Performance fix: replace WorkerPool sleeping with condition variable

src/include/threadpool/mono_queue_pool.h

@@ -83,7 +83,11 @@ inline void MonoQueuePool::SubmitTask(const F &func) {
   if (!is_running_) {
     Startup();
   }
-  task_queue_.Enqueue(std::move(func));
+  {
+    std::lock_guard<std::mutex> lock(worker_pool_.cv_lock);
+    task_queue_.Enqueue(std::move(func));
+  }
+  worker_pool_.not_empty.notify_one();
 }
 
 inline MonoQueuePool &MonoQueuePool::GetInstance() {

src/include/threadpool/worker_pool.h

@@ -13,7 +13,9 @@
 #pragma once
 
 #include <atomic>
+#include <condition_variable>
 #include <functional>
+#include <mutex>
 #include <string>
 #include <thread>
 #include <vector>
@@ -52,6 +54,9 @@ class WorkerPool {
    */
   uint32_t NumWorkers() const { return num_workers_; }
 
+  std::mutex cv_lock;
+  std::condition_variable not_empty;
+
  private:
   // The name of this pool
   std::string pool_name_;

src/threadpool/worker_pool.cpp

@@ -10,6 +10,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <chrono>
+
 #include "threadpool/worker_pool.h"
 
 #include "common/logger.h"
@@ -20,22 +22,18 @@ namespace threadpool {
 namespace {
 
 void WorkerFunc(std::string thread_name, std::atomic_bool *is_running,
-                TaskQueue *task_queue) {
-  constexpr auto kMinPauseTime = std::chrono::microseconds(1);
-  constexpr auto kMaxPauseTime = std::chrono::microseconds(1000);
-
+                TaskQueue *task_queue, std::mutex *cv_lock,
+                std::condition_variable *not_empty) {
   LOG_INFO("Thread %s starting ...", thread_name.c_str());
 
-  auto pause_time = kMinPauseTime;
   while (is_running->load() || !task_queue->IsEmpty()) {
+    std::unique_lock<std::mutex> lock(*cv_lock);
+    not_empty->wait_for(lock, std::chrono::seconds(1),
+                        [&] { return !task_queue->IsEmpty(); });
     std::function<void()> task;
-    if (!task_queue->Dequeue(task)) {
-      // Polling with exponential back-off
-      std::this_thread::sleep_for(pause_time);
-      pause_time = std::min(pause_time * 2, kMaxPauseTime);
-    } else {
+    if (task_queue->Dequeue(task)) {
+      lock.unlock();
       task();
-      pause_time = kMinPauseTime;
     }
   }
 
@@ -56,7 +54,8 @@ void WorkerPool::Startup() {
   if (is_running_.compare_exchange_strong(running, true)) {
     for (size_t i = 0; i < num_workers_; i++) {
       std::string name = pool_name_ + "-worker-" + std::to_string(i);
-      workers_.emplace_back(WorkerFunc, name, &is_running_, &task_queue_);
+      workers_.emplace_back(WorkerFunc, name, &is_running_, &task_queue_,
+                            &cv_lock, &not_empty);
     }
   }
 }