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Subject: Re: [OMPI users] MPI_Reduce performance
From: Eugene Loh (eugene.loh_at_[hidden])
Date: 2010-09-09 12:38:21


Gus Correa wrote:

> More often than not some components lag behind (regardless of how
> much you tune the number of processors assigned to each component),
> slowing down the whole scheme.
> The coupler must sit and wait for that late component,
> the other components must sit and wait for the coupler,
> and the (vicious) "positive feedback" cycle that
> Ashley mentioned goes on and on.

I think "sit and wait" is the "typical" scenario that Dick mentions.
Someone lags, so someone else has to wait.

In contrast, the "feedback" cycle Ashley mentions is where someone lags
and someone else keeps racing ahead, pumping even more data at the
laggard, forcing the laggard ever further behind.