[Support] Simplify and optimize ThreadPool

[MaskRay]

Summary

• Merge QueueLock and CompletionLock.
• Avoid spurious CompletionCondition.notify_all() when ActiveThreads is greater than 0.
• Use default member initializers.

[mehdi_amini]

Isn't this technically pessimizing some cases by sharing a lock (and so increasing potential contention here)?

llvm/lib/Support/ThreadPool.cpp

57

Can you expand this over different variables/statements? Or at minima add explicit parentheses? I am sure the compiler parses it, but not me :)

60

Update the comment here

[MaskRay]

Address comments

[MaskRay]

In D78856#2003373, @mehdi_amini wrote:

Isn't this technically pessimizing some cases by sharing a lock (and so increasing potential contention here)?

No. Only the thread(s) calling ThreadPool::Wait() waits on CompletionCondition. When it gets stuck, it will not awake spuriously. Note, before, we took locks 3 times in the loop body. Now it is 2...
I believe this version is strictly better.

[MaskRay]

Avoid spurious wake-up of waiter

[aganea]

llvm/lib/Support/ThreadPool.cpp

57

Is it worth generalizing the notify condition between this and ThreadPool::wait() below, to ease future maintenance/comprehension?

[mehdi_amini]

No. Only the thread(s) calling ThreadPool::Wait() waits on CompletionCondition. When it gets stuck, it will not awake spuriously. Note, before, we took locks 3 times in the loop body. Now it is 2...

I wasn't worried about contention with the waiting threads, but between working threads: they are taking the QueueLock more often now (it was taken once or twice per iteration of the loop body before and is now taken an extra time).
I am not sure we'd be able to measure any difference here, just curious how you thought about this tradeoff!

Originally I think I started writing all this code with ActiveThreads being an atomic so that the working threads can increment/decrement it without taking any lock: however adding the "wait()" then I couldn't see how to avoid taking the CompletionLock because of how std::condition_variable is setup.
That said since the waiting thread takes the QueueLock with your change, would leaving the ActiveThreads atomic allow to not take the Queue lock on task completion?
I'm wondering about something like this:

// Notify task completion if this is the last active thread, in case
// someone waits on ThreadPool::wait().
bool Notify = --ActiveThreads == 0;
if (Notify)
  CompletionCondition.notify_all();

llvm/lib/Support/ThreadPool.cpp

57

Can you add parentheses or split the expression?

Notify = (--ActiveThreads == 0) && Tasks.empty();

or

--ActiveThreads;
Notify = !ActiveThreads && Tasks.empty();

The latter avoid the side effect on ActiveThreads in the expression, making the second line "pure" and easier to read.

[MaskRay]

In D78856#2003725, @mehdi_amini wrote:

No. Only the thread(s) calling ThreadPool::Wait() waits on CompletionCondition. When it gets stuck, it will not awake spuriously. Note, before, we took locks 3 times in the loop body. Now it is 2...

I wasn't worried about contention with the waiting threads, but between working threads: they are taking the QueueLock more often now (it was taken once or twice per iteration of the loop body before and is now taken an extra time).
I am not sure we'd be able to measure any difference here, just curious how you thought about this tradeoff!

There are two stages, before and after the execution of as task. The original code sequence used different locks. I suppose this is what you meant by less contention: a worker in the first stage (holding QueueLock while not holding CompletionLock) cannot contend with a worker in the second stage (holding CompletionLock)..

But note that:

• there is still a window when a worker in the first stage can hold CompletionLock. The window is small, but it still exists.
• the spurious signalling of a condition variable (CompletionCondition) can cause contention because a condition variable has an internal lock.

Originally I think I started writing all this code with ActiveThreads being an atomic so that the working threads can increment/decrement it without taking any lock: however adding the "wait()" then I couldn't see how to avoid taking the CompletionLock because of how std::condition_variable is setup.
That said since the waiting thread takes the QueueLock with your change, would leaving the ActiveThreads atomic allow to not take the Queue lock on task completion?
I'm wondering about something like this:

// Notify task completion if this is the last active thread, in case
// someone waits on ThreadPool::wait().
bool Notify = --ActiveThreads == 0;
if (Notify)
  CompletionCondition.notify_all();

The suggested code sequence has the problem of lost signals. It happens because a waiter thread can change the observed state between waiter's testing and blocking.

  worker: pop one item from Tasks and Tasks is now empty
waiter: lock QueueLock
waiter: test that `!ActiveThreads && Tasks.empty()` is false and decide to unlock and block
  worker: decrement ActiveThreads to zero
  worker: signal QueueLock  # the signal is lost
waiter: unlock QueueLock and block

With an atomic ActiveThreads, an alternative code sequence is possible, but I'm not sure it is better:

bool Notify = --ActiveThreads == 0;
{ lock_guard guard(QueueLock); Notify &= Tasks.empty(); }
if (Notify) CompletionCondition.notify_all();

llvm/lib/Support/ThreadPool.cpp

57

Is it worth generalizing the notify condition between this and ThreadPool::wait() below, to ease future maintenance/comprehension?

Sorry that I am not following this suggestion. Can you elaborate?

[MaskRay]

Avoid --ActiveThreads in a complex expression

[aganea]

llvm/lib/Support/ThreadPool.cpp

57

I meant a function, to be used by the code here and the code below in wait(), simply to signify (to users who read the code) that it should check exactly the same things?

private: bool workCompletedNoLock() { return !ActiveThreads && Tasks.empty(); }

[MaskRay]

Place !ActiveThreads && Tasks.empty() in a new member function

llvm/lib/Support/ThreadPool.cpp

57

Gotcha. Good idea!

[mehdi_amini]

In D78856#2003762, @MaskRay wrote:

In D78856#2003725, @mehdi_amini wrote:

No. Only the thread(s) calling ThreadPool::Wait() waits on CompletionCondition. When it gets stuck, it will not awake spuriously. Note, before, we took locks 3 times in the loop body. Now it is 2...

I wasn't worried about contention with the waiting threads, but between working threads: they are taking the QueueLock more often now (it was taken once or twice per iteration of the loop body before and is now taken an extra time).
I am not sure we'd be able to measure any difference here, just curious how you thought about this tradeoff!

There are two stages, before and after the execution of as task. The original code sequence used different locks. I suppose this is what you meant by less contention: a worker in the first stage (holding QueueLock while not holding CompletionLock) cannot contend with a worker in the second stage (holding CompletionLock)..

Yes this is what I meant, the "second stage" lock contends with the first stage queue manipulation now, while it didn't before. It also contends with any thread trying to enqueue.
Overall for a given execution there will be quite a lot more (almost doubled?) locking/unlocking of the QueueLock right?

• the spurious signalling of a condition variable (CompletionCondition) can cause contention because a condition variable has an internal lock.

We're getting into implementation / platform specific, but I thought that signaling was mostly gonna map to one or two futex syscall?

The suggested code sequence has the problem of lost signals. It happens because a waiter thread can change the observed state between waiter's testing and blocking.
...

Thanks! This is likely the sequence that lead me to add this lock at the time... IIRC you can avoid this with low-level use of futex but there isn't a portable solution.

Anyway, I'm fine with this change overall, I don't think this thread pool implementation is optimized for very small tasks anyway.

[MaskRay]

Anyway, I'm fine with this change overall, I don't think this thread pool implementation is optimized for very small tasks anyway.

May I get a LGTM? :)

llvm/include/llvm/Support/ThreadPool.h

private:
 
   /// Tasks waiting for execution in the pool.
   std::queue<PackagedTaskTy> Tasks;
 
   /// Locking and signaling for accessing the Tasks queue.
   std::mutex QueueLock;
   std::condition_variable QueueCondition;
 
-  /// Locking and signaling for job completion
-  std::mutex CompletionLock;
+  /// Signaling for job completion
   std::condition_variable CompletionCondition;
 
   /// Keep track of the number of thread actually busy
-  std::atomic<unsigned> ActiveThreads;
+  unsigned ActiveThreads = 0;
 
 #if LLVM_ENABLE_THREADS // avoids warning for unused variable
   /// Signal for the destruction of the pool, asking thread to exit.
-  bool EnableFlag;
+  bool EnableFlag = true;
 #endif
 
   unsigned ThreadCount;
 };
 }
 
 #endif // LLVM_SUPPORT_THREAD_POOL_H

llvm/lib/Support/ThreadPool.cpp

 #include "llvm/Support/Threading.h"
 #include "llvm/Support/raw_ostream.h"
 
 using namespace llvm;
 
 #if LLVM_ENABLE_THREADS
 
 ThreadPool::ThreadPool(ThreadPoolStrategy S)
-    : ActiveThreads(0), EnableFlag(true),
-      ThreadCount(S.compute_thread_count()) {
+    : ThreadCount(S.compute_thread_count()) {
   // Create ThreadCount threads that will loop forever, wait on QueueCondition
   // for tasks to be queued or the Pool to be destroyed.
   Threads.reserve(ThreadCount);
   for (unsigned ThreadID = 0; ThreadID < ThreadCount; ++ThreadID) {
     Threads.emplace_back([S, ThreadID, this] {
       S.apply_thread_strategy(ThreadID);
       while (true) {
         PackagedTaskTy Task;
         {
           std::unique_lock<std::mutex> LockGuard(QueueLock);
           // Wait for tasks to be pushed in the queue
           QueueCondition.wait(LockGuard,
                               [&] { return !EnableFlag || !Tasks.empty(); });
           // Exit condition
           if (!EnableFlag && Tasks.empty())
             return;
           // Yeah, we have a task, grab it and release the lock on the queue
 
           // We first need to signal that we are active before popping the queue
           // in order for wait() to properly detect that even if the queue is
           // empty, there is still a task in flight.
-          {
-            std::unique_lock<std::mutex> LockGuard(CompletionLock);
-            ++ActiveThreads;
-          }
           Task = std::move(Tasks.front());
           Tasks.pop();
         }
         // Run the task we just grabbed
         Task();
 
+        bool Notify;
         {
           // Adjust `ActiveThreads`, in case someone waits on ThreadPool::wait()
-          std::unique_lock<std::mutex> LockGuard(CompletionLock);
-          --ActiveThreads;
+          std::lock_guard<std::mutex> LockGuard(QueueLock);
+          Notify = --ActiveThreads == 0;
         }
-
         // Notify task completion, in case someone waits on ThreadPool::wait()
+        if (Notify)
           CompletionCondition.notify_all();
       }
     });
   }
 }
 
 void ThreadPool::wait() {
   // Wait for all threads to complete and the queue to be empty
-  std::unique_lock<std::mutex> LockGuard(CompletionLock);
+  std::unique_lock<std::mutex> LockGuard(QueueLock);
   // The order of the checks for ActiveThreads and Tasks.empty() matters because
   // any active threads might be modifying the Tasks queue, and this would be a
   // race.
   CompletionCondition.wait(LockGuard,
                            [&] { return !ActiveThreads && Tasks.empty(); });
 }
 
 std::shared_future<void> ThreadPool::asyncImpl(TaskTy Task) {
ThreadPool::~ThreadPool() {
   for (auto &Worker : Threads)
     Worker.join();
 }
 
 #else // LLVM_ENABLE_THREADS Disabled
 
 // No threads are launched, issue a warning if ThreadCount is not 0
 ThreadPool::ThreadPool(ThreadPoolStrategy S)
-    : ActiveThreads(0), ThreadCount(S.compute_thread_count()) {
+    : ThreadCount(S.compute_thread_count()) {
   if (ThreadCount != 1) {
     errs() << "Warning: request a ThreadPool with " << ThreadCount
            << " threads, but LLVM_ENABLE_THREADS has been turned off\n";
   }
 }
 
 void ThreadPool::wait() {
   // Sequential implementation running the tasks