linux-kernel - Re: Disabling in-memory write cache for x86-64 in Linux II

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Message-ID: <20131101142426.1065.25534.stgit@dhcp-10-30-17-2.sw.ru>
Date:	Fri, 01 Nov 2013 18:25:56 +0400
From:	Maxim Patlasov <MPatlasov@...allels.com>
To:	karl.kiniger@....ge.com
Cc:	jack@...e.cz, linux-kernel@...r.kernel.org, t.artem@...os.com,
	mgorman@...e.de, tytso@....edu, akpm@...ux-foundation.org,
	fengguang.wu@...el.com, torvalds@...ux-foundation.org,
	mpatlasov@...allels.com
Subject: Re: Disabling in-memory write cache for x86-64 in Linux II

On Thu 31-10-13 14:26:12, Karl Kiniger wrote:
> On Tue 131029, Jan Kara wrote:
> > On Fri 25-10-13 11:15:55, Karl Kiniger wrote:
> > > On Fri 131025, Linus Torvalds wrote:
> ....
> > > Is it currently possible to somehow set above values per block device?
> >   Yes, to some extent. You can set /sys/block/<device>/bdi/max_ratio to
> > the maximum proportion the device's dirty data can take from the total
> > amount. The caveat currently is that this setting only takes effect after
> > we have more than (dirty_background_ratio + dirty_ratio)/2 dirty data in
> > total because that is an amount of dirty data when we start to throttle
> > processes. So if the device you'd like to limit is the only one which is
> > currently written to, the limiting doesn't have a big effect.
>
> Thanks for the info - thats was I am looking for.
>
> You are right that the limiting doesn't have a big effect right now:
>
> on my  4x speed  DVD+RW on /dev/sr0, x86_64, 4GB,
> Fedora19:
>
> max_ratio set to 100  - about 500MB buffered, sync time 2:10 min.
> max_ratio set to 1    - about 330MB buffered, sync time 1:23 min.
>
> ... way too much buffering.

"strictlimit" feature must fit your and Artem's needs quite well. The feature
enforces per-BDI dirty limits even if the global dirty limit is not reached
yet. I'll send a patch adding knob to turn it on/off.

Thanks,
Maxim
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