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Commit 8927f66c authored by Wu Fengguang's avatar Wu Fengguang
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writeback: dirty position control - bdi reserve area 



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: default avatarWu Fengguang <fengguang.wu@intel.com>
parent 57fc978c

mm/page-writeback.c

+15 −0
Original line number Diff line number Diff line
@@ -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;

}