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/*
*
* Copyright 2016 gRPC authors.
*
* 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 "src/cpp/thread_manager/thread_manager.h"
#include <climits>
#include <mutex>
#include <grpc/support/log.h>
#include "src/core/lib/gprpp/thd.h"
namespace grpc {
ThreadManager::WorkerThread::WorkerThread(ThreadManager* thd_mgr)
: thd_mgr_(thd_mgr) {
// Make thread creation exclusive with respect to its join happening in
// ~WorkerThread().
thd_ = grpc_core::Thread(
"grpcpp_sync_server",
[](void* th) { static_cast<ThreadManager::WorkerThread*>(th)->Run(); },
this);
thd_.Start();
}
void ThreadManager::WorkerThread::Run() {
thd_mgr_->MainWorkLoop();
thd_mgr_->MarkAsCompleted(this);
}
ThreadManager::WorkerThread::~WorkerThread() {
// Don't join until the thread is fully constructed.
thd_.Join();
}
ThreadManager::ThreadManager(int min_pollers, int max_pollers)
: shutdown_(false),
num_pollers_(0),
min_pollers_(min_pollers),
max_pollers_(max_pollers == -1 ? INT_MAX : max_pollers),
num_threads_(0) {}
ThreadManager::~ThreadManager() {
{
std::lock_guard<std::mutex> lock(mu_);
GPR_ASSERT(num_threads_ == 0);
}
CleanupCompletedThreads();
}
void ThreadManager::Wait() {
std::unique_lock<std::mutex> lock(mu_);
while (num_threads_ != 0) {
shutdown_cv_.wait(lock);
}
}
void ThreadManager::Shutdown() {
std::lock_guard<std::mutex> lock(mu_);
shutdown_ = true;
}
bool ThreadManager::IsShutdown() {
std::lock_guard<std::mutex> lock(mu_);
return shutdown_;
}
void ThreadManager::MarkAsCompleted(WorkerThread* thd) {
{
std::lock_guard<std::mutex> list_lock(list_mu_);
completed_threads_.push_back(thd);
}
std::lock_guard<std::mutex> lock(mu_);
num_threads_--;
if (num_threads_ == 0) {
shutdown_cv_.notify_one();
}
}
void ThreadManager::CleanupCompletedThreads() {
std::list<WorkerThread*> completed_threads;
{
// swap out the completed threads list: allows other threads to clean up
// more quickly
std::unique_lock<std::mutex> lock(list_mu_);
completed_threads.swap(completed_threads_);
}
for (auto thd : completed_threads) delete thd;
}
void ThreadManager::Initialize() {
{
std::unique_lock<std::mutex> lock(mu_);
num_pollers_ = min_pollers_;
num_threads_ = min_pollers_;
}
for (int i = 0; i < min_pollers_; i++) {
// Create a new thread (which ends up calling the MainWorkLoop() function
new WorkerThread(this);
}
}
void ThreadManager::MainWorkLoop() {
while (true) {
void* tag;
bool ok;
WorkStatus work_status = PollForWork(&tag, &ok);
std::unique_lock<std::mutex> lock(mu_);
// Reduce the number of pollers by 1 and check what happened with the poll
num_pollers_--;
bool done = false;
switch (work_status) {
case TIMEOUT:
// If we timed out and we have more pollers than we need (or we are
// shutdown), finish this thread
if (shutdown_ || num_pollers_ > max_pollers_) done = true;
break;
case SHUTDOWN:
// If the thread manager is shutdown, finish this thread
done = true;
break;
case WORK_FOUND:
// If we got work and there are now insufficient pollers, start a new
// one
if (!shutdown_ && num_pollers_ < min_pollers_) {
num_pollers_++;
num_threads_++;
// Drop lock before spawning thread to avoid contention
lock.unlock();
new WorkerThread(this);
} else {
// Drop lock for consistency with above branch
lock.unlock();
}
// Lock is always released at this point - do the application work
DoWork(tag, ok);
// Take the lock again to check post conditions
lock.lock();
// If we're shutdown, we should finish at this point.
if (shutdown_) done = true;
break;
}
// If we decided to finish the thread, break out of the while loop
if (done) break;
// Otherwise go back to polling as long as it doesn't exceed max_pollers_
//
// **WARNING**:
// There is a possibility of threads thrashing here (i.e excessive thread
// shutdowns and creations than the ideal case). This happens if max_poller_
// count is small and the rate of incoming requests is also small. In such
// scenarios we can possibly configure max_pollers_ to a higher value and/or
// increase the cq timeout.
//
// However, not doing this check here and unconditionally incrementing
// num_pollers (and hoping that the system will eventually settle down) has
// far worse consequences i.e huge number of threads getting created to the
// point of thread-exhaustion. For example: if the incoming request rate is
// very high, all the polling threads will return very quickly from
// PollForWork() with WORK_FOUND. They all briefly decrement num_pollers_
// counter thereby possibly - and briefly - making it go below min_pollers;
// This will most likely result in the creation of a new poller since
// num_pollers_ dipped below min_pollers_.
//
// Now, If we didn't do the max_poller_ check here, all these threads will
// go back to doing PollForWork() and the whole cycle repeats (with a new
// thread being added in each cycle). Once the total number of threads in
// the system crosses a certain threshold (around ~1500), there is heavy
// contention on mutexes (the mu_ here or the mutexes in gRPC core like the
// pollset mutex) that makes DoWork() take longer to finish thereby causing
// new poller threads to be created even faster. This results in a thread
// avalanche.
if (num_pollers_ < max_pollers_) {
num_pollers_++;
} else {
break;
}
};
CleanupCompletedThreads();
// If we are here, either ThreadManager is shutting down or it already has
// enough threads.
}
} // namespace grpc
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