Subject: Re: [RFC][PATCH] Per file dirty limit throttling
From: Peter Zijlstra <peterz@infradead.org>
To: Nikanth Karthikesan <knikanth@suse.de>
Cc: Wu Fengguang <fengguang.wu@intel.com>, Bill Davidsen <davidsen@tmr.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Jens Axboe <axboe@kernel.dk>,
        Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.cz>
In-Reply-To: <201008171039.23701.knikanth@suse.de>
References: <201008160949.51512.knikanth@suse.de>
	 <1281956742.1926.1217.camel@laptop>  <201008171039.23701.knikanth@suse.de>
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Date: Tue, 17 Aug 2010 10:24:35 +0200
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On Tue, 2010-08-17 at 10:39 +0530, Nikanth Karthikesan wrote:
> Oh, nice.  Per-task limit is an elegant solution, which should help during 
> most of the common cases.
> 
> But I just wonder what happens, when
> 1. The dirtier is multiple co-operating processes
> 2. Some app like a shell script, that repeatedly calls dd with seek and skip? 
> People do this for data deduplication, sparse skipping etc..
> 3. The app dies and comes back again. Like a VM that is rebooted, and 
> continues writing to a disk backed by a file on the host.
> 
> Do you think, in those cases this might still be useful? 

Those cases do indeed defeat the current per-task-limit, however I think
the solution to that is to limit the amount of writeback done by each
blocked process.

Jan Kara had some good ideas in that department.
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