Improve multi-threaded mate finding

vondele · mcostalba · commit d5f883ab29d4 · 2017-08-26T09:53:34.000+02:00
If any thread found a 'mate in x' stop the search. Previously only
mainThread would do so. Requires the bestThread selection to be
adjusted to always prefer mate scores, even if the search depth is less.

I've tried to collect some data for this patch. On 30 cores, mate finding
seems 5-30% faster on average. It is not so easy to get numbers for this,
as the time to find a mate fluctuates significantly with multi-threaded runs,
so it is an average over 100 searches for the same position. Furthermore,
hash size and position make a difference as well.

Bench: 5965302
diff --git a/src/search.cpp b/src/search.cpp
@@ -308,7 +308,9 @@ void MainThread::search() {
           Depth depthDiff = th->completedDepth - bestThread->completedDepth;
           Value scoreDiff = th->rootMoves[0].score - bestThread->rootMoves[0].score;
 
-          if (scoreDiff > 0 && depthDiff >= 0)
+          // Select the thread with the best score, always if it is a mate
+          if (    scoreDiff > 0
+              && (depthDiff >= 0 || th->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY))
               bestThread = th;
       }
   }
@@ -455,29 +457,27 @@ void Thread::search() {
           // Sort the PV lines searched so far and update the GUI
           std::stable_sort(rootMoves.begin(), rootMoves.begin() + PVIdx + 1);
 
-          if (!mainThread)
-              continue;
-
-          if (Threads.stop || PVIdx + 1 == multiPV || Time.elapsed() > 3000)
+          if (    mainThread
+              && (Threads.stop || PVIdx + 1 == multiPV || Time.elapsed() > 3000))
               sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
       }
 
       if (!Threads.stop)
           completedDepth = rootDepth;
 
+      // Have we found a "mate in x"?
+      if (   Limits.mate
+          && bestValue >= VALUE_MATE_IN_MAX_PLY
+          && VALUE_MATE - bestValue <= 2 * Limits.mate)
+          Threads.stop = true;
+
       if (!mainThread)
           continue;
 
       // If skill level is enabled and time is up, pick a sub-optimal best move
       if (skill.enabled() && skill.time_to_pick(rootDepth))
           skill.pick_best(multiPV);
 
-      // Have we found a "mate in x"?
-      if (   Limits.mate
-          && bestValue >= VALUE_MATE_IN_MAX_PLY
-          && VALUE_MATE - bestValue <= 2 * Limits.mate)
-          Threads.stop = true;
-
       // Do we have time for the next iteration? Can we stop searching now?
       if (Limits.use_time_management())
       {
