[Mpi-forum] additional keys for MPI_COMM_SPLIT_TYPE
Schulz, Martin
schulzm at llnl.gov
Sun Mar 24 15:00:19 CDT 2013
Hi Jeff, all,
On Mar 24, 2013, at 12:36 PM, Jeff Hammond <jhammond at alcf.anl.gov> wrote:
> Martin encouraged me to socialize this with the Forum. The idea here
> seems broader than just one working group so I'm sending to the entire
> Forum for feedback.
I still think it would be good to have one WG take responsibility for this discussion. It seems to fit best into the collectives WG (and/or the communicator/groups chapter). Torsten: as the lead for the collectives group, does this sound reasonable to you?
Thanks,
Martin
>
> See https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/372. The text
> is included below in case it makes it easier to read on your
> phone...because I know this is that urgent :-)
>
> Jeff
>
>
> ---------- Forwarded message ----------
> From: MPI Forum <mpi-forum at lists.mpi-forum.org>
> Date: Sun, Mar 24, 2013 at 1:50 PM
> Subject: [MPI Forum] #372: additional keys for MPI_COMM_SPLIT_TYPE
> To:
>
>
> #372: additional keys for MPI_COMM_SPLIT_TYPE
> -------------------------------------+-------------------------------------
> Reporter: | Owner:
> jhammond | Status: new
> Type: | Milestone:
> Enhancements to standard | 2013/03/11 Chicago, USA
> Priority: | Keywords:
> Forum feedback requested | Author: Bill Gropp: 0
> Version: MPI | Author: Adam Moody: 0
> <next> | Author: Dick Treumann: 0
> Implementation status: | Author: George Bosilca: 0
> Waiting | Author: Bronis de Supinski: 0
> Author: Rich Graham: 0 | Author: Jeff Squyres: 0
> Author: Torsten Hoefler: 0 | Author: Rolf Rabenseifner: 0
> Author: Jesper Larsson Traeff: 0 |
> Author: David Solt: 0 |
> Author: Rajeev Thakur: 0 |
> Author: Alexander Supalov: 0 |
> -------------------------------------+-------------------------------------
> {{{MPI_COMM_SPLIT_TYPE}}} currently supports only on key,
> {{{MPI_COMM_TYPE_SHARED}}}, which refers to shared memory domains, as
> supported by shared memory windows ({{{MPI_WIN_ALLOCATE_SHARED}}}).
>
> This ticket proposes additional keys that appear useful enough to justify
> standardization.
>
> The first key address the need for users to have a portable way of
> querying properties of the filesystem. This key requires the user to
> specify the specific file path of interest using an MPI_Info object. The
> communicator returned represents the set of processes that can write to a
> single instance of that file path. For a local disk, it is likely (but
> not necessary) that this communicator be the same as returned by
> {{{MPI_COMM_TYPE_SHARED}}}. On the other hand, globally shared
> filesystems will return a duplicate of {{{MPI_COMM_WORLD}}}. When the
> implementation cannot determine the answer, the resulting communicator is
> {{{MPI_COMM_NULL}}} and users cannot assume any information about the
> path.
>
> To be perfectly honest, the choice of {{{key = MPI_COMM_TYPE_SHARED}}} to
> be only used for shared-memory is unfortunately, because we could have
> instead used this key for anything that could be shared and let the
> differences be enumerated by {{{MPI_Info}}}. For exampled, shared memory
> windows could use {{{(key,value)=("memory","shared")}}} (granted, this is
> a somewhat silly set of options, but it would naturally permit one to use
> e.g. {{{(key,value)=("filepath","/home")}}} and
> {{{(key,value)=("device","gpu0")}}}. We could implement the new set of
> options using the existing key if we standardize {{{MPI_INFO_NULL}}} to
> mean "shared memory" but that isn't particularly appealing.
>
> In addition to {{{MPI_COMM_TYPE_FILEPATH}}}, which is illustrated below, I
> think that {{{MPI_COMM_TYPE_DEVICE}}} is useful to standardize as a key
> without specifying the possible options that the {{{MPI_Info}}} can take,
> with the exception of noting that, if this key is used with
> {{{MPI_INFO_NULL}}}, the result is always {{{MPI_COMM_NULL}}}. Devices
> that implementations could define include coprocessors (aka accelerators
> or GPUs), NICs (e.g. in the case of multi-rail IB systems) and any number
> of other machine-specific cases. The purpose of standardizing the key is
> to encourage implementations to take the most portable route when trying
> to implement this type of feature, thus confining all of the non-portable
> aspects to the definition of the {{{(key,value)}}} pair in the info
> object.
>
> I have also considered {{{MPI_COMM_TYPE_LOCALE}}}, which would also allow
> the implementation to define info keys to specific, e.g. subcomms sharing
> an IO node on Cray or Blue Gene, but this could just as easily be
> implemented using the {{{MPI_COMM_TYPE_DEVICE}}} key. Furthermore, since
> devices can be specified as a filepath (they often have an associated with
> a {{{/dev/something}}} on Linux systems), there is no compelling reason to
> add more than one key. This is, of course, the reason why overloading
> {{{MPI_COMM_TYPE_SHARED}}} via info objects seems like the most
> straightforward choice except in regards to backwards compatibility.
>
> What do people think? Can we augment {{{MPI_COMM_TYPE_SHARED}}} with info
> keys and leave the case of {{{MPI_INFO_NULL}}} to mean shared memory, do
> we break backwards compatibility or do we add a new key that has the
> desired catch-all properties?
>
> Here is an example program illustrating the use of the proposed
> functionality for {{{MPI_COMM_TYPE_FILEPATH}}}:
> {{{
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> #include <mpi.h>
>
> int main( int argc, char *argv[] )
> {
> int rank;
> MPI_Info i1, i2, i3, i4, i5;
> MPI_Comm c0, c1, c2, c3, c4, c5;
> int result, happy=0;
>
> MPI_Init(&argc,&argv);
> MPI_Comm_rank(MPI_COMM_WORLD, &rank);
>
> MPI_Info_create( &i1 );
> MPI_Info_create( &i2 );
> MPI_Info_create( &i3 );
> MPI_Info_create( &i4 );
> MPI_Info_create( &i5 );
>
> MPI_Info_set( i1, (char*)"path", (char*)"/global-shared-fs" );
> MPI_Info_set( i2, (char*)"path", (char*)"/proc-local-fs" );
> MPI_Info_set( i3, (char*)"path", (char*)"/node-local-fs" );
> MPI_Info_set( i4, (char*)"path", (char*)"/proc-zero-fs" );
> MPI_Info_set( i5, (char*)"path", (char*)"/dev/rand" );
>
> MPI_Comm_split_type( MPI_COMM_WORLD, MPI_COMM_TYPE_SHARED, 0,
> MPI_INFO_NULL, &c0 );
>
> MPI_Comm_split_type( MPI_COMM_WORLD, MPI_COMM_TYPE_FILEPATH, 0, i1,
> &c1 );
> MPI_Comm_split_type( MPI_COMM_WORLD, MPI_COMM_TYPE_FILEPATH, 0, i2,
> &c2 );
> MPI_Comm_split_type( MPI_COMM_WORLD, MPI_COMM_TYPE_FILEPATH, 0, i3,
> &c3 );
> MPI_Comm_split_type( MPI_COMM_WORLD, MPI_COMM_TYPE_FILEPATH, 0, i4,
> &c4 );
> MPI_Comm_split_type( MPI_COMM_WORLD, MPI_COMM_TYPE_FILEPATH, 0, i5,
> &c5 );
>
> /* a globally visible shared filesystem should result in a comm that
> is equivalent to world */
> MPI_Comm_compare( MPI_COMM_WORLD, c1, &result );
> if ( result == MPI_CONGRUENT) happy++;
>
> /* a process-local filesystem should result in MPI_COMM_SELF */
> MPI_Comm_compare( MPI_COMM_SELF, c2, &result );
> if ( result == MPI_CONGRUENT) happy++;
>
> /* a filesystem shared within the node is likely to result in a
> communicator equivalent
> to the one that supports shared memory, provided shared memory is
> available */
> MPI_Comm_compare( c0, c3, &result );
> if ( result == MPI_CONGRUENT) happy++;
>
> /* the /proc-zero-fs is only visible from rank 0 of world... */
> if (rank==0) {
> MPI_Comm_compare( MPI_COMM_SELF, c4, &result );
> if ( result == MPI_CONGRUENT) happy++;
> } else {
> if ( c4 == MPI_COMM_NULL) happy++;
> }
>
> /* the sharable nature of /dev/rand is probably a meaningless concept
> so
> we expect the implementation to return MPI_COMM_NULL for c5 */
> if ( c5 == MPI_COMM_NULL) happy++;
>
> MPI_Comm_free( &c1 );
> MPI_Comm_free( &c2 );
> MPI_Comm_free( &c3 );
> MPI_Comm_free( &c4 );
> MPI_Comm_free( &c5 );
>
> MPI_Info_free( &i1 );
> MPI_Info_free( &i2 );
> MPI_Info_free( &i3 );
> MPI_Info_free( &i4 );
> MPI_Info_free( &i5 );
>
> MPI_Finalize( );
> return 0;
> }
> }}}
>
> --
> Ticket URL: <https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/372>
> MPI Forum <https://svn.mpi-forum.org/>
> MPI Forum
>
>
> --
> Jeff Hammond
> Argonne Leadership Computing Facility
> University of Chicago Computation Institute
> jhammond at alcf.anl.gov / (630) 252-5381
> http://www.linkedin.com/in/jeffhammond
> https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond
> _______________________________________________
> mpi-forum mailing list
> mpi-forum at lists.mpi-forum.org
> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi-forum
________________________________________________________________________
Martin Schulz, schulzm at llnl.gov, http://people.llnl.gov/schulzm
CASC @ Lawrence Livermore National Laboratory, Livermore, USA
More information about the mpi-forum
mailing list