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    SubjectRe: [PATCH 3/3] writeback: Do not congestion sleep when there are no congested BDIs
    On Fri, Aug 27, 2010 at 03:13:16PM +1000, Dave Chinner wrote:
    > On Thu, Aug 26, 2010 at 04:14:16PM +0100, Mel Gorman wrote:
    > > If congestion_wait() is called with no BDIs congested, the caller will
    > > sleep for the full timeout and this is an unnecessary sleep.
    > That, I think, is an invalid assumption. congestion_wait is used in
    > some places as a backoff mechanism that waits for some IO work to be
    > done, with congestion disappearing being a indication that progress
    > has been made and so we can retry sooner than the entire timeout.

    As it's write IO rather than some IO, I wonder if that's really the
    right thing to do. However, I accept your (and others) point that
    converting all congestion_wait() callers may be too much of a change.

    > For example, if _xfs_buf_lookup_pages() fails to allocate page cache
    > pages for a buffer, it will kick the xfsbufd to writeback dirty
    > buffers (so they can be freed) and immediately enter
    > congestion_wait(). If there isn't congestion when we enter
    > congestion_wait(), we still want to give the xfsbufds a chance to
    > clean some pages before we retry the allocation for the new buffer.
    > Removing the congestion_wait() sleep behaviour will effectively
    > _increase_ memory pressure with XFS on fast disk subsystems because
    > it now won't backoff between failed allocation attempts...
    > Perhaps a congestion_wait_iff_congested() variant is needed for the
    > VM? I can certainly see how it benefits the VM from a latency
    > perspective, but it is the opposite behaviour that is expected in
    > other places...

    I'm added a wait_iff_congested() and updated a few of the VM callers. I changed
    a fairly minimum number of what appeared to be the obvious ones to change.


    Mel Gorman
    Part-time Phd Student Linux Technology Center
    University of Limerick IBM Dublin Software Lab

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      Last update: 2010-08-27 11:37    [W:0.021 / U:63.680 seconds]
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