[commit: ghc] master: small optimisation for the program in #5367: if the worker thread (78e6b61)

[Simon Marlow]

Repository : ssh://darcs.haskell.org//srv/darcs/ghc

On branch  : master

http://hackage.haskell.org/trac/ghc/changeset/78e6b615329dd8ea4527b499107048693c87f895

>---------------------------------------------------------------

commit 78e6b615329dd8ea4527b499107048693c87f895
Author: Simon Marlow <marlowsd at gmail.com>
Date:   Wed Aug 3 13:41:33 2011 +0100

    small optimisation for the program in #5367: if the worker thread
    being woken already has its wakeup flag set, don't bother signalling
    its condition variable again.

>---------------------------------------------------------------

 rts/Capability.c |   12 +++++++-----
 1 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/rts/Capability.c b/rts/Capability.c
index 91c5e2d..57c75e6 100644
--- a/rts/Capability.c
+++ b/rts/Capability.c
@@ -366,11 +366,13 @@ giveCapabilityToTask (Capability *cap USED_IF_DEBUG, Task *task)
                cap->no, task->incall->tso ? "bound task" : "worker",
                (void *)task->id);
     ACQUIRE_LOCK(&task->lock);
-    task->wakeup = rtsTrue;
-    // the wakeup flag is needed because signalCondition() doesn't
-    // flag the condition if the thread is already runniing, but we want
-    // it to be sticky.
-    signalCondition(&task->cond);
+    if (task->wakeup == rtsFalse) {
+        task->wakeup = rtsTrue;
+        // the wakeup flag is needed because signalCondition() doesn't
+        // flag the condition if the thread is already runniing, but we want
+        // it to be sticky.
+        signalCondition(&task->cond);
+    }
     RELEASE_LOCK(&task->lock);
 }
 #endif