From c5858ff9aef05c4b6f8423d9a9060296bb826fde Mon Sep 17 00:00:00 2001
From: Marco Costalba <mcostalba@gmail.com>
Date: Mon, 25 Jan 2010 14:06:35 +0100
Subject: [PATCH] In split() release the lock before slow search stack copy

Once we have allocated our slave threads and we have removed
master from available threads we can safely remove the lock
so that the lenghty search stack copy operation will not
impact lock contention.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
---
 src/search.cpp | 31 +++++++++++++++++++------------
 1 file changed, 19 insertions(+), 12 deletions(-)

diff --git a/src/search.cpp b/src/search.cpp
index 1c271561..f259c8fb 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -2950,7 +2950,6 @@ namespace {
     assert(ActiveThreads > 1);
 
     SplitPoint* splitPoint;
-    int i;
 
     lock_grab(&MPLock);
 
@@ -2983,34 +2982,42 @@ namespace {
     splitPoint->cpus = 1;
     splitPoint->pos = &p;
     splitPoint->parentSstack = sstck;
-    for (i = 0; i < ActiveThreads; i++)
+    for (int i = 0; i < ActiveThreads; i++)
         splitPoint->slaves[i] = 0;
 
-    // Copy the tail of current search stack to the master thread
-    memcpy(splitPoint->sstack[master] + ply - 1, sstck + ply - 1, 3 * sizeof(SearchStack));
+    Threads[master].idle = false;
     Threads[master].splitPoint = splitPoint;
 
-    // Make copies of the current position and search stack for each thread
-    for (i = 0; i < ActiveThreads && splitPoint->cpus < MaxThreadsPerSplitPoint; i++)
+    // Allocate available threads setting idle flag to false
+    for (int i = 0; i < ActiveThreads && splitPoint->cpus < MaxThreadsPerSplitPoint; i++)
         if (thread_is_available(i, master))
         {
-            memcpy(splitPoint->sstack[i] + ply - 1, sstck + ply - 1, 3 * sizeof(SearchStack));
+            Threads[i].idle = false;
             Threads[i].splitPoint = splitPoint;
             splitPoint->slaves[i] = 1;
             splitPoint->cpus++;
         }
 
+    assert(splitPoint->cpus > 1);
+
+    // We can release the lock because master and slave threads are already booked
+    lock_release(&MPLock);
+
+    // Copy the tail of current search stack to the master thread
+    memcpy(splitPoint->sstack[master] + ply - 1, sstck + ply - 1, 3 * sizeof(SearchStack));
+
     // Tell the threads that they have work to do. This will make them leave
-    // their idle loop.
-    for (i = 0; i < ActiveThreads; i++)
+    // their idle loop. Also copy search stack tail for each slave thread.
+    for (int i = 0; i < ActiveThreads; i++)
+    {
         if (i == master || splitPoint->slaves[i])
         {
             Threads[i].workIsWaiting = true;
-            Threads[i].idle = false;
             Threads[i].stop = false;
         }
-
-    lock_release(&MPLock);
+        if (splitPoint->slaves[i])
+            memcpy(splitPoint->sstack[i] + ply - 1, sstck + ply - 1, 3 * sizeof(SearchStack));
+    }
 
     // Everything is set up. The master thread enters the idle loop, from
     // which it will instantly launch a search, because its workIsWaiting
-- 
2.39.2