[mpich-commits] [mpich] MPICH primary repository branch, master, updated. v3.1.3-1-g7a36603

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http://git.mpich.org/mpich.git/commitdiff/7a36603163cd917fe89a39cad668e5a7b6f917cb

commit 7a36603163cd917fe89a39cad668e5a7b6f917cb
Author: Joe Ratterman <jratt at us.ibm.com>
Date:   Mon May 2 13:00:29 2011 -0500

    When decrementing a ref-count to 0, do not bother with atomics.
    
      If the ref-count is 1, we *should* be the only thread holding a
      reference.  It is therefore legal to skip atomics.  If that is
      insufficient justificaion, consider that there are only three things
      a concurrent thread should be doing:
    
        1) Reading: Not a problem, since the store is atomic.
        2) Decrementing: Illegal; it would go negative using pure atomics.
        3) Incrementing: Illegal; we are about to destroy the object.
    
    Add #ifdef guard for use of 'aggressive counter optimizations'
    
    (ibm) 20a558c3802fb3e138eb6b3c786a434538efffdd
    
    Signed-off-by: Joe Ratterman <jratt at us.ibm.com>
    Signed-off-by: Michael Blocksome <blocksom at us.ibm.com>
    
    Updated by Pavan Balaji to use an OPA atomic load instead of a simple
    load operation.
    
    Fixes #1835
    
    Signed-off-by: Pavan Balaji <balaji at anl.gov>
    Signed-off-by: William Gropp <wgropp at illinois.edu>

diff --git a/src/include/mpihandlemem.h b/src/include/mpihandlemem.h
index 1847031..1ab6752 100644
--- a/src/include/mpihandlemem.h
+++ b/src/include/mpihandlemem.h
@@ -300,10 +300,25 @@ typedef OPA_int_t MPIU_Handle_ref_count;
         MPIU_HANDLE_LOG_REFCOUNT_CHANGE(objptr_, "incr"); \
         MPIU_HANDLE_CHECK_REFCOUNT(objptr_,"incr");       \
     } while (0)
+
+/* Special optimization: when the reference count touches one, we are
+ * about to free the object.  In this case, it is illegal for another
+ * thread to either increment the reference (i.e., create a new object
+ * that depends on this object) or decrement the references (i.e.,
+ * free the object simultaneously).  We, however, have to ensure that
+ * our load is atomic.  OPA does this for us, while optimizing it to a
+ * simple load for cases where the architecture already provides us
+ * with load atomicity and no need for load memory barriers. */
 #define MPIU_Object_release_ref_always(objptr_,inuse_ptr)               \
     do {                                                                \
-        int got_zero_ = OPA_decr_and_test_int(&((objptr_)->ref_count)); \
-        *(inuse_ptr) = got_zero_ ? 0 : 1;                               \
+        if (likely(OPA_load_int((objptr_)->ref_count.v) == 1)) {        \
+            (objptr_)->ref_count.v = 0;                                 \
+            *(inuse_ptr) = 0;                                           \
+        }                                                               \
+        else {                                                          \
+            int got_zero_ = OPA_decr_and_test_int(&((objptr_)->ref_count)); \
+            *(inuse_ptr) = got_zero_ ? 0 : 1;                           \
+        }                                                               \
         MPIU_HANDLE_LOG_REFCOUNT_CHANGE(objptr_, "decr");               \
         MPIU_HANDLE_CHECK_REFCOUNT(objptr_,"decr");                     \
     } while (0)

-----------------------------------------------------------------------

Summary of changes:
 src/include/mpihandlemem.h |   19 +++++++++++++++++--
 1 files changed, 17 insertions(+), 2 deletions(-)


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