Do not sleep, but yield

During the search, do not block on condition variable, but instead use std::this_thread::yield().

Clear gain with 16 threads. Again results vary highly depending on hardware, but on average it's a clear gain.

ELO: 12.17 +-4.3 (95%) LOS: 100.0%
Total: 7998 W: 1407 L: 1127 D: 5464

There is no functional change in single thread mode

Resolves #294

src/search.cpp

@@ -1595,8 +1595,25 @@ void Thread::idle_loop() {
 
   assert(!this_sp || (this_sp->master == this && searching));
 
-  while (!exit)
+  while (   !exit
+         && !(this_sp && this_sp->slavesMask.none()))
   {
+      // If there is nothing to do, sleep.
+      while(   !exit
+            && !(this_sp && this_sp->slavesMask.none())
+            && !searching)
+      {
+          if (   !this_sp 
+              && !Threads.main()->thinking)
+          {
+              std::unique_lock<Mutex> lk(mutex);
+              while (!exit && !Threads.main()->thinking)
+                    sleepCondition.wait(lk);
+          }
+          else
+              std::this_thread::yield();
+      }
+
       // If this thread has been assigned work, launch a search
       while (searching)
       {
@@ -1639,15 +1656,6 @@ void Thread::idle_loop() {
           sp->allSlavesSearching = false;
           sp->nodes += pos.nodes_searched();
 
-          // Wake up the master thread so to allow it to return from the idle
-          // loop in case we are the last slave of the split point.
-          if (this != sp->master && sp->slavesMask.none())
-          {
-              assert(!sp->master->searching);
-
-              sp->master->notify_one();
-          }
-
           // After releasing the lock we can't access any SplitPoint related data
           // in a safe way because it could have been released under our feet by
           // the sp master.
@@ -1711,21 +1719,6 @@ void Thread::idle_loop() {
               sp->mutex.unlock();
           }
       }
-
-      // Avoid races with notify_one() fired from last slave of the split point
-      std::unique_lock<Mutex> lk(mutex);
-
-      // If we are master and all slaves have finished then exit idle_loop
-      if (this_sp && this_sp->slavesMask.none())
-      {
-          assert(!searching);
-          break;
-      }
-
-      // If we are not searching, wait for a condition to be signaled instead of
-      // wasting CPU time polling for work.
-      if (!searching && !exit)
-          sleepCondition.wait(lk);
   }
 }
 

src/thread.cpp

@@ -184,8 +184,6 @@ void Thread::split(Position& pos, Stack* ss, Value alpha, Value beta, Value* bes
       }
 
       slave->allocMutex.unlock();
-
-      slave->notify_one(); // Could be sleeping
   }
 
   // Everything is set up. The master thread enters the idle loop, from which
@@ -375,5 +373,7 @@ void ThreadPool::start_thinking(const Position& pos, const LimitsType& limits,
           RootMoves.push_back(RootMove(m));
 
   main()->thinking = true;
-  main()->notify_one(); // Starts main thread
+
+  for (Thread* th : *this)
+      th->notify_one();
 }