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/*
* Copyright (C) 2012 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
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#include "thread_pool.h"
#include "base/casts.h"
#include "base/stl_util.h"
#include "runtime.h"
#include "thread-inl.h"
namespace art {
static constexpr bool kMeasureWaitTime = false;
ThreadPoolWorker::ThreadPoolWorker(ThreadPool* thread_pool, const std::string& name,
size_t stack_size)
: thread_pool_(thread_pool),
name_(name) {
std::string error_msg;
stack_.reset(MemMap::MapAnonymous(name.c_str(), nullptr, stack_size, PROT_READ | PROT_WRITE,
false, false, &error_msg));
CHECK(stack_.get() != nullptr) << error_msg;
const char* reason = "new thread pool worker thread";
pthread_attr_t attr;
CHECK_PTHREAD_CALL(pthread_attr_init, (&attr), reason);
CHECK_PTHREAD_CALL(pthread_attr_setstack, (&attr, stack_->Begin(), stack_->Size()), reason);
CHECK_PTHREAD_CALL(pthread_create, (&pthread_, &attr, &Callback, this), reason);
CHECK_PTHREAD_CALL(pthread_attr_destroy, (&attr), reason);
}
ThreadPoolWorker::~ThreadPoolWorker() {
CHECK_PTHREAD_CALL(pthread_join, (pthread_, nullptr), "thread pool worker shutdown");
}
void ThreadPoolWorker::Run() {
Thread* self = Thread::Current();
Task* task = nullptr;
thread_pool_->creation_barier_.Wait(self);
while ((task = thread_pool_->GetTask(self)) != nullptr) {
task->Run(self);
task->Finalize();
}
}
void* ThreadPoolWorker::Callback(void* arg) {
ThreadPoolWorker* worker = reinterpret_cast<ThreadPoolWorker*>(arg);
Runtime* runtime = Runtime::Current();
CHECK(runtime->AttachCurrentThread(worker->name_.c_str(), true, nullptr, false));
// Do work until its time to shut down.
worker->Run();
runtime->DetachCurrentThread();
return nullptr;
}
void ThreadPool::AddTask(Thread* self, Task* task) {
MutexLock mu(self, task_queue_lock_);
tasks_.push_back(task);
// If we have any waiters, signal one.
if (started_ && waiting_count_ != 0) {
task_queue_condition_.Signal(self);
}
}
ThreadPool::ThreadPool(const char* name, size_t num_threads)
: name_(name),
task_queue_lock_("task queue lock"),
task_queue_condition_("task queue condition", task_queue_lock_),
completion_condition_("task completion condition", task_queue_lock_),
started_(false),
shutting_down_(false),
waiting_count_(0),
start_time_(0),
total_wait_time_(0),
// Add one since the caller of constructor waits on the barrier too.
creation_barier_(num_threads + 1),
max_active_workers_(num_threads) {
Thread* self = Thread::Current();
while (GetThreadCount() < num_threads) {
const std::string worker_name = StringPrintf("%s worker thread %zu", name_.c_str(),
GetThreadCount());
threads_.push_back(new ThreadPoolWorker(this, worker_name, ThreadPoolWorker::kDefaultStackSize));
}
// Wait for all of the threads to attach.
creation_barier_.Wait(self);
}
void ThreadPool::SetMaxActiveWorkers(size_t threads) {
MutexLock mu(Thread::Current(), task_queue_lock_);
CHECK_LE(threads, GetThreadCount());
max_active_workers_ = threads;
}
ThreadPool::~ThreadPool() {
{
Thread* self = Thread::Current();
MutexLock mu(self, task_queue_lock_);
// Tell any remaining workers to shut down.
shutting_down_ = true;
// Broadcast to everyone waiting.
task_queue_condition_.Broadcast(self);
completion_condition_.Broadcast(self);
}
// Wait for the threads to finish.
STLDeleteElements(&threads_);
}
void ThreadPool::StartWorkers(Thread* self) {
MutexLock mu(self, task_queue_lock_);
started_ = true;
task_queue_condition_.Broadcast(self);
start_time_ = NanoTime();
total_wait_time_ = 0;
}
void ThreadPool::StopWorkers(Thread* self) {
MutexLock mu(self, task_queue_lock_);
started_ = false;
}
Task* ThreadPool::GetTask(Thread* self) {
MutexLock mu(self, task_queue_lock_);
while (!IsShuttingDown()) {
const size_t thread_count = GetThreadCount();
// Ensure that we don't use more threads than the maximum active workers.
const size_t active_threads = thread_count - waiting_count_;
// <= since self is considered an active worker.
if (active_threads <= max_active_workers_) {
Task* task = TryGetTaskLocked();
if (task != nullptr) {
return task;
}
}
++waiting_count_;
if (waiting_count_ == GetThreadCount() && tasks_.empty()) {
// We may be done, lets broadcast to the completion condition.
completion_condition_.Broadcast(self);
}
const uint64_t wait_start = kMeasureWaitTime ? NanoTime() : 0;
task_queue_condition_.Wait(self);
if (kMeasureWaitTime) {
const uint64_t wait_end = NanoTime();
total_wait_time_ += wait_end - std::max(wait_start, start_time_);
}
--waiting_count_;
}
// We are shutting down, return nullptr to tell the worker thread to stop looping.
return nullptr;
}
Task* ThreadPool::TryGetTask(Thread* self) {
MutexLock mu(self, task_queue_lock_);
return TryGetTaskLocked();
}
Task* ThreadPool::TryGetTaskLocked() {
if (started_ && !tasks_.empty()) {
Task* task = tasks_.front();
tasks_.pop_front();
return task;
}
return nullptr;
}
void ThreadPool::Wait(Thread* self, bool do_work, bool may_hold_locks) {
if (do_work) {
Task* task = nullptr;
while ((task = TryGetTask(self)) != nullptr) {
task->Run(self);
task->Finalize();
}
}
// Wait until each thread is waiting and the task list is empty.
MutexLock mu(self, task_queue_lock_);
while (!shutting_down_ && (waiting_count_ != GetThreadCount() || !tasks_.empty())) {
if (!may_hold_locks) {
completion_condition_.Wait(self);
} else {
completion_condition_.WaitHoldingLocks(self);
}
}
}
size_t ThreadPool::GetTaskCount(Thread* self) {
MutexLock mu(self, task_queue_lock_);
return tasks_.size();
}
WorkStealingWorker::WorkStealingWorker(ThreadPool* thread_pool, const std::string& name,
size_t stack_size)
: ThreadPoolWorker(thread_pool, name, stack_size), task_(nullptr) {}
void WorkStealingWorker::Run() {
Thread* self = Thread::Current();
Task* task = nullptr;
WorkStealingThreadPool* thread_pool = down_cast<WorkStealingThreadPool*>(thread_pool_);
while ((task = thread_pool_->GetTask(self)) != nullptr) {
WorkStealingTask* stealing_task = down_cast<WorkStealingTask*>(task);
{
CHECK(task_ == nullptr);
MutexLock mu(self, thread_pool->work_steal_lock_);
// Register that we are running the task
++stealing_task->ref_count_;
task_ = stealing_task;
}
stealing_task->Run(self);
// Mark ourselves as not running a task so that nobody tries to steal from us.
// There is a race condition that someone starts stealing from us at this point. This is okay
// due to the reference counting.
task_ = nullptr;
bool finalize;
// Steal work from tasks until there is none left to steal. Note: There is a race, but
// all that happens when the race occurs is that we steal some work instead of processing a
// task from the queue.
while (thread_pool->GetTaskCount(self) == 0) {
WorkStealingTask* steal_from_task = nullptr;
{
MutexLock mu(self, thread_pool->work_steal_lock_);
// Try finding a task to steal from.
steal_from_task = thread_pool->FindTaskToStealFrom();
if (steal_from_task != nullptr) {
CHECK_NE(stealing_task, steal_from_task)
<< "Attempting to steal from completed self task";
steal_from_task->ref_count_++;
} else {
break;
}
}
if (steal_from_task != nullptr) {
// Task which completed earlier is going to steal some work.
stealing_task->StealFrom(self, steal_from_task);
{
// We are done stealing from the task, lets decrement its reference count.
MutexLock mu(self, thread_pool->work_steal_lock_);
finalize = !--steal_from_task->ref_count_;
}
if (finalize) {
steal_from_task->Finalize();
}
}
}
{
MutexLock mu(self, thread_pool->work_steal_lock_);
// If nobody is still referencing task_ we can finalize it.
finalize = !--stealing_task->ref_count_;
}
if (finalize) {
stealing_task->Finalize();
}
}
}
WorkStealingWorker::~WorkStealingWorker() {}
WorkStealingThreadPool::WorkStealingThreadPool(const char* name, size_t num_threads)
: ThreadPool(name, 0),
work_steal_lock_("work stealing lock"),
steal_index_(0) {
while (GetThreadCount() < num_threads) {
const std::string worker_name = StringPrintf("Work stealing worker %zu", GetThreadCount());
threads_.push_back(new WorkStealingWorker(this, worker_name,
ThreadPoolWorker::kDefaultStackSize));
}
}
WorkStealingTask* WorkStealingThreadPool::FindTaskToStealFrom() {
const size_t thread_count = GetThreadCount();
for (size_t i = 0; i < thread_count; ++i) {
// TODO: Use CAS instead of lock.
++steal_index_;
if (steal_index_ >= thread_count) {
steal_index_-= thread_count;
}
WorkStealingWorker* worker = down_cast<WorkStealingWorker*>(threads_[steal_index_]);
WorkStealingTask* task = worker->task_;
if (task) {
// Not null, we can probably steal from this worker.
return task;
}
}
// Couldn't find something to steal.
return nullptr;
}
WorkStealingThreadPool::~WorkStealingThreadPool() {}
} // namespace art