[PATCH 1/5] writeback: balanced_rate cannot exceed write bandwidth

[Wu Fengguang]

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 <fengguang.wu@xxxxxxxxx>
---
 mm/page-writeback.c |    5 +++++
 1 file changed, 5 insertions(+)

--- linux-next.orig/mm/page-writeback.c	2011-11-17 20:18:03.000000000 +0800
+++ linux-next/mm/page-writeback.c	2011-11-17 20:18:23.000000000 +0800
@@ -804,6 +804,11 @@ 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:


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