Don't leave and reenter guile mode if mutex is available

On Aug 5, 2009, at 10:06, Ken Raeburn wrote:
> (1) In scm_pthread_mutex_lock, we leave and re-enter guile mode so
> that we don't block the thread while in guile mode.  But we could
> use pthread_mutex_trylock first, and avoid the costs scm_leave_guile
> seems to incur on the Mac.  If we can't acquire the lock, it should
> return immediately, and then we can do the expensive, blocking
> version.  A quick, hack version of this changed my run time for
> A(3,8) from 17.5s to 14.5s, saving about 17%; sigaltstack and
> sigprocmask are still in the picture, because they're called from
> scm_catch_with_pre_unwind_handler.  I'll work up a nicer patch
> later.

Ah, we already had scm_i_pthread_mutex_trylock lying around; that made
things easy.
A second timing test with A(3,9) and this version of the patch (based
on 1.9.1) shows the same improvement.

* libguile/threads.c (scm_pthread_mutex_lock): Try the mutex before
  leaving and reentering guile mode.

1 files changed, 9 insertions, 4 deletions

diff --git a/libguile/threads.c b/libguile/threads.c
index 1721e0b28..e468f2fd5 100644
--- a/libguile/threads.c
+++ b/libguile/threads.c
@@ -1826,10 +1826,15 @@ scm_std_select (int nfds,
 int
 scm_pthread_mutex_lock (scm_i_pthread_mutex_t *mutex)
 {
-  scm_t_guile_ticket t = scm_leave_guile ();
-  int res = scm_i_pthread_mutex_lock (mutex);
-  scm_enter_guile (t);
-  return res;
+  if (scm_i_pthread_mutex_trylock (mutex) == 0)
+    return 0;
+  else
+    {
+      scm_t_guile_ticket t = scm_leave_guile ();
+      int res = scm_i_pthread_mutex_lock (mutex);
+      scm_enter_guile (t);
+      return res;
+    }
 }
 
 static void