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    SubjectRe: PERF: performance tests with the split LRU VM in -mm
    On Mon, 28 Jul 2008 10:57:42 -0400
    Rik van Riel <> wrote:

    > On Thu, 24 Jul 2008 22:25:10 -0400
    > Rik van Riel <> wrote:
    > > TEST 1: dd if=/dev/sda of=/dev/null bs=1M
    > >
    > > kernel speed swap used
    > >
    > > 2.6.26 111MB/s 500kB
    > > -mm 110MB/s 59MB (ouch, system noticably slower)
    > > noforce 111MB/s 128kB
    > > stream 108MB/s 0 (slight regression, not sure why yet)
    > >
    > > This patch shows that the split LRU VM in -mm has a problem
    > > with large streaming IOs: the working set gets pushed out of
    > > memory, which makes doing anything else during the big streaming
    > > IO kind of painful.
    > >
    > > However, either of the two patches posted fixes that problem,
    > > though at a slight performance penalty for the "stream" patch.
    > OK, the throughput number with this test turns out not to mean
    > nearly as much as I thought.
    > Switching off CPU frequency scaling, pinning the CPUs at the
    > highest speed, resulted in a throughput of only 102MB/s.
    > My suspicion is that faster running code on the CPU results
    > in IOs being sent down to the device faster, resulting in
    > smaller IOs and lower throughput.
    > This would be promising for the "stream" patch, which makes
    > choosing between the two patches harder :)
    > Andrew, what is your preference between:
    > and

    Boy. They both seem rather hacky special-cases. But that doesn't mean
    that they're undesirable hacky special-cases. I guess the second one
    looks a bit more "algorithmic" and a bit less hacky-special-case. But
    it all depends on testing..

    On a different topic, these:


    have been floating about in -mm for ages, awaiting demonstration that
    they're a net benefit. But all of this new page-reclaim rework was
    built on top of those two patches and incorporates and retains them.

    I could toss them out, but that would require some rework and would
    partially invalidate previous testing and who knows, they _might_ be
    good patches. Or they might not be.

    What are your thoughts?

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