linux-kernel - Re: Local DoS through write heavy I/O on CFQ & Deadline

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Date:	Fri, 12 Oct 2012 16:58:39 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Alex Bligh <alex@...x.org.uk>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Local DoS through write heavy I/O on CFQ & Deadline

On Fri 12-10-12 15:48:34, Alex Bligh wrote:
> 
> 
> --On 12 October 2012 15:30:45 +0200 Michal Hocko <mhocko@...e.cz> wrote:
> 
> >>Full info, including logs and scripts can be found at:
> >>  https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1064521
> >
> >You seem to have 8G of RAM and dirty_ratio=20 resp.
> >dirty_background_ratio=10 which means that 1.5G worth of dirty data
> >until writer gets throttled which is a lot. Background writeback starts
> >at 800M which is probably not sufficient as well. Have you tried to set
> >dirty_bytes at a reasonable value (wrt. to your storage)?
> 
> This is for an appliance install where we have no idea how much
> memory the box has in advance other than 'at least 4G' so it
> is difficult to tune by default.
> 
> However, I don't think that would solve the problem as the zcat/dd
> can always generate data faster than it can be written to disk unless
> or until it is throttled, which it never is. 

Once dirty_ratio (resp. dirty_bytes) limit is hit then the process which
writes gets throttled. If this is not the case then there is a bug in
the throttling code.

> Isn't the only thing that is going to change that it ends up
> triggering the writeback earlier?

Set the limit lowe?
 
> Happy to test etc - what would you suggest, dirty_ratio=5,
> dirty_background_ratio=2 ?

These are measured in percentage. On the other hand if you use
dirty_bytes resp. dirty_background_bytes then you get absolute numbers
independent on the amount of memory.

-- 
Michal Hocko
SUSE Labs
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