Neighborhood Collectives

[dalcinl]

@jsquyres I've found serious issues in the implementation of neighborhood collectives. These issues showed up after a memory allocation optimization I've implemented in mpi4py in commit c2eaa292, which failed badly with Open MPI 2.0.1 in this Bitbucket Pipelines build.

After checking the source code in ompi/v2.x, I found a major issue for the case of intracommunicators with a topology. The code simply do not check the topology to determine the number or sources and destinations, and use ompi_comm_size() instead. This leads to wrong checks and reads after end of arrays.

In mpi4py, I'm using the following code to determine the number of sources and destinations. This piece of code only handles intracommunicators, but could be extended to intercommunicators.

BTW, the wording of the MPI standard seems to require a topology for neighborhood collectives, although supporting intracomms and intercomms seems possible and the result should be equivalent to regular allgather and alltoall. Or maybe the right thing to do is to just error in case of intercomms or intracomms with no topology.

Am I missing something?

PS: MPICH seems to implement neighbor collectives just for Cartesian/graph topologies, the helper routine MPIR_Topo_canon_nhb_count is quite similar to what I'm using in mpi4py (though the MPICH one fails in case of no topology). @roblatham00 Any comments to you competitors :-) about this?

$[dalcinl@localhost mpich.git]$ git grep MPIR_Topo_canon_nhb_count
src/include/mpir_topo.h:int MPIR_Topo_canon_nhb_count(MPIR_Comm *comm_ptr, int *indegree, int *outdegree, int *weighted);
src/mpi/topo/inhb_allgather.c:    mpi_errno = MPIR_Topo_canon_nhb_count(comm_ptr, &indegree, &outdegree, &weighted);
src/mpi/topo/inhb_allgatherv.c:    mpi_errno = MPIR_Topo_canon_nhb_count(comm_ptr, &indegree, &outdegree, &weighted);
src/mpi/topo/inhb_alltoall.c:    mpi_errno = MPIR_Topo_canon_nhb_count(comm_ptr, &indegree, &outdegree, &weighted);
src/mpi/topo/inhb_alltoallv.c:    mpi_errno = MPIR_Topo_canon_nhb_count(comm_ptr, &indegree, &outdegree, &weighted);
src/mpi/topo/inhb_alltoallw.c:    mpi_errno = MPIR_Topo_canon_nhb_count(comm_ptr, &indegree, &outdegree, &weighted);
src/mpi/topo/topoutil.c:#define FUNCNAME MPIR_Topo_canon_nhb_count
src/mpi/topo/topoutil.c:int MPIR_Topo_canon_nhb_count(MPIR_Comm *comm_ptr, int *indegree, int *outdegree, int *weighted)

[dalcinl]

Oh, sorry. The problem is just in MPI_Neighbor_allgatherv() and MPI_Ineighbor_allgatherv(). You guys should implement it using ompi_comm_neighbors_count(), like you do in MPI_Neighbor_alltoallv().

[ggouaillardet]

@dalcinl can you please point to the test case ?
/* a C version is ideal, but i guess i will be able to figure it out with a python version */

i briefly check the source code, and https://github.com/open-mpi/ompi/blob/v2.x/ompi/mca/coll/libnbc/nbc_ineighbor_allgatherv.c nor https://github.com/open-mpi/ompi/blob/v2.x/ompi/mca/coll/basic/coll_basic_neighbor_alltoallv.c use　ompi_comm_size()
do you have some other piece(s) of code in mind ?

i do not think Neighbor collective work on intercomm.
in Open MPI, MPI_Cart_create MPI_Graph_create MPI_Dist_graph_create nor MPI_Dist_graph_create_adjacent can accept an inter communicator, so i do not think you can
create an intercomm with a topology.

[hjelmn]

Are are attempting a neighborhood collective on a communicator without a topology?

[ggouaillardet]

@hjelmn this erroneous case is tested when checking params

if (! OMPI_COMM_IS_TOPO(comm)) {
OMPI_ERRHANDLER_INVOKE(MPI_COMM_WORLD, MPI_ERR_TOPOLOGY, FUNC_NAME);

[hjelmn]

@ggouaillardet That is only enabled if the mpi parameter check is turned on. That is not on by default.

[hjelmn]

Hate that close and comment button.

[jsquyres]

@hjelmn I don't think you're right -- I believe the MPI param check is set to "runtime" by default, and is enabled at runtime.

[dalcinl]

A reproducer is a little hard to write. Moreover, I'm still not sure why my Python tests are failing, maybe there are additional issues. Graph topologies seem to work, but I get errors with Cartesian topologies. I think there is a bug in Open MPI in the case of Cartesian topologies, but I have to investigate further and open a new issue.

Meanwhile, please see the following, you still have to fix neighbor allgatherv for reads past the end of the recvcount array.

See the C reproducer at the end. The code is a bit contrived, but it is attempting to send and receive zero length messages. I get the following running under valgrind control. Indeed, valgrind complains in the loop checking for the recvcounts array.

[dalcinl@kw14821 openmpi]$ mpicc ngh_allgatherv.c 
[dalcinl@kw14821 openmpi]$ mpiexec -n 2 valgrind -q ./a.out 
==17973== Invalid read of size 4
==17973==    at 0x4EC9592: PMPI_Neighbor_allgatherv (pneighbor_allgatherv.c:125)
==17973==    by 0x400B74: main (in /home/dalcinl/Devel/BUGS-MPI/openmpi/a.out)
==17973==  Address 0x131ad104 is 0 bytes after a block of size 4 alloc'd
==17973==    at 0x4C2BBAD: malloc (vg_replace_malloc.c:299)
==17973==    by 0x400B1B: main (in /home/dalcinl/Devel/BUGS-MPI/openmpi/a.out)
==17973== 
==17974== Invalid read of size 4
==17974==    at 0x4EC9592: PMPI_Neighbor_allgatherv (pneighbor_allgatherv.c:125)
==17974==    by 0x400B74: main (in /home/dalcinl/Devel/BUGS-MPI/openmpi/a.out)
==17974==  Address 0x6a6e7a4 is 0 bytes after a block of size 4 alloc'd
==17974==    at 0x4C2BBAD: malloc (vg_replace_malloc.c:299)
==17974==    by 0x400B1B: main (in /home/dalcinl/Devel/BUGS-MPI/openmpi/a.out)
==17974== 

#include <mpi.h>
#include <stdlib.h>
#include <assert.h>

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

  int size,rank;
  MPI_Comm_size(MPI_COMM_WORLD, &size);
  MPI_Comm_rank(MPI_COMM_WORLD, &rank);

  int left  = rank!=0      ? rank-1 : size-1;
  int right = rank!=size-1 ? rank+1 : 0;

  MPI_Comm comm;
  int indegree  = 1, sources[]      = { left  };
  int outdegree = 1, destinations[] = { right };
  MPI_Dist_graph_create_adjacent(MPI_COMM_WORLD,
                                 indegree,  sources,      MPI_UNWEIGHTED,
                                 outdegree, destinations, MPI_UNWEIGHTED,
                                 MPI_INFO_NULL, 0, &comm); 

  int sendbuf[2] = { rank, -1 };
  int sendcount  = 0;
  int recvbuf[2] = { -1, -1};
  int *recvcounts = malloc(1*sizeof(int)); recvcounts[0] = 0;
  int *recvdispls = malloc(1*sizeof(int)); recvdispls[0] = 0;
  MPI_Neighbor_allgatherv(sendbuf, sendcount,              MPI_INT,
                          recvbuf, recvcounts, recvdispls, MPI_INT,
                          comm);
  assert(recvbuf[0] == -1);
  assert(recvbuf[1] == -1);

  MPI_Comm_free(&comm);
  MPI_Finalize();
  return 0;
}

[hjelmn]

Ah, ok. This is a bug up in the binding itself. Looks like a C&P error.

[hjelmn]

Shows how well-used these functions are :D.