[Beowulf] Performance characterising a HPC application

Richard Walsh rbw at ahpcrc.org
Thu Mar 29 06:27:40 PDT 2007

Hey Patrick,

Patrick Geoffray wrote:
> Message aggregation would be much more beneficial in the context of
> UPC, where the compiler will likely generates many small grain
> communications to the same remote process. However, as Greg pointed
> out, MPI applications often already aggregate messages themselves
> (this is something even "challenged" application people understand
> easily). 
Right ...
> I would bet that UPC could more efficiently leverage a strided or
> vector communication primitive instead of message aggregation. I don't
> know if GasNet provides one, I know ARMCI does. 
Having the alternative of compiling to a pseudo-vector pipelined
GASnet/ARMCI primitive (hiding latency in the pipeline) >or< an
aggregation primitive (amortizing latency over a large data block) would
seem to be a good thing depending on the number and distribution of
remote memory references in your given kernel.  The latency bubbles in
interconnect- mediated remote memory references are of course much
larger than in a hardware mediated global addresses space remote
reference.  This might change the effectiveness of pipelined based
latency hiding.   From what I know the Berkeley UPC
compiler team is focused on optimization through aggregation rather than
pipelining.  Regardless, at this point our more GUPS-like direct remote
memory reference patterns in our UPC codes, which perform well on the
Cray X1E, must be manually aggregated to achieve performance on a cluster.




Richard B. Walsh

Project Manager
Network Computing Services, Inc.
Army High Performance Computing Research Center (AHPCRC)
rbw at ahpcrc.org  |  612.337.3467

>  "The world is given to me only once, not one existing and one
>   perceived. The subject and object are but one."
>   Erwin Schroedinger

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