RE: Synchronization article on developerWorks

[Jeff Sturm <jsturm at one-point dot com>]

On Tue, 24 Jul 2001, Boehm, Hans wrote:
> I just tried Tom's program on my Pentium II/300 running RH6.2.  I see a
> bigger difference between 3.0 and 3.1.  I have some local changes in my
> tree, but I don't think they effect the empty or staticEmpty test.

I noticed the tests vary considerably between runs.  Also, the asserts in
natObject.cc are not disabled; that helps a bit.

Dynamic linking incurs some cost that a JIT can probably eliminate.

> 4) Contended synchronization might be handled better.  If so linuxthreads
> should really do the same.

Last I checked, linuxthreads didn't spin on a mutex... after one
test_and_set failure it falls into a nanosleep.  That may have changed in
newer releases.

I'm curious if the path for reentrant monitors couldn't be shortened
somewhat.  In _Jv_MonitorExit we have

  if (__builtin_expect(light_thr_id == self, true))
    {
      count = he -> light_count;
      if (__builtin_expect((address & ~HEAVY) == addr, true))
        {
          if (count != 0)
            {
              // We held the lightweight lock all along.  Thus the values
              // we saw for light_thr_id and light_count must have been
              // valid.
              he -> light_count = count - 1;
              return;

No compare_and_swap here.  I guess if a thread evaluates "light_thr_id ==
self" true it knows it holds the lock.  Why can't _Jv_MonitorEnter do the
same and potentially avoid the CAS?  Am I missing something?

Index: natObject.cc
===================================================================
RCS file: /cvs/gcc/gcc/libjava/java/lang/natObject.cc,v
retrieving revision 1.17
diff -u -r1.17 natObject.cc
--- natObject.cc        2001/07/23 03:51:17     1.17
+++ natObject.cc        2001/07/25 04:46:49
@@ -768,8 +769,8 @@

   assert(!(addr & FLAGS));
 retry:
-  if (__builtin_expect(compare_and_swap(&(he -> address),
-                                       0, addr),true))
+  if (__builtin_expect (he->light_thr_id != self
+      && compare_and_swap(&(he -> address), 0, addr), true))
     {
       assert(he -> light_thr_id == INVALID_THREAD_ID);
       assert(he -> light_count == 0);