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Subject: Re: [OMPI devel] shared-memory allocations
From: Paul H. Hargrove (PHHargrove_at_[hidden])
Date: 2008-12-13 17:46:01


To expand slightly on Patrick's last comment:

> Cache prefetching is slightly
> more efficient on local socket, so closer to reader may be a bit better.

Ideally one polls from cache, but in the event that the line is evicted the
next poll after the eviction will pay a lower cost if the memory is near to
the reader.

-Paul

Patrick Geoffray wrote:
> Richard Graham wrote:
>> Yes - it is polling volatile memory, so has to load from memory on
>> every read.
>
> Actually, it will poll in cache, and only load from memory when the
> cache coherency protocol invalidates the cache line. Volatile semantic
> only prevents compiler optimizations.
>
> It does not matter much where the pages are (closer to reader or
> receiver) on NUMAs, as long as they are equally distributed among all
> sockets (ie the choice is consistent). Cache prefetching is slightly
> more efficient on local socket, so closer to reader may be a bit better.
>
> Patrick
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-- 
Paul H. Hargrove                          PHHargrove_at_[hidden]
Future Technologies Group
HPC Research Department                   Tel: +1-510-495-2352
Lawrence Berkeley National Laboratory     Fax: +1-510-486-6900