Get rid of timer thread

Unfortunately std::condition_variable::wait_for()
is not accurate in general case and the timer thread
can wake up also after tens or even hundreds of
millisecs after time has elapsded. CPU load, process
priorities, number of concurrent threads, even from
other processes, will have effect upon it.

Even official documentation says: "This function may
block for longer than timeout_duration due to scheduling
or resource contention delays."

So retire timer and use a polling scheme based on a
local thread counter that counts search() calls and
a small trick to keep polling frequency constant,
independently from the number of threads.

Tested for no regression at very fast TC 2+0.05 th 7:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 32969 W: 6720 L: 6620 D: 19629

TC 2+0.05 th 1:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 7765 W: 1917 L: 1765 D: 4083

And at STC TC, both single thread
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 15587 W: 3036 L: 2905 D: 9646

And with 7 threads
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 8149 W: 1367 L: 1227 D: 5555

bench: 8639247

src/search.cpp

@@ -141,6 +141,7 @@ namespace {
   Value value_from_tt(Value v, int ply);
   void update_pv(Move* pv, Move move, Move* childPv);
   void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt);
+  void check_time();
 
 } // namespace
 
@@ -298,14 +299,10 @@ void MainThread::think() {
           }
       }
 
-      Threads.timer->run = true;
-      Threads.timer->notify_one(); // Start the recurring timer
-
       search(true); // Let's start searching!
 
-      // Stop the threads and the timer
+      // Stop the threads
       Signals.stop = true;
-      Threads.timer->run = false;
 
       // Wait until all threads have finished
       for (Thread* th : Threads)
@@ -585,6 +582,20 @@ namespace {
     bestValue = -VALUE_INFINITE;
     ss->ply = (ss-1)->ply + 1;
 
+    // Check for available remaining time
+    if (thisThread->resetCallsCnt.load(std::memory_order_relaxed))
+    {
+        thisThread->resetCallsCnt = false;
+        thisThread->callsCnt = 0;
+    }
+    if (++thisThread->callsCnt > 4096)
+    {
+        for (Thread* th : Threads)
+            th->resetCallsCnt = true;
+
+        check_time();
+    }
+
     // Used to send selDepth info to GUI
     if (PvNode && thisThread->maxPly < ss->ply)
         thisThread->maxPly = ss->ply;
@@ -1455,6 +1466,43 @@ moves_loop: // When in check search starts from here
     return best;
   }
 
+
+  // check_time() is used to print debug info and, more importantly, to detect
+  // when we are out of available time and thus stop the search.
+
+  void check_time() {
+
+    static TimePoint lastInfoTime = now();
+
+    int elapsed = Time.elapsed();
+    TimePoint tick = Limits.startTime + elapsed;
+
+    if (tick - lastInfoTime >= 1000)
+    {
+        lastInfoTime = tick;
+        dbg_print();
+    }
+
+    // An engine may not stop pondering until told so by the GUI
+    if (Limits.ponder)
+        return;
+
+    if (Limits.use_time_management())
+    {
+        bool stillAtFirstMove =    Signals.firstRootMove.load(std::memory_order_relaxed)
+                               && !Signals.failedLowAtRoot.load(std::memory_order_relaxed)
+                               &&  elapsed > Time.available() * 3 / 4;
+
+        if (stillAtFirstMove || elapsed > Time.maximum() - 10)
+            Signals.stop = true;
+    }
+    else if (Limits.movetime && elapsed >= Limits.movetime)
+        Signals.stop = true;
+
+    else if (Limits.nodes && Threads.nodes_searched() >= Limits.nodes)
+            Signals.stop = true;
+  }
+
 } // namespace
 
 
@@ -1565,40 +1613,3 @@ bool RootMove::extract_ponder_from_tt(Position& pos)
 
     return false;
 }
-
-
-/// TimerThread::check_time() is called by when the timer triggers. It is used
-/// to print debug info and, more importantly, to detect when we are out of
-/// available time and thus stop the search.
-
-void TimerThread::check_time() {
-
-  static TimePoint lastInfoTime = now();
-  int elapsed = Time.elapsed();
-
-  if (now() - lastInfoTime >= 1000)
-  {
-      lastInfoTime = now();
-      dbg_print();
-  }
-
-  // An engine may not stop pondering until told so by the GUI
-  if (Limits.ponder)
-      return;
-
-  if (Limits.use_time_management())
-  {
-      bool stillAtFirstMove =    Signals.firstRootMove
-                             && !Signals.failedLowAtRoot
-                             &&  elapsed > Time.available() * 3 / 4;
-
-      if (   stillAtFirstMove
-          || elapsed > Time.maximum() - 2 * TimerThread::Resolution)
-          Signals.stop = true;
-  }
-  else if (Limits.movetime && elapsed >= Limits.movetime)
-      Signals.stop = true;
-
-  else if (Limits.nodes && Threads.nodes_searched() >= Limits.nodes)
-          Signals.stop = true;
-}

src/search.h

@@ -93,7 +93,7 @@ struct LimitsType {
 /// typically in an async fashion e.g. to stop the search by the GUI.
 
 struct SignalsType {
-  std::atomic<bool> stop, stopOnPonderhit, firstRootMove, failedLowAtRoot;
+  std::atomic_bool stop, stopOnPonderhit, firstRootMove, failedLowAtRoot;
 };
 
 typedef std::unique_ptr<std::stack<StateInfo>> StateStackPtr;

src/thread.cpp

@@ -66,7 +66,7 @@ void ThreadBase::notify_one() {
 
 // ThreadBase::wait() set the thread to sleep until 'condition' turns true
 
-void ThreadBase::wait(std::atomic<bool>& condition) {
+void ThreadBase::wait(std::atomic_bool& condition) {
 
   std::unique_lock<Mutex> lk(mutex);
   sleepCondition.wait(lk, [&]{ return bool(condition); });
@@ -74,7 +74,7 @@ void ThreadBase::wait(std::atomic<bool>& condition) {
 
 
 // ThreadBase::wait_while() set the thread to sleep until 'condition' turns false
-void ThreadBase::wait_while(std::atomic<bool>& condition) {
+void ThreadBase::wait_while(std::atomic_bool& condition) {
 
   std::unique_lock<Mutex> lk(mutex);
   sleepCondition.wait(lk, [&]{ return !condition; });
@@ -86,34 +86,14 @@ void ThreadBase::wait_while(std::atomic<bool>& condition) {
 
 Thread::Thread() {
 
-  searching = false;
+  searching = resetCallsCnt = false;
-  maxPly = 0;
+  maxPly = callsCnt = 0;
   history.clear();
   counterMoves.clear();
   idx = Threads.size(); // Starts from 0
 }
 
 
-// TimerThread::idle_loop() is where the timer thread waits Resolution milliseconds
-// and then calls check_time(). When not searching, thread sleeps until it's woken up.
-
-void TimerThread::idle_loop() {
-
-  while (!exit)
-  {
-      std::unique_lock<Mutex> lk(mutex);
-
-      if (!exit)
-          sleepCondition.wait_for(lk, std::chrono::milliseconds(run ? Resolution : INT_MAX));
-
-      lk.unlock();
-
-      if (!exit && run)
-          check_time();
-  }
-}
-
-
 // Thread::idle_loop() is where the thread is parked when it has no work to do
 
 void Thread::idle_loop() {
@@ -174,7 +154,6 @@ void MainThread::join() {
 
 void ThreadPool::init() {
 
-  timer = new_thread<TimerThread>();
   push_back(new_thread<MainThread>());
   read_uci_options();
 }
@@ -185,9 +164,6 @@ void ThreadPool::init() {
 
 void ThreadPool::exit() {
 
-  delete_thread(timer); // As first because check_time() accesses threads data
-  timer = nullptr;
-
   for (Thread* th : *this)
       delete_thread(th);
 

src/thread.h

@@ -44,12 +44,12 @@ struct ThreadBase : public std::thread {
   virtual ~ThreadBase() = default;
   virtual void idle_loop() = 0;
   void notify_one();
-  void wait(std::atomic<bool>& b);
+  void wait(std::atomic_bool& b);
-  void wait_while(std::atomic<bool>& b);
+  void wait_while(std::atomic_bool& b);
 
   Mutex mutex;
   ConditionVariable sleepCondition;
-  std::atomic<bool> exit;
+  std::atomic_bool exit;
 };
 
 
@@ -68,8 +68,8 @@ struct Thread : public ThreadBase {
   Material::Table materialTable;
   Endgames endgames;
   size_t idx, PVIdx;
-  int maxPly;
+  int maxPly, callsCnt;
-  std::atomic<bool> searching;
+  std::atomic_bool searching, resetCallsCnt;
 
   Position rootPos;
   Search::RootMoveVector rootMoves;
@@ -80,25 +80,14 @@ struct Thread : public ThreadBase {
 };
 
 
-/// MainThread and TimerThread are derived classes used to characterize the two
+/// MainThread is a derived classes used to characterize the the main one
-/// special threads: the main one and the recurring timer.
 
 struct MainThread : public Thread {
   MainThread() { thinking = true; } // Avoid a race with start_thinking()
   virtual void idle_loop();
   void join();
   void think();
-  std::atomic<bool> thinking;
+  std::atomic_bool thinking;
-};
-
-struct TimerThread : public ThreadBase {
-
-  static const int Resolution = 5; // Millisec between two check_time() calls
-
-  virtual void idle_loop();
-  void check_time();
-
-  bool run = false;
 };
 
 
@@ -115,7 +104,6 @@ struct ThreadPool : public std::vector<Thread*> {
   void read_uci_options();
   void start_thinking(const Position&, const Search::LimitsType&, Search::StateStackPtr&);
   int64_t nodes_searched();
-  TimerThread* timer;
 };
 
 extern ThreadPool Threads;