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From: Greg Lindahl (greg.lindahl_at_[hidden])
Date: 2006-11-08 13:51:29


On Wed, Nov 08, 2006 at 12:25:20PM +0000, Miguel Figueiredo Mascarenhas Sousa Filipe wrote:

> > Unless sharing data repeatedly leads you to false sharing and a loss
> > in performance.
>
> what does that mean.. I did not understand that.

Google indexes a bunch of good webpages on "false sharing".

> > For many programs with both MPI and shared memory implementations, the
> > MPI version runs faster on SMPs and numa-CCs. Why? See the previous
> > paragraph...
>
> Of course it does..its faster to copy data in main memory than it is
> to do it thought any kind of network interface.

MPI implementations typically copy data in main memory when it is
talking on the same node. If it's faster to use the network interface,
and sometimes it is due to DMA engines, then your friendly MPI
implementer is likely to be taking advantage of it.

> But, for instance.. try to benchmark real applications with a MPI and
> posix threads implementations in the same numa-cc or big SMP machine..
> my bet is that posix threads implementation is going to be faster..

And your bet is often wrong.

-- greg