writeback: dirty position control - bdi reserve area (8927f66c) · Commits · 戴 / test

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>

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;
		}

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