[mpich-discuss] MPI scatter algorithms for BlueGene/P

[Khalid Hasanov]

Jeff, thanks a lot for your detailed answer. It makes much more sense now.

Best regards,
Khalid

On Tue, Mar 3, 2015 at 4:11 AM, Jeff Hammond <jeff.science at gmail.com> wrote:

> Those operations were never specially optimized on Blue Gene.  The
> operations that were optimized heavily on Blue Gene were/are Barrier,
> Bcast, Allreduce, and Alltoall(v).  Most of the other collective
> optimizations were derivatives of that.  For example, one can
> implement Allgather as a series of Bcasts.  And Reduce was optimized
> as a side effect of Allreduce; in some cases, Allreduce was faster
> than Reduce, which is counter-intuitive.
>
> One optimization that Blue Gene had for Gather was to use Reduce with
> BOR (on BG/P) and SUM (on BG/Q, for float types at least).  This
> turned Gather of count=1 into Reduce of count=nproc (size of
> communicator), but it was very faster for short messages.
>
> There aren't many good optimizations for Scatter.  MPICH has most of
> the generic ones, as you might expect.  It is possible that Scatter as
> Alltoallv with only one non-zero in the count vector was faster than
> MPICH, but this would surprise me.
>
> In any case, all MPI collectives on BG benefit from very good
> network-to-processor balance, minimal rendezvous (connectionless HW
> and SW), trivial virtual-to-physical translation, good bisection
> bandwidth of _electrically isolated_ torus networks, etc.
>
> If you have a more specific question, I might have a better answer.
>
> Best,
>
> Jeff
>
> On Sun, Mar 1, 2015 at 4:36 PM, Khalid Hasanov <xalid.h at gmail.com> wrote:
> > Hello,
> >
> > First of all, I am not sure if this group is the right place for this
> > question. If not I apologize for asking unrelated question.
> >
> > I read two papers about optimizing MPI collective communications on BG/L
> and
> > BG/P.
> > (Optimization of MPI Collective Communication on BlueGene/L Systems
> > and MPI Collective Communications on The Blue Gene/P Supercomputer:
> > Algorithms and Optimizations respectively). However, these two papers do
> not
> > mention anything about MPI scatter and gather operations, I wonder if
> these
> > two collective operations have been optimized for BlueGene or they use
> > exactly the same algorithms from MPICH. Any reference appreciated.
> >
> >
> > Best regards,
> > Khalid
> >
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>
>
> --
> Jeff Hammond
> jeff.science at gmail.com
> http://jeffhammond.github.io/
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