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    SubjectRe: regression: 100% io-wait with 2.6.24-rcX
    ----- Original Message ----
    > From: Mike Snitzer <snitzer@gmail.com>
    > To: Fengguang Wu <wfg@mail.ustc.edu.cn>
    > Cc: Peter Zijlstra <peterz@infradead.org>; jplatte@naasa.net; Ingo Molnar <mingo@elte.hu>; linux-kernel@vger.kernel.org; "linux-ext4@vger.kernel.org" <linux-ext4@vger.kernel.org>; Linus Torvalds <torvalds@linux-foundation.org>; Andrew Morton <akpm@li>
    > Sent: Tuesday, January 15, 2008 10:13:22 PM
    > Subject: Re: regression: 100% io-wait with 2.6.24-rcX
    >
    > On Jan 14, 2008 7:50 AM, Fengguang Wu wrote:
    > > On Mon, Jan 14, 2008 at 12:41:26PM +0100, Peter Zijlstra wrote:
    > > >
    > > > On Mon, 2008-01-14 at 12:30 +0100, Joerg Platte wrote:
    > > > > Am Montag, 14. Januar 2008 schrieb Fengguang Wu:
    > > > >
    > > > > > Joerg, this patch fixed the bug for me :-)
    > > > >
    > > > > Fengguang, congratulations, I can confirm that your patch
    > fixed
    >
    the bug! With
    > > > > previous kernels the bug showed up after each reboot. Now,
    > when
    >
    booting the
    > > > > patched kernel everything is fine and there is no longer
    > any
    >
    suspicious
    > > > > iowait!
    > > > >
    > > > > Do you have an idea why this problem appeared in 2.6.24?
    > Did
    >
    somebody change
    > > > > the ext2 code or is it related to the changes in the scheduler?
    > > >
    > > > It was Fengguang who changed the inode writeback code, and I
    > guess
    >
    the
    > > > new and improved code was less able do deal with these funny corner
    > > > cases. But he has been very good in tracking them down and
    > solving
    >
    them,
    > > > kudos to him for that work!
    > >
    > > Thank you.
    > >
    > > In particular the bug is triggered by the patch named:
    > > "writeback: introduce writeback_control.more_io to
    > indicate
    >
    more io"
    > > That patch means to speed up writeback, but unfortunately its
    > > aggressiveness has disclosed bugs in reiserfs, jfs and now ext2.
    > >
    > > Linus, given the number of bugs it triggered, I'd recommend revert
    > > this patch(git commit
    > 2e6883bdf49abd0e7f0d9b6297fc3be7ebb2250b).
    >
    Let's
    > > push it back to -mm tree for more testings?
    >
    > Fengguang,
    >
    > I'd like to better understand where your writeback work stands
    > relative to 2.6.24-rcX and -mm. To be clear, your changes in
    > 2.6.24-rc7 have been benchmarked to provide a ~33% sequential write
    > performance improvement with ext3 (as compared to 2.6.22, CFS could be
    > helping, etc but...). Very impressive!
    >
    > Given this improvement it is unfortunate to see your request to revert
    > 2e6883bdf49abd0e7f0d9b6297fc3be7ebb2250b but it is understandable if
    > you're not confident in it for 2.6.24.
    >
    > That said, you recently posted an -mm patchset that first reverts
    > 2e6883bdf49abd0e7f0d9b6297fc3be7ebb2250b and then goes on to address
    > the "slow writes for concurrent large and small file writes" bug:
    > http://lkml.org/lkml/2008/1/15/132
    >
    > For those interested in using your writeback improvements in
    > production sooner rather than later (primarily with ext3); what
    > recommendations do you have? Just heavily test our own 2.6.24 + your
    > evolving "close, but not ready for merge" -mm writeback patchset?
    >
    Hi Fengguang, Mike,

    I can add myself to Mikes question. It would be good to know a "roadmap" for the writeback changes. Testing 2.6.24-rcX so far has been showing quite nice improvement of the overall writeback situation and it would be sad to see this [partially] gone in 2.6.24-final. Linus apparently already has reverted "...2250b". I will definitely repeat my tests with -rc8. and report.

    Cheers
    Martin






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