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    SubjectRe: [PATCH 7/8] writeback: sync old inodes first in background writeback
    Hi Mel,

    On Thu, Jul 22, 2010 at 05:42:09PM +0800, Mel Gorman wrote:
    > On Thu, Jul 22, 2010 at 04:52:10PM +0800, Wu Fengguang wrote:
    > > > Some insight on how the other writeback changes that are being floated
    > > > around might affect the number of dirty pages reclaim encounters would also
    > > > be helpful.
    > >
    > > Here is an interesting related problem about the wait_on_page_writeback() call
    > > inside shrink_page_list():
    > >
    > > http://lkml.org/lkml/2010/4/4/86

    I guess you've got the answers from the above thread, anyway here is
    the brief answers to your questions.

    > >
    > > The problem is, wait_on_page_writeback() is called too early in the
    > > direct reclaim path, which blocks many random/unrelated processes when
    > > some slow (USB stick) writeback is on the way.
    > >
    > > A simple dd can easily create a big range of dirty pages in the LRU
    > > list. Therefore priority can easily go below (DEF_PRIORITY - 2) in a
    > > typical desktop, which triggers the lumpy reclaim mode and hence
    > > wait_on_page_writeback().
    > >
    >
    > Lumpy reclaim is for high-order allocations. A simple dd should not be
    > triggering it regularly unless there was a lot of forking going on at the
    > same time.

    dd could create the dirty file fast enough, so that no other processes
    are injecting pages into the LRU lists besides dd itself. So it's
    creating a large range of hard-to-reclaim LRU pages which will trigger
    this code

    + else if (sc->order && priority < DEF_PRIORITY - 2)
    + lumpy_reclaim = 1;


    > Also, how would a random or unrelated process get blocked on
    > writeback unless they were also doing high-order allocations? What was the
    > source of the high-order allocations?

    sc->order is 1 on fork().

    Thanks,
    Fengguang


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