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Subject: [OMPI devel] Problem of memory lost in MPI_Type_create_hindexed() with count = 1 (patch proposed)
From: Pascal Deveze (Pascal.Deveze_at_[hidden])
Date: 2011-04-14 10:48:01


Calling MPI_Type_create_hindexed(int count, int array_of_blocklengths[],
            MPI_Aint array_of_displacements[], MPI_Datatype oldtype,
            MPI_Datatype *newtype)
with a count parameter of 1 causes a loss of memory detected by valgrind :

==2053== 576 (448 direct, 128 indirect) bytes in 1 blocks are definitely
lost in loss record 157 of 182
==2053== at 0x4C2415D: malloc (vg_replace_malloc.c:195)
==2053== by 0x4E7CEC7: opal_obj_new (opal_object.h:469)
==2053== by 0x4E7D134: ompi_datatype_create (ompi_datatype_create.c:71)
==2053== by 0x4E7D58E: ompi_datatype_create_hindexed
(ompi_datatype_create_indexed.c:89)
==2053== by 0x4EA74D0: PMPI_Type_create_hindexed
(ptype_create_hindexed.c:75)
==2053== by 0x401A5C: main (in /home_nfs/xxx/type_create_hindexed)

This can be reproduced with the following trivial code:
=====================================
#include "mpi.h"

MPI_Datatype newtype;
int lg[3];
MPI_Aint disp[3];

int main(int argc, char **argv) {
    MPI_Init(&argc,&argv);

    disp[0] = (MPI_Aint)disp;
    disp[1] = (MPI_Aint)disp+1;
    lg[0] = 5;
    lg[1] = 5;

    MPI_Type_create_hindexed(1, lg, disp, MPI_BYTE, &newtype);
    MPI_Type_free(&newtype);

    MPI_Finalize();
}
======================================
If MPI_Type_create_hindexed() is called with a count parameter greater
1, valgrind does not detect any lost record.

Patch proposed:

hg diff ompi/datatype/ompi_datatype_create_indexed.c
diff -r a2d94a70f474 ompi/datatype/ompi_datatype_create_indexed.c
--- a/ompi/datatype/ompi_datatype_create_indexed.c Wed Mar 30
18:47:31 2011 +0200
+++ b/ompi/datatype/ompi_datatype_create_indexed.c Thu Apr 14
16:16:08 2011 +0200
@@ -91,11 +91,6 @@
     dLength = pBlockLength[0];
     endat = disp + dLength * extent;
 
- if( 1 >= count ) {
- pdt = ompi_datatype_create( oldType->super.desc.used + 2 );
- /* multiply by count to make it zero if count is zero */
- ompi_datatype_add( pdt, oldType, count * dLength, disp, extent );
- } else {
         for( i = 1; i < count; i++ ) {
             if( endat == pDisp[i] ) {
                 /* contiguous with the previsious */
@@ -109,7 +104,6 @@
             }
         }
         ompi_datatype_add( pdt, oldType, dLength, disp, extent );
- }
     *newType = pdt;
     return OMPI_SUCCESS;
 }

Explanation:
    The case (0 == count) was resolved before by returning.
    The problem is that, in the case ( 1 >= count ),
ompi_datatype_create() is called again (it has been just called before).
    In fact the case (1 == count) is not different of the case (1 <
count), so it is possible to just avoid the if-else statement.

We need a patch for OpenMPI 1.5 branch.