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Subject[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 <fengguang.wu@intel.com>
---
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:



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