[PATCH 10/13] writeback: make reasonable gap between the dirty/background thresholds

[Wu Fengguang]

The change is virtually a no-op for the majority users that use the
default 10/20 background/dirty ratios. For others don't know why they
are setting background ratio close enough to dirty ratio. Someone must
set background ratio equal to dirty ratio, but no one seems to notice or
complain that it's then silently halved under the hood..

Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 mm/page-writeback.c |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

--- linux-next.orig/mm/page-writeback.c	2010-11-15 13:12:50.000000000 +0800
+++ linux-next/mm/page-writeback.c	2010-11-15 13:13:42.000000000 +0800
@@ -403,8 +403,15 @@ void global_dirty_limits(unsigned long *
 	else
 		background = (dirty_background_ratio * available_memory) / 100;
 
-	if (background >= dirty)
-		background = dirty / 2;
+	/*
+	 * Ensure at least 1/4 gap between background and dirty thresholds, so
+	 * that when dirty throttling starts at (background + dirty)/2, it's at
+	 * the entrance of bdi soft throttle threshold, so as to avoid being
+	 * hard throttled.
+	 */
+	if (background > dirty - dirty * 2 / BDI_SOFT_DIRTY_LIMIT)
+		background = dirty - dirty * 2 / BDI_SOFT_DIRTY_LIMIT;
+
 	tsk = current;
 	if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
 		background += background / 4;


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