Improve smp performance for high number of threads

Balance threads between split points.

There are huge differences between different machines and autopurging makes it very difficult to measure the improvement in fishtest, but the following was recorded for 16 threads at 15+0.05:

    For Bravone (1000 games): 0 ELO
    For Glinscott (1000 games): +20 ELO
    For bKingUs (1000 games): +50 ELO
    For fastGM (1500 games): +50 ELO

The change was regression for no one, and a big improvement for some, so it should be fine to commit it.
Also for 8 threads at 15+0.05 we measured a statistically significant improvement:
ELO: 6.19 +-3.9 (95%) LOS: 99.9%
Total: 10325 W: 1824 L: 1640 D: 6861

Finally it was verified that there was no (significant) regression for

4 threads:
ELO: 0.09 +-2.8 (95%) LOS: 52.4%
Total: 19908 W: 3422 L: 3417 D: 13069

2 threads:
ELO: 0.38 +-3.0 (95%) LOS: 60.0%
Total: 19044 W: 3480 L: 3459 D: 12105

1 thread:
ELO: -1.27 +-2.1 (95%) LOS: 12.3%
Total: 40000 W: 7829 L: 7975 D: 24196

Resolves #258

src/search.cpp

@@ -1042,7 +1042,9 @@ moves_loop: // When in check and at SpNode search starts from here
           &&  Threads.size() >= 2
           &&  depth >= Threads.minimumSplitDepth
           &&  (   !thisThread->activeSplitPoint
-               || !thisThread->activeSplitPoint->allSlavesSearching)
+               || !thisThread->activeSplitPoint->allSlavesSearching
+               || (   int(Threads.size()) > MAX_SLAVES_PER_SPLITPOINT
+                   && thisThread->activeSplitPoint->slavesCount == MAX_SLAVES_PER_SPLITPOINT))
           &&  thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
       {
           assert(bestValue > -VALUE_INFINITE && bestValue < beta);
@@ -1587,6 +1589,11 @@ void Thread::idle_loop() {
           // Try to late join to another split point if none of its slaves has
           // already finished.
           if (Threads.size() > 2)
+          {
+              SplitPoint *bestSp = NULL;
+              int bestThread = 0;
+              int bestScore = INT_MAX;
+
               for (size_t i = 0; i < Threads.size(); ++i)
               {
                   const int size = Threads[i]->splitPointsSize; // Local copy
@@ -1594,26 +1601,42 @@ void Thread::idle_loop() {
 
                   if (   sp
                       && sp->allSlavesSearching
+                      && sp->slavesCount < MAX_SLAVES_PER_SPLITPOINT
                       && available_to(Threads[i]))
                   {
-                      // Recheck the conditions under lock protection
-                      Threads.mutex.lock();
-                      sp->mutex.lock();
+                      int score = sp->spLevel * 256 * 256 + sp->slavesCount * 256 - sp->depth * 1;
 
-                      if (   sp->allSlavesSearching
-                          && available_to(Threads[i]))
+                      if (score < bestScore)
                       {
-                           sp->slavesMask.set(idx);
-                           activeSplitPoint = sp;
-                           searching = true;
+                           bestSp = sp;
+                           bestThread = i;
+                           bestScore = score;
                       }
-
-                      sp->mutex.unlock();
-                      Threads.mutex.unlock();
-
-                      break; // Just a single attempt
                   }
               }
+
+              if (bestSp)
+              {
+                  sp = bestSp;
+ 
+                  // Recheck the conditions under lock protection
+                  Threads.mutex.lock();
+                  sp->mutex.lock();
+
+                  if (   sp->allSlavesSearching
+                      && sp->slavesCount < MAX_SLAVES_PER_SPLITPOINT
+                      && available_to(Threads[bestThread]))
+                  {
+                      sp->slavesMask.set(idx);
+                      sp->slavesCount++;
+                      activeSplitPoint = sp;
+                      searching = true;
+                  }
+
+                  sp->mutex.unlock();
+                  Threads.mutex.unlock();
+              }
+          }
       }
 
       // Grab the lock to avoid races with Thread::notify_one()

src/thread.cpp

@@ -154,7 +154,9 @@ void Thread::split(Position& pos, Stack* ss, Value alpha, Value beta, Value* bes
 
   sp.masterThread = this;
   sp.parentSplitPoint = activeSplitPoint;
+  sp.spLevel = activeSplitPoint ? activeSplitPoint->spLevel + 1 : 0;
   sp.slavesMask = 0, sp.slavesMask.set(idx);
+  sp.slavesCount = 1;
   sp.depth = depth;
   sp.bestValue = *bestValue;
   sp.bestMove = *bestMove;
@@ -182,9 +184,11 @@ void Thread::split(Position& pos, Stack* ss, Value alpha, Value beta, Value* bes
 
   Thread* slave;
 
-  while ((slave = Threads.available_slave(this)) != NULL)
+  while (    sp.slavesCount < MAX_SLAVES_PER_SPLITPOINT 
+         && (slave = Threads.available_slave(this)) != NULL)
   {
       sp.slavesMask.set(slave->idx);
+      sp.slavesCount++;
       slave->activeSplitPoint = &sp;
       slave->searching = true; // Slave leaves idle_loop()
       slave->notify_one(); // Could be sleeping

src/thread.h

@@ -33,6 +33,7 @@ struct Thread;
 
 const int MAX_THREADS = 128;
 const int MAX_SPLITPOINTS_PER_THREAD = 8;
+const int MAX_SLAVES_PER_SPLITPOINT = 4;
 
 /// Mutex and ConditionVariable struct are wrappers of the low level locking
 /// machinery and are modeled after the corresponding C++11 classes.
@@ -72,6 +73,7 @@ struct SplitPoint {
   const Position* pos;
   Search::Stack* ss;
   Thread* masterThread;
+  int spLevel;
   Depth depth;
   Value beta;
   int nodeType;
@@ -84,6 +86,7 @@ struct SplitPoint {
   // Shared variable data
   Mutex mutex;
   std::bitset<MAX_THREADS> slavesMask;
+  int slavesCount;
   volatile bool allSlavesSearching;
   volatile uint64_t nodes;
   volatile Value alpha;