From: Wu Fengguang <fengguang.wu@intel.com>
Date: Sun, 12 Jun 2011 01:32:32 +0000 (-0600)
Subject: writeback: control dirty pause time
X-Git-Tag: v3.2-rc1~23^2~7
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=57fc978cfb61ed40a7bbfe5a569359159ba31abd;p=pandora-kernel.git

writeback: control dirty pause time

The dirty pause time shall ultimately be controlled by adjusting
nr_dirtied_pause, since there is relationship

	pause = pages_dirtied / task_ratelimit

Assuming

	pages_dirtied ~= nr_dirtied_pause
	task_ratelimit ~= dirty_ratelimit

We get

	nr_dirtied_pause ~= dirty_ratelimit * desired_pause

Here dirty_ratelimit is preferred over task_ratelimit because it's
more stable.

It's also important to limit possible large transitional errors:

- bw is changing quickly
- pages_dirtied << nr_dirtied_pause on entering dirty exceeded area
- pages_dirtied >> nr_dirtied_pause on btrfs (to be improved by a
  separate fix, but still expect non-trivial errors)

So we end up using the above formula inside clamp_val().

The best test case for this code is to run 100 "dd bs=4M" tasks on
btrfs and check its pause time distribution.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
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