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: Wu Fengguang <fengguang.wu@intel.com>
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 6a8bb69..325f753 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -599,6 +599,7 @@
*/
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 @@
} 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;
}