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    SubjectRe: [Bug #12809] iozone regression with 2.6.29-rc6
    On Mon, Mar 16, 2009 at 01:03:42PM +0800, Lin, Ming wrote:
    > On Sun, 2009-03-15 at 08:27 +0800, Linus Torvalds wrote:
    > >
    > > On Sat, 14 Mar 2009, Rafael J. Wysocki wrote:
    > > >
    > > > The following bug entry is on the current list of known regressions
    > > > from 2.6.28. Please verify if it still should be listed and let me know
    > > > (either way).
    > > >
    > > >
    > > > Bug-Entry : http://bugzilla.kernel.org/show_bug.cgi?id=12809
    > > > Subject : iozone regression with 2.6.29-rc6
    > > > Submitter : Lin Ming <ming.m.lin@intel.com>
    > > > Date : 2009-02-27 9:13 (16 days old)
    > > > First-Bad-Commit: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=1cf6e7d83bf334cc5916137862c920a97aabc018
    > > > References : http://marc.info/?l=linux-kernel&m=123572630504360&w=4
    > > > Handled-By : Wu Fengguang <fengguang.wu@intel.com>
    > >
    > > I suspect that I should just raise the default dirty limits. Wu reported
    > > that it fixed the regression, and while he picked some rather high
    > > percentages, I think we could certainly raise the rather aggressive
    > > default ones.
    > >
    > > After all, those default percentages were picked (a) with the old dirty
    > > logic and (b) largely at random and (c) designed to be aggressive. In
    > > particular, that (a) means that having fixed some of the dirty accounting,
    > > maybe the real bug is now that it was always too aggressive, just hidden
    > > by an accounting issue.
    > >
    > > If we raised the default ratio from 5/10 to 10/20, what happens to the
    > > iozone regression?
    >
    > echo 10 > /proc/sys/vm/dirty_background_ratio
    > echo 20 > /proc/sys/vm/dirty_ratio
    >
    > It fixed the regression of iozone (filesize 1200M) on 4P dual-core HT
    > machine(8G mem).

    A quick&coarse calculation: 8G * 15% = 1200M.
    This means an iozone process dirtying 1200M data won't be write-blocked.

    So the thresholds of 10/20 are just about enough for fixing this regression.

    Thanks,
    Fengguang



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