On Wed, 2011-09-28 at 22:02 +0800, Wu Fengguang wrote:
/me attempts to swap back neurons related to writeback
> After lots of experiments, I end up with this bdi reserve point
>
> + x_intercept = bdi_thresh / 2 + MIN_WRITEBACK_PAGES;
>
> together with this chunk to avoid a bdi stuck in bdi_thresh=0 state:
>
> @@ -590,6 +590,7 @@ static unsigned long bdi_position_ratio(
> */
> if (unlikely(bdi_thresh > thresh))
> bdi_thresh = thresh;
> + bdi_thresh = max(bdi_thresh, (limit - dirty) / 8);
> /*
> * scale global setpoint to bdi's:
> * bdi_setpoint = setpoint * bdi_thresh / thresh
So you cap bdi_thresh at a minimum of (limit-dirty)/8 which can be
pretty close to 0 if we have a spike in dirty or a negative spike in
writeout bandwidth (sudden seeks or whatnot).
> The above changes are good enough to keep reasonable amount of bdi
> dirty pages, so the bdi underrun flag ("[PATCH 11/18] block: add bdi
> flag to indicate risk of io queue underrun") is dropped.
That sounds like goodness ;-)
> I also tried various bdi freerun patches, however the results are not
> satisfactory. Basically the bdi reserve area approach (this patch)
> yields noticeably more smooth/resilient behavior than the
> freerun/underrun approaches. I noticed that the bdi underrun flag
> could lead to sudden surge of dirty pages (especially if not
> safeguarded by the dirty_exceeded condition) in the very small
> window..
OK, so let me try and parse this magic:
+ x_intercept = bdi_thresh / 2 + MIN_WRITEBACK_PAGES;
+ if (bdi_dirty < x_intercept) {
+ if (bdi_dirty > x_intercept / 8) {
+ pos_ratio *= x_intercept;
+ do_div(pos_ratio, bdi_dirty);
+ } else
+ pos_ratio *= 8;
+ }
So we set our target some place north of MIN_WRITEBACK_PAGES: if we're
short we add a factor of: x_intercept/bdi_dirty.
Now, since bdi_dirty < x_intercept, this is > 1 and thus we promote more
dirties.
Additionally we don't let the factor get larger than 8 to avoid silly
large fluctuations (8 already seems quite generous to me).
Now I guess the only problem is when nr_bdi * MIN_WRITEBACK_PAGES ~
limit, at which point things go pear shaped.
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