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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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