Allow threads to sleep when available

By mean of an an UCI option it is possible let the available
threads to sleep, this should help with Hyper Threading although
is not the best solution when number of threads equals number
of available cores.

Option is disabled by default.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/search.cpp

@@ -263,6 +263,7 @@ namespace {
   // Multi-threads related variables
   Depth MinimumSplitDepth;
   int MaxThreadsPerSplitPoint;
+  bool UseSleepingThreads;
   ThreadsManager ThreadsMgr;
 
   // Node counters, used only by thread[0] but try to keep in different cache
@@ -455,6 +456,7 @@ bool think(Position& pos, bool infinite, bool ponder, int time[], int increment[
   MaxThreadsPerSplitPoint = Options["Maximum Number of Threads per Split Point"].value<int>();
   MultiPV                 = Options["MultiPV"].value<int>();
   UseLogFile              = Options["Use Search Log"].value<bool>();
+  UseSleepingThreads      = Options["Use Sleeping Threads"].value<bool>();
 
   if (UseLogFile)
       LogFile.open(Options["Search Log Filename"].value<std::string>().c_str(), std::ios::out | std::ios::app);
@@ -466,7 +468,7 @@ bool think(Position& pos, bool infinite, bool ponder, int time[], int increment[
   if (newActiveThreads != ThreadsMgr.active_threads())
   {
       ThreadsMgr.set_active_threads(newActiveThreads);
-      init_eval(ThreadsMgr.active_threads());
+      init_eval(newActiveThreads);
   }
 
   // Wake up needed threads
@@ -2199,6 +2201,9 @@ split_point_start: // At split points actual search starts from here
 
     assert(threadID >= 0 && threadID < MAX_THREADS);
 
+    int i;
+    bool allFinished = false;
+
     while (true)
     {
         // Slave threads can exit as soon as AllThreadsShouldExit raises,
@@ -2212,19 +2217,30 @@ split_point_start: // At split points actual search starts from here
 
         // If we are not thinking, wait for a condition to be signaled
         // instead of wasting CPU time polling for work.
-        while (threadID >= ActiveThreads || threads[threadID].state == THREAD_INITIALIZING)
+        while (   threadID >= ActiveThreads || threads[threadID].state == THREAD_INITIALIZING
+               || (UseSleepingThreads && threads[threadID].state == THREAD_AVAILABLE))
         {
-            assert(!sp);
-            assert(threadID != 0);
+            assert(!sp || UseSleepingThreads);
+            assert(threadID != 0 || UseSleepingThreads);
 
-            if (AllThreadsShouldExit)
-                break;
-
-            threads[threadID].state = THREAD_AVAILABLE;
+            if (threads[threadID].state == THREAD_INITIALIZING)
+                threads[threadID].state = THREAD_AVAILABLE;
 
+            // Grab the lock to avoid races with wake_sleeping_thread()
             lock_grab(&WaitLock);
 
-            if (threadID >= ActiveThreads || threads[threadID].state == THREAD_INITIALIZING)
+            // If we are master and all slaves have finished do not go to sleep
+            for (i = 0; sp && i < ActiveThreads && !sp->slaves[i]; i++) {}
+            allFinished = (i == ActiveThreads);
+
+            if (allFinished || AllThreadsShouldExit)
+            {
+                lock_release(&WaitLock);
+                break;
+            }
+
+            // Do sleep here after retesting sleep conditions
+            if (threadID >= ActiveThreads || threads[threadID].state == THREAD_AVAILABLE)
                 cond_wait(&WaitCond[threadID], &WaitLock);
 
             lock_release(&WaitLock);
@@ -2245,20 +2261,25 @@ split_point_start: // At split points actual search starts from here
 
             if (tsp->pvNode)
                 search<PV, true>(pos, ss, tsp->alpha, tsp->beta, tsp->depth, tsp->ply);
-            else {
+            else
                 search<NonPV, true>(pos, ss, tsp->alpha, tsp->beta, tsp->depth, tsp->ply);
-            }
+
             assert(threads[threadID].state == THREAD_SEARCHING);
 
             threads[threadID].state = THREAD_AVAILABLE;
+
+            // Wake up master thread so to allow it to return from the idle loop in
+            // case we are the last slave of the split point.
+            if (UseSleepingThreads && threadID != tsp->master && threads[tsp->master].state == THREAD_AVAILABLE)
+                wake_sleeping_thread(tsp->master);
         }
 
         // If this thread is the master of a split point and all slaves have
         // finished their work at this split point, return from the idle loop.
-        int i = 0;
-        for ( ; sp && i < ActiveThreads && !sp->slaves[i]; i++) {}
+        for (i = 0; sp && i < ActiveThreads && !sp->slaves[i]; i++) {}
+        allFinished = (i == ActiveThreads);
 
-        if (i == ActiveThreads)
+        if (allFinished)
         {
             // Because sp->slaves[] is reset under lock protection,
             // be sure sp->lock has been released before to return.
@@ -2468,6 +2489,7 @@ split_point_start: // At split points actual search starts from here
 
     // Initialize the split point object
     splitPoint.parent = masterThread.splitPoint;
+    splitPoint.master = master;
     splitPoint.stopRequest = false;
     splitPoint.ply = ply;
     splitPoint.depth = depth;
@@ -2517,6 +2539,9 @@ split_point_start: // At split points actual search starts from here
             assert(i == master || threads[i].state == THREAD_BOOKED);
 
             threads[i].state = THREAD_WORKISWAITING; // This makes the slave to exit from idle_loop()
+
+            if (UseSleepingThreads && i != master)
+                wake_sleeping_thread(i);
         }
 
     // Everything is set up. The master thread enters the idle loop, from

src/thread.h

@@ -55,6 +55,7 @@ struct SplitPoint {
   bool pvNode, mateThreat;
   Value beta;
   int ply;
+  int master;
   Move threatMove;
   SearchStack sstack[MAX_THREADS][PLY_MAX_PLUS_2];
 

src/ucioption.cpp

@@ -94,6 +94,7 @@ void init_uci_options() {
   Options["Minimum Split Depth"] = Option(4, 4, 7);
   Options["Maximum Number of Threads per Split Point"] = Option(5, 4, 8);
   Options["Threads"] = Option(1, 1, MAX_THREADS);
+  Options["Use Sleeping Threads"] = Option(false);
   Options["Hash"] = Option(32, 4, 8192);
   Options["Clear Hash"] = Option(false, "button");
   Options["Ponder"] = Option(true);