[petsc-users] performance surprise

[Jed Brown]

On Sat, Jan 21, 2012 at 10:50, Dominik Szczerba <dominik at itis.ethz.ch>wrote:

> Jed, Barry: thanks for the responses. I was aware of the related FAQ
> entry but somehow thought only standard desktop SMPs were concerned. I
> never suspected interconnect communication to be better than internal
> memory bus...
>

You are conflating memory bandwidth with network bandwidth. You need memory
bandwidth if you have a working set larger than cache, but each socket only
has enough memory bandwidth for one or two cores, so adding more cores
doesn't speed up your program. Network performance is a separate issue.
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