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    SubjectRe: [PATCH 17/18] writeback: fix dirtied pages accounting on redirty
    On Wed, Sep 07, 2011 at 08:22:22AM +0800, Jan Kara wrote:
    > On Tue 06-09-11 18:18:56, Peter Zijlstra wrote:
    > > On Sun, 2011-09-04 at 09:53 +0800, Wu Fengguang wrote:
    > > > De-account the accumulative dirty counters on page redirty.
    > > >
    > > > Page redirties (very common in ext4) will introduce mismatch between
    > > > counters (a) and (b)
    > > >
    > > > a) NR_DIRTIED, BDI_DIRTIED, tsk->nr_dirtied
    > > > b) NR_WRITTEN, BDI_WRITTEN
    > > >
    > > > This will introduce systematic errors in balanced_rate and result in
    > > > dirty page position errors (ie. the dirty pages are no longer balanced
    > > > around the global/bdi setpoints).
    > > >
    > >
    > > So wtf is ext4 doing? Shouldn't a page stay dirty until its written out?
    > >
    > > That is, should we really frob around this behaviour or fix ext4 because
    > > its on crack?
    > Fengguang, could you please verify your findings with recent kernel? I
    > believe ext4 got fixed in this regard some time ago already (and yes, old
    > delalloc writeback code in ext4 was terrible).

    Jan, attached is the results for 3.1-rc4, before and after this patchset.
    The test case is ext4, 1 dd, bs=4k, dirty_bytes=1GB.

    Judging from global_dirtied_written.png, the dirtied/written lines are
    still departing away from each other...

    Thanks,
    Fengguang
    [unhandled content-type:image/png][unhandled content-type:image/png][unhandled content-type:image/png][unhandled content-type:image/png][unhandled content-type:image/png][unhandled content-type:image/png]
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