linux-kernel - Re: [RFC][PATCH] Per file dirty limit throttling

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Message-ID: <20100818140856.GE28417@balbir.in.ibm.com>
Date:	Wed, 18 Aug 2010 19:38:56 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Nikanth Karthikesan <knikanth@...e.de>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Bill Davidsen <davidsen@....com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Jens Axboe <axboe@...nel.dk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.cz>
Subject: Re: [RFC][PATCH] Per file dirty limit throttling

* Peter Zijlstra <peterz@...radead.org> [2010-08-18 11:58:56]:

> On Wed, 2010-08-18 at 14:52 +0530, Nikanth Karthikesan wrote:
> > On Tuesday 17 August 2010 13:54:35 Peter Zijlstra wrote:
> > > 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.
> > > 
> > 
> > Blocked on what? Sorry, I do not understand.
> 
> balance_dirty_pages(), by limiting the work done there (or actually, the
> amount of page writeback completions you wait for -- starting IO isn't
> that expensive), you can also affect the time it takes, and therefore
> influence the impact.
>

There is an ongoing effort to look at per-cgroup dirty limits and I
honestly think it would be nice to do it at that level first. We need
it there as a part of the overall I/O controller. As a specialized
need it could handle your case as well. 

-- 
	Three Cheers,
	Balbir
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