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    SubjectRe: [PATCH 11/18] block: add bdi flag to indicate risk of io queue underrun
    On Tue, Sep 06, 2011 at 10:22:48PM +0800, Peter Zijlstra wrote:
    > On Sun, 2011-09-04 at 09:53 +0800, Wu Fengguang wrote:
    > > +++ linux-next/mm/page-writeback.c 2011-08-31 14:40:58.000000000 +0800
    > > @@ -1067,6 +1067,9 @@ static void balance_dirty_pages(struct a
    > > nr_dirty, bdi_thresh, bdi_dirty,
    > > start_time);
    > >
    > > + if (unlikely(!dirty_exceeded && bdi_async_underrun(bdi)))
    > > + break;
    > > +
    > > dirty_ratelimit = bdi->dirty_ratelimit;
    > > pos_ratio = bdi_position_ratio(bdi, dirty_thresh,
    > > background_thresh, nr_dirty,
    >
    > So dirty_exceeded looks like:
    >
    >
    > 1109 dirty_exceeded = (bdi_dirty > bdi_thresh) ||
    > 1110 (nr_dirty > dirty_thresh);
    >
    > Would it make sense to write it as:
    >
    > if (nr_dirty > dirty_thresh ||
    > (nr_dirty > freerun && bdi_dirty > bdi_thresh))
    > dirty_exceeded = 1;
    >
    > So that we don't actually throttle bdi thingies when we're still in the
    > freerun area?

    Sounds not necessary -- (nr_dirty > freerun) is implicitly true
    because there is a big break early in the loop:

    if (nr_dirty > freerun)
    break;

    Thanks,
    Fengguang


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