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    SubjectRe: [RFC PATCH 0/3] Avoid the use of congestion_wait under zone pressure
    On Mon, Mar 15, 2010 at 01:09:35PM -0700, Andrew Morton wrote:
    > On Mon, 15 Mar 2010 13:34:50 +0100
    > Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com> wrote:
    >
    > > c) If direct reclaim did reasonable progress in try_to_free but did not
    > > get a page, AND there is no write in flight at all then let it try again
    > > to free up something.
    > > This could be extended by some kind of max retry to avoid some weird
    > > looping cases as well.
    > >
    > > d) Another way might be as easy as letting congestion_wait return
    > > immediately if there are no outstanding writes - this would keep the
    > > behavior for cases with write and avoid the "running always in full
    > > timeout" issue without writes.
    >
    > They're pretty much equivalent and would work. But there are two
    > things I still don't understand:
    >
    > 1: Why is direct reclaim calling congestion_wait() at all? If no
    > writes are going on there's lots of clean pagecache around so reclaim
    > should trivially succeed. What's preventing it from doing so?
    >
    > 2: This is, I think, new behaviour. A regression. What caused it?
    >

    I looked at this a bit closer using an iozone test very similar to
    Christian's. Despite buying a number of disks, I still can't reproduce his
    problem but I instrumented congestion_wait counts and times similar to
    what he did.

    2.6.29-instrument:congestion_waittime 990
    2.6.30-instrument:congestion_waittime 2823
    2.6.31-instrument:congestion_waittime 193169
    2.6.32-instrument:congestion_waittime 228890
    2.6.33-instrument:congestion_waittime 785529
    2.6.34-rc1-instrument:congestion_waittime 797178

    So in the problem window, there was *definite* increases in the time spent
    in congestion_wait and the number of times it was called. I'll look
    closer at this tomorrow and Monday and see can I pin down what is
    happening.

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


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