linux-kernel - Re: [PATCH 4/5] writeback: per task dirty rate limit

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Date:	Tue, 09 Aug 2011 20:35:06 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	linux-fsdevel@...r.kernel.org,
	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 4/5] writeback: per task dirty rate limit

On Sat, 2011-08-06 at 16:44 +0800, Wu Fengguang wrote:
> 
> Add two fields to task_struct.
> 
> 1) account dirtied pages in the individual tasks, for accuracy
> 2) per-task balance_dirty_pages() call intervals, for flexibility
> 
> The balance_dirty_pages() call interval (ie. nr_dirtied_pause) will
> scale near-sqrt to the safety gap between dirty pages and threshold.
> 
> XXX: The main problem of per-task nr_dirtied is, if 10k tasks start
> dirtying pages at exactly the same time, each task will be assigned a
> large initial nr_dirtied_pause, so that the dirty threshold will be
> exceeded long before each task reached its nr_dirtied_pause and hence
> call balance_dirty_pages(). 

Right, so why remove the per-cpu threshold? you can keep that as a bound
on the number of out-standing dirty pages.

Loosing that bound is actually a bad thing (TM), since you could have
configured a tight dirty limit and lock up your machine this way.
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