[mpich-discuss] MPI_Allgatherv complexity

Justin Luitjens luitjens at gmail.com
Mon Nov 2 18:17:37 CST 2009

There is likely a decent amount of variance in the distribution.

On Mon, Nov 2, 2009 at 5:15 PM, Pavan Balaji <balaji at mcs.anl.gov> wrote:

> On 11/02/2009 06:11 PM, Justin Luitjens wrote:
> > Here is a graph of the time for an allgather as a function of message
> > size.  We would like to scale up to 98K processors but as you can see
> > the time for all gather dramatically increases at 49K processors.  At
> > 98K the time increases by around a factor of 4 for the few datapoints
> > that I have tested (they are not in the graph).
> What's the message size pattern like? Is everyone using about the same
> size messages, or is there a lot of skew? For the second case, we added
> a new algorithm to fix this performance issue, though I'm not sure Cray
> picked it up yet.
>  -- Pavan
> --
> Pavan Balaji
> http://www.mcs.anl.gov/~balaji <http://www.mcs.anl.gov/%7Ebalaji>
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