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    SubjectRe: The performance and behaviour of the anti-fragmentation related patches
    Andrew Morton wrote:
    > On Fri, 2 Mar 2007 09:23:49 -0800 (PST) Christoph Lameter <> wrote:
    >> On Fri, 2 Mar 2007, Andrew Morton wrote:
    >>>> Linux is *not* happy on 256GB systems. Even on some 32GB systems
    >>>> the swappiness setting *needs* to be tweaked before Linux will even
    >>>> run in a reasonable way.
    >>> Please send testcases.
    >> It is not happy if you put 256GB into one zone.
    > Oh come on. What's the workload? What happens? system time? user time?
    > kernel profiles?

    I can't share all the details, since a lot of the problems are customer

    One particular case is a 32GB system with a database that takes most
    of memory. The amount of actually freeable page cache memory is in
    the hundreds of MB. With swappiness at the default level of 60, kswapd
    ends up eating most of a CPU, and other tasks also dive into the pageout
    code. Even with swappiness as high as 98, that system still has
    problems with the CPU use in the pageout code!

    Another typical problem is that people want to back up their database
    servers. During the backup, parts of the working set get evicted from
    the VM and performance is horrible.

    A third scenario is where a system has way more RAM than swap, and not
    a whole lot of freeable page cache. In this case, the VM ends up
    spending WAY too much CPU time scanning and shuffling around essentially
    unswappable anonymous memory and tmpfs files.

    I have briefly characterized some of these working sets on:

    One thing I do not yet have are easily runnable test cases. I know
    the problems that happen because customers run into them, but it is
    not as easy to reproduce on test systems...

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