Date: Fri, 12 Oct 2012 15:30:45 +0200
From: Michal Hocko <mhocko@suse.cz>
To: Alex Bligh <alex@alex.org.uk>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Local DoS through write heavy I/O on CFQ & Deadline
Message-ID: <20121012133044.GA10115@dhcp22.suse.cz>
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On Thu 11-10-12 13:23:32, Alex Bligh wrote:
> We have noticed significant I/O scheduling issues on both the CFQ and the
> deadline scheduler where a non-root user can starve any other process of
> any I/O for minutes at a time. The problem is more serious using CFQ but is
> still an effective local DoS vector using Deadline.
> 
> A simple way to generate the problem is:
> 
>   dd if=/dev/zero of=- bs=1M count=50000 | dd if=- of=myfile bs=1M count=50000
> 
[...]
> 
> 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)?
-- 
Michal Hocko
SUSE Labs
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