linux-kernel - Re: [PATCH 03/18] writeback: dirty rate control

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Date:	Thu, 29 Sep 2011 19:57:12 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Greg Thelen <gthelen@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Andrea Righi <arighi@...eler.com>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 03/18] writeback: dirty rate control

A minor fix to this patch.

While testing the fio mmap workload, bdi->dirty_ratelimit is observed
to be knocked down to 1 and then brought up high in regular intervals.

The showed up problem is, it took long delays to bring up
bdi->dirty_ratelimit due to the round-down problem of the below
task_ratelimit calculation: when dirty_ratelimit=1 and pos_ratio = 1.5,
the resulted task_ratelimit will be 1, which fooled stops the logic
from increasing dirty_ratelimit as long as pos_ratio < 2. The below
change (from round-down to round-up) can nicely fix this problem.

Thanks,
Fengguang
---

--- linux-next.orig/mm/page-writeback.c	2011-09-24 15:52:11.000000000 +0800
+++ linux-next/mm/page-writeback.c	2011-09-24 15:52:11.000000000 +0800
@@ -766,6 +766,7 @@ static void bdi_update_dirty_ratelimit(s
 	 */
 	task_ratelimit = (u64)dirty_ratelimit *
 					pos_ratio >> RATELIMIT_CALC_SHIFT;
+	task_ratelimit++; /* it helps rampup dirty_ratelimit from tiny values */
 
 	/*
 	 * A linear estimation of the "balanced" throttle rate. The theory is,
--
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