[mpich-discuss] Memory alignment with MPI_Alloc_mem

[Jeff Hammond]

If you are going to suballocate from a slab yourself, you can handle
alignment yourself easy enough, no?  Do I not understand what you mean
here.  And what sort of alignment do you want?  Are you trying to
align to 32/64 bytes because of AVX or some other x86 feature on Cray
XC30 or do you want page alignment?

But what do you really want to achieve?  While it is usually
beneficial to use pre-registered buffers on RDMA networks, good MPI
implementations have a page-registration cache.  If, as you say, you
are suballocating from a slab, Cray MPI should have the backing pages
in the registration cache after you use them as MPI buffers.

You can maximize the efficiency of the page registration cache by
using large pages.  Search for intro_hugepages using 'man' or on the
Internet to learn the specifics of this.  I suspect that using large
pages will induce much of the benefit you hoped to achieve with an
explicitly-registering MPI_Alloc_mem.

If you really want to max out RDMA on Cray networks, you need to use
DMAPP.  I have some simple examples and pointers to docs here:
https://github.com/jeffhammond/HPCInfo/tree/master/dmapp.  I have more
examples other places that I'll migrate to that location if requested.

If you're interested in portability, MPI-3 RMA is a good abstraction
for RDMA networks.  Some implementations do a better job than others
at exposing this relationship.  Cray MPI has a DMAPP back-end for RMA
now, although it is not active by default.  You could also try Torsten
Hoefler's foMPI
[http://spcl.inf.ethz.ch/Research/Parallel_Programming/foMPI/].

Best,

Jeff

On Sat, Feb 14, 2015 at 2:37 PM, Marcin Zalewski
<marcin.zalewski at gmail.com> wrote:
> I am using Cray MPT, and I would like to allocate a large region of
> memory from which, in turn, I will allocate buffers to be used with
> MPI. I am wondering if there is any benefit from allocating that heap
> with MPI_Alloc_mem. I would hope that it could be pre-registered for
> RDMA, speeding things up. However, I need this memory to have a
> specific alignment. Is there a general way in MPICH or maybe a
> specific way for MPT to request alignment with MPI_Alloc_mem?
>
> Thanks,
> Marcin
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-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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