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    SubjectRe: [PATCH][5/?] count writeback pages in nr_scanned
    Rik van Riel <riel@redhat.com> wrote:
    >
    > On Thu, 6 Jan 2005, Nick Piggin wrote:
    >
    > > I think what Andrea is worried about is that blk_congestion_wait is
    > > fairly vague, and can be a source of instability in the scanning
    > > implementation.
    >
    > The recent OOM kill problem has been happening:
    > 1) with cache pressure on lowmem only, due to a block device write
    > 2) with no block congestion at all
    > 3) with pretty much all pageable lowmme pages in writeback state

    You must have a wild number of requests configured in the queue. Is this
    CFQ?

    I've done testing with "all of memory under writeback" before and it went
    OK. It's certainly a design objective to handle this well. But that
    testing was before we broke it.

    > It appears the VM has trouble dealing with the situation where
    > there is no block congestion to wait on...

    It's misnamed. We don't "wait for the queue to come out of congestion".
    We simply throttle until a write completes, or, rarely, timeout.

    The bug which you fixed would cause the VM to scan itself to death without
    throttling. Did the fix fix things?
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