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Subject: Re: [OMPI devel] System V Shared Memory for Open MPI: Request forCommunity Input and Testing
From: Paul H. Hargrove (PHHargrove_at_[hidden])
Date: 2010-06-11 05:36:16


Chris,

  I think that "reclaiming pages from the page cache" is the PROBLEM,
not the solution. If I understand you correctly a lower value of
"swappiness" means that the ANONYMOUS pages of an application's stack
and heap are less likely to be subject to swap I/O. However, the
concern here is for the pages of an mmap()ed file (though an unlinked
one). So, my expectation is that the page cache is their "owner" rather
than the application. If that is an incorrect understanding, I would
appreciate being corrected.

-Paul

Christopher Samuel wrote:
>
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> On 10/06/10 18:43, Paul H. Hargrove wrote:
>
> > When the file is on a real (not tmpfs or other ramdisk) I am 95% certain
> > that this is an artifact of the Linux swapper/pager behavior which is
> > thinking it is being smart by "swapping ahead". Even when there is no
> > memory pressure that requires swapping, Linux starts queuing swap I/O
> > for pages to keep the number of "clean" pages up when possible.
>
> I believe you can tweak that behaviour through the VM subsystem
> using /proc/sys/vm/swappiness, it defaults to 60 but lower values
> are meant to make the kernel less likely to swap out applications
> and instead concentrate on reclaiming pages from the page cache.
>
> cheers,
> Chris
> - --
> Christopher Samuel - Senior Systems Administrator
> VLSCI - Victorian Life Sciences Computational Initiative
> Email: samuel_at_[hidden] Phone: +61 (0)3 903 55545
> http://www.vlsci.unimelb.edu.au/
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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