[PATCH 12/18] writeback: balanced_rate cannot exceed write bandwidth

Add an upper limit to balanced_rate according to the below inequality.
This filters out some rare but huge singular points, which at least
enables more readable gnuplot figures.

When there are N dd dirtiers,

	balanced_dirty_ratelimit = write_bw / N

So it holds that

	balanced_dirty_ratelimit <= write_bw

Signed-off-by: Wu Fengguang <redacted>
---
 mm/page-writeback.c |    5 +++++
 1 file changed, 5 insertions(+)

--- linux-next.orig/mm/page-writeback.c	2011-08-29 19:14:22.000000000 +0800
+++ linux-next/mm/page-writeback.c	2011-08-29 19:20:36.000000000 +0800

@@ -828,6 +828,11 @@ static void bdi_update_dirty_ratelimit(s
 	 */
 	balanced_dirty_ratelimit = div_u64((u64)task_ratelimit * write_bw,
 					   dirty_rate | 1);
+	/*
+	 * balanced_dirty_ratelimit ~= (write_bw / N) <= write_bw
+	 */
+	if (unlikely(balanced_dirty_ratelimit > write_bw))
+		balanced_dirty_ratelimit = write_bw;
 
 	/*
 	 * We could safely do this and return immediately:


--

To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>