[mpich-discuss] MPI_Allgatherv complexity

Pavan Balaji balaji at mcs.anl.gov
Mon Nov 2 18:15:52 CST 2009

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

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