Re: [OMPI devel] MPI_Request_get_status and opal_progress [PATCH]

[Shaun Jackman]

Hi Jeff,

Attached is an updated patch, which moves the loop label before 
opal_atomic_mb as is done in req_test.c.

Cheers,
Shaun

Jeff Squyres wrote:
On Sep 14, 2009, at 5:12 PM, Shaun Jackman wrote:

Two questions. Should the loop label recheck_request_status come
before or after the call to opal_atomic_mb?


I believe that a 2nd mb is necessary.  But I think it could be put  
into the "if" block at the end, and leave the label where it is.  But  
see below.

Is it necessary to check
request->req_state a second time, or is it only necessary to check
request->req_complete the second time?


In THREAD_MULTIPLE, I guess the state could change.  But in non- 
THREAD_MULTIPLE, it's superfluous to check a 2nd time because a  
request can't have changed into INACTIVE simply due to a call to  
opal_progress().  I notice that req_test.c checks the whole enchilada  
again (including the mb and state change).  Since we're currently not  
distinguishing between THREAD_MULTIPLE and not in these functions, I  
think we need to be conservative and check the state again.

I think this means that our TEST/WAIT functions are not well-tuned for  
threading (or, at least, there are optimizations that could be made  
based on the thread level).

George / Brian -- got opinions here?

2009-09-14  Shaun Jackman  <sjack...@gmail.com>

        * ompi/mpi/c/request_get_status.c (MPI_Request_get_status):
        If opal_progress is called then check the status of the request
        before returning. opal_progress is called only once. This logic
        parallels MPI_Test (ompi_request_default_test).

--- ompi/mpi/c/request_get_status.c.orig        2008-11-04 12:56:27.000000000 
-0800
+++ ompi/mpi/c/request_get_status.c     2009-09-24 15:30:09.995850000 -0700
@@ -41,6 +41,10 @@
 int MPI_Request_get_status(MPI_Request request, int *flag,
                            MPI_Status *status) 
 {
+#if OMPI_ENABLE_PROGRESS_THREADS == 0
+    int do_it_once = 0;
+#endif
+
     MEMCHECKER(
         memchecker_request(&request);
     );
@@ -57,6 +61,9 @@
         }
     }
 
+#if OMPI_ENABLE_PROGRESS_THREADS == 0
+ recheck_request_status:
+#endif
     opal_atomic_mb();
     if( (request == MPI_REQUEST_NULL) || (request->req_state == 
OMPI_REQUEST_INACTIVE) ) {
         *flag = true;
@@ -78,9 +85,17 @@
         }
         return MPI_SUCCESS;
     }
-    *flag = false;
 #if OMPI_ENABLE_PROGRESS_THREADS == 0
-    opal_progress();
+    if( 0 == do_it_once ) {
+        /**
+         * If we run the opal_progress then check the status of the request 
before
+         * leaving. We will call the opal_progress only once per call.
+         */
+        opal_progress();
+        do_it_once++;
+        goto recheck_request_status;
+    }
 #endif
+    *flag = false;
     return MPI_SUCCESS;
 }