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 <[email protected]> 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 _______________________________________________ discuss mailing list [email protected] To manage subscription options or unsubscribe: https://lists.mpich.org/mailman/listinfo/discuss

-- Jeff Hammond [email protected] http://jeffhammond.github.io/ _______________________________________________ discuss mailing list [email protected] To manage subscription options or unsubscribe: https://lists.mpich.org/mailman/listinfo/discuss