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    SubjectRe: [PATCH 3/3] writeback: Do not congestion sleep when there are no congested BDIs
    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.

    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...

    Cheers,

    Dave.
    --
    Dave Chinner
    david@fromorbit.com


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