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    SubjectRe: [PATCH 08/13] writeback: quit throttling when bdi dirty pages dropped low
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    On Wed, 2010-11-17 at 12:27 +0800, Wu Fengguang wrote:

    > @@ -578,6 +579,25 @@ static void balance_dirty_pages(struct a
    > bdi_stat(bdi, BDI_WRITEBACK);
    > }
    >
    > + /*
    > + * bdi_thresh takes time to ramp up from the initial 0,
    > + * especially for slow devices.
    > + *
    > + * It's possible that at the moment dirty throttling starts,
    > + * bdi_dirty = nr_dirty
    > + * = (background_thresh + dirty_thresh) / 2
    > + * >> bdi_thresh
    > + * Then the task could be blocked for a dozen second to flush
    > + * all the exceeded (bdi_dirty - bdi_thresh) pages. So offer a
    > + * complementary way to break out of the loop when 250ms worth
    > + * of dirty pages have been cleaned during our pause time.
    > + */
    > + if (nr_dirty < dirty_thresh &&
    > + bdi_prev_dirty - bdi_dirty >
    > + bdi->write_bandwidth >> (PAGE_CACHE_SHIFT + 2))
    > + break;
    > + bdi_prev_dirty = bdi_dirty;
    > +
    > if (bdi_dirty >= bdi_thresh) {
    > pause = HZ/10;
    > goto pause;


    So we're testing to see if during our pause time (<=100ms) we've written
    out 250ms worth of pages (given our current bandwidth estimation),
    right?

    (1/4th of bandwidth in bytes/s is bytes per 0.25s)

    (and in your recent patches you've changed the bw to pages/s so I take
    it the PAGE_CACHE_SIZE will be gone from all these sites).




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