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writeback: dirty position control - bdi reserve area

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Keep a minimal pool of dirty pages for each bdi, so that the disk IO
queues won't underrun. Also gently increase a small bdi_thresh to avoid
it stuck in 0 for some light dirtied bdi.

It's particularly useful for JBOD and small memory system.

It may result in (pos_ratio > 1) at the setpoint and push the dirty
pages high. This is more or less intended because the bdi is in the
danger of IO queue underflow.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
This commit is contained in:
Wu Fengguang 2011-08-04 22:16:46 -06:00
parent 57fc978cfb
commit 8927f66c4e
1 changed files with 15 additions and 0 deletions
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15
mm/page-writeback.c
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@ -599,6 +599,7 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
*/
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
@ -622,6 +623,20 @@ static unsigned long bdi_position_ratio(struct backing_dev_info *bdi,
} else
pos_ratio /= 4;
/*
* bdi reserve area, safeguard against dirty pool underrun and disk idle
* It may push the desired control point of global dirty pages higher
* than setpoint.
*/
x_intercept = bdi_thresh / 2;
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;
}
return pos_ratio;
}