Implement proper stop signalling from root node

Previous behavior was to wait on all nodes to finish their search on their own TM and aggregate to root node via a blocking MPI_Allreduce call. This seems to be problematic.

In this commit a proper non-blocking signalling barrier was implemented to use TM from root node to control the cluster search, and disable TM on all non-root nodes.

Also includes some cosmetic fix to the nodes/NPS display.

src/cluster.cpp

@@ -37,10 +37,13 @@ namespace Cluster {
 
 static int world_rank = MPI_PROC_NULL;
 static int world_size = 0;
+static bool stop_signal = false;
+static MPI_Request reqStop = MPI_REQUEST_NULL;
 
 static MPI_Comm InputComm = MPI_COMM_NULL;
 static MPI_Comm TTComm = MPI_COMM_NULL;
 static MPI_Comm MoveComm = MPI_COMM_NULL;
+static MPI_Comm StopComm = MPI_COMM_NULL;
 
 static MPI_Datatype TTEntryDatatype = MPI_DATATYPE_NULL;
 static std::vector<TTEntry> TTBuff;
@@ -104,6 +107,7 @@ void init() {
   MPI_Comm_dup(MPI_COMM_WORLD, &InputComm);
   MPI_Comm_dup(MPI_COMM_WORLD, &TTComm);
   MPI_Comm_dup(MPI_COMM_WORLD, &MoveComm);
+  MPI_Comm_dup(MPI_COMM_WORLD, &StopComm);
 }
 
 void finalize() {
@@ -131,6 +135,32 @@ bool getline(std::istream& input, std::string& str) {
   return state;
 }
 
+void sync_start() {
+  stop_signal = false;
+
+  // Start listening to stop signal
+  if (!is_root())
+      MPI_Ibarrier(StopComm, &reqStop);
+}
+
+void sync_stop() {
+  if (is_root()) {
+      if (!stop_signal && Threads.stop) {
+          // Signal the cluster about stopping
+          stop_signal = true;
+          MPI_Ibarrier(StopComm, &reqStop);
+          MPI_Wait(&reqStop, MPI_STATUS_IGNORE);
+      }
+  }
+  else {
+      int flagStop;
+      // Check if we've received any stop signal
+      MPI_Test(&reqStop, &flagStop, MPI_STATUS_IGNORE);
+      if (flagStop)
+          Threads.stop = true;
+  }
+}
+
 int size() {
   return world_size;
 }

src/cluster.h

@@ -69,6 +69,8 @@ inline bool is_root() { return rank() == 0; }
 void save(Thread* thread, TTEntry* tte,
           Key k, Value v, Bound b, Depth d, Move m, Value ev);
 void reduce_moves(MoveInfo& mi);
+void sync_start();
+void sync_stop();
 
 #else
 
@@ -86,6 +88,8 @@ inline void save(Thread* thread, TTEntry* tte,
   tte->save(k, v, b, d, m, ev);
 }
 inline void reduce_moves(MoveInfo&) { }
+inline void sync_start() { }
+inline void sync_stop() { }
 
 #endif /* USE_MPI */
 

src/main.cpp

@@ -27,7 +27,6 @@
 #include "tt.h"
 #include "uci.h"
 #include "syzygy/tbprobe.h"
-#include "cluster.h"
 
 namespace PSQT {
   void init();

src/search.cpp

@@ -234,12 +234,15 @@ void MainThread::search() {
   Threads.stopOnPonderhit = true;
 
   while (!Threads.stop && (Threads.ponder || Limits.infinite))
-  {} // Busy wait for a stop or a ponder reset
+  { } // Busy wait for a stop or a ponder reset
 
   // Stop the threads if not already stopped (also raise the stop if
   // "ponderhit" just reset Threads.ponder).
   Threads.stop = true;
 
+  // Finish any outstanding barriers.
+  Cluster::sync_stop();
+
   // Wait until all threads have finished
   for (Thread* th : Threads)
       if (th != this)
@@ -292,8 +295,8 @@ void MainThread::search() {
 
   previousScore = static_cast<Value>(mi.score);
 
-  // Send again PV info if we have a new best thread
   if (Cluster::is_root()) {
+      // Send again PV info if we have a new best thread
       if (bestThread != this)
           sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
 
@@ -1608,6 +1611,9 @@ void MainThread::check_time() {
   if (Threads.ponder)
       return;
 
+  // Check if root has reached a stop barrier
+  Cluster::sync_stop();
+
   if (   (Limits.use_time_management() && elapsed > Time.maximum() - 10)
       || (Limits.movetime && elapsed >= Limits.movetime)
       || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
@@ -1653,8 +1659,8 @@ string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
       if (!tb && i == pvIdx)
           ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
 
-      ss << " nodes "    << nodesSearched
+      ss << " nodes "    << nodesSearched * Cluster::size()
-         << " nps "      << nodesSearched * 1000 / elapsed;
+         << " nps "      << nodesSearched * Cluster::size() * 1000 / elapsed;
 
       if (elapsed > 1000) // Earlier makes little sense
           ss << " hashfull " << TT.hashfull();

src/search.h

@@ -26,6 +26,7 @@
 #include "misc.h"
 #include "movepick.h"
 #include "types.h"
+#include "cluster.h"
 
 class Position;
 
@@ -89,7 +90,7 @@ struct LimitsType {
   }
 
   bool use_time_management() const {
-    return !(mate | movetime | depth | nodes | perft | infinite);
+    return Cluster::is_root() && !(mate | movetime | depth | nodes | perft | infinite);
   }
 
   std::vector<Move> searchmoves;

src/thread.cpp

@@ -163,6 +163,8 @@ void ThreadPool::start_thinking(Position& pos, StateListPtr& states,
   main()->wait_for_search_finished();
 
   stopOnPonderhit = stop = false;
+  Cluster::sync_start();
+
   ponder = ponderMode;
   Search::Limits = limits;
   Search::RootMoves rootMoves;