitor id due to 3 bit monitor_chunks_ index, and gain it back from
// the 3 bit alignment constraint on monitors:
static_assert(kMaxListSize * kChunkSize < (1 << LockWord::kMonitorIdSize),
"Monitor id bits don't fit");
static_assert(IsPowerOfTwo(kMaxListSize), "kMaxListSize must be power of 2");
// Array of pointers to lists (again arrays) of pointers to chunks containing monitors.
// Zeroth entry points to a list (array) of kInitialChunkStorage pointers to chunks.
// Each subsequent list as twice as large as the preceding one.
// Monitor Ids are interpreted as follows:
// Top 3 bits (of 28): index into monitor_chunks_.
// Next 16 bits: index into the chunk list, i.e. monitor_chunks_[i].
// Last 9 bits: offset within chunk, expressed as multiple of kMonitorAlignment.
// If we set kInitialChunkStorage to 512, this would allow us to use roughly 128K chunks of
// monitors, which is 0.5GB of monitors. With this maximum setting, the largest chunk list
// contains 64K entries, and we make full use of the available index space. With a
// kInitialChunkStorage value of 256, this is proportionately reduced to 0.25GB of monitors.
// Updates to monitor_chunks_ are guarded by allocated_monitor_ids_lock_ .
// No field in this entire data structure is ever updated once a monitor id whose lookup
// requires it has been made visible to another thread. Thus readers never race with
// updates, in spite of the fact that they acquire no locks.
uintptr_t* monitor_chunks_[kMaxChunkLists]; // uintptr_t is really a Monitor* .
// Highest currently used index in monitor_chunks_ . Used for newly allocated chunks.
size_t current_chunk_list_index_ GUARDED_BY(Locks::allocated_monitor_ids_lock_);
// Number of chunk pointers stored in monitor_chunks_[current_chunk_list_index_] so far.
size_t num_chunks_ GUARDED_BY(Locks::allocated_monitor_ids_lock_);
// After the initial allocation, this is always equal to
// ChunkListCapacity(current_chunk_list_index_).
size_t current_chunk_list_capacity_ GUARDED_BY(Locks::allocated_monitor_ids_lock_);
typedef TrackingAllocator<uint8_t, kAllocatorTagMonitorPool> Allocator;
Allocator allocator_;
// Start of free list of monitors.
// Note: these point to the right memory regions, but do *not* denote initialized objects.
Monitor* first_free_ GUARDED_BY(Locks::allocated_monitor_ids_lock_);
} // namespace art