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    SubjectRe: 2.6.22 -mm merge plans -- vm bugfixes
    Nick Piggin wrote:
    > Hugh Dickins wrote:
    >
    >> On Tue, 1 May 2007, Nick Piggin wrote:
    >
    >
    >>> There were concerns that we could do this more cheaply, but I think it
    >>> is important to start with a base that is simple and more likely to
    >>> be correct and build on that. My testing didn't show any obvious
    >>> problems with performance.
    >>
    >>
    >>
    >> I don't see _problems_ with performance, but I do consistently see the
    >> same kind of ~5% degradation in lmbench fork, exec, sh, mmap latency
    >> and page fault tests on SMP, several machines, just as I did last year.
    >
    >
    > OK. I did run some tests at one stage which didn't show a regression
    > on my P4, however I don't know that they were statistically significant.
    > I'll try a couple more runs and post numbers.

    I didn't have enough time tonight to get means/stddev, etc, but the runs
    are pretty stable.

    Patch tested was just the lock page one.

    SMP kernel, tasks bound to 1 CPU:

    P4 Xeon
    pagefault fork exec
    2.6.21 1.67-1.69 140.7-142.0 449.5-460.8
    +patch 1.75-1.77 144.0-145.5 456.2-463.0

    So it's taken on nearly 5% on pagefault, but looks like less than 2% on
    fork, so not as bad as your numbers (phew).

    G5
    pagefault fork exec
    2.6.21 1.49-1.51 164.6-170.8 741.8-760.3
    +patch 1.71-1.73 175.2-180.8 780.5-794.2

    Bigger hit there.

    Page faults can be improved a tiny bit by not using a test and clear op
    in unlock_page (less barriers for the G5).

    I don't think that's really a blocker problem for a merge, but I wonder
    what we can do to improve it. Lockless pagecache shaves quite a bit of
    straight line find_get_page performance there.

    Going to a non-sleeping lock might be one way to go in the long term, but
    it would require quite a lot of restructuring.

    --
    SUSE Labs, Novell Inc.
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