[Mpi-forum] MPI-3 One-Sided Communications
rlgraham at ornl.gov
Fri Apr 24 08:28:51 CDT 2009
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On 4/24/09 9:20 AM, "Jeff Hammond" <jeff.science at gmail.com> wrote:
> On Thu, Apr 23, 2009 at 8:17 PM, Anthony Skjellum <tony at verarisoft.com> wrote:
>> Hi, I am not familiar with you project. Tell us more.
> Vinod should be the one to tell you about ARMCI in detail since he
> wrote a good portion of it. I just use it. There are also a number
> of papers available (see below).
>> In order to achieve lowest latency (or overhead, depending on your
>> optimization, or a blend thereof), a protocol designer might not want to pay
>> for fixed costs that once amortized over long transfers yields more long
>> transfer performance. Classic fixed vs variable cost situation. The classic
>> trades in two-sided are zero copy for long and two copy for short. If you
>> have evidence to the contrary, great... Lots of people reported in the past
>> the effect of a tradeoff as I described coming from the complex semantics of
>> the mpi one sided api.
> I think the trade-off comes from the implementation not the one-sided
> model itself. I agree achieving low-latency with a clunky
> implementation requires a compromise elsewhere.
>> One also has to ask if we can get even lower latency with less complex
>> protocol too imho.
> ARMCI is quite minimal and sits very close the hardware layer. GASNet
> is also very lightweight, perhaps as much as can be achieved without
> compromising portability.
> Einstein's "Make everything as simple as possible, but not simpler" is
> the right guiding principle for HPC, in contrast to the standard model
> of "make everything as general as possible."
>> Do you get both lower latency than two sided short msgs and higher bandwidth
>> than two sided?
> Someone else should have data on this. If it's out-dated, I'll do
> fresh timings on current hardware this summer.
>> What is the baseline of performance for two sided? Is it optimized? :-)
> The ARMCI performance page notes that the vendor MPI implementation
> was used in many of the cases and it can be assumed in others.
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