linux-kernel - Re: Debugging system freezes on filesystem writes

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Date:	Wed, 7 Nov 2012 17:17:30 +0100
From:	Jan Kara <jack@...e.cz>
To:	Marcus Sundman <marcus@...ox.fi>
Cc:	linux-kernel@...r.kernel.org, jack@...e.cz
Subject: Re: Debugging system freezes on filesystem writes

On Fri 02-11-12 04:19:24, Marcus Sundman wrote:
> On 01.11.2012 21:01, Jan Kara wrote:
> >On Mon 29-10-12 00:39:46, Marcus Sundman wrote:
> >   Hello,
> >
> >>I have a big problem with the system freezing and would appreciate
> >>any help on debugging this and pinpointing where exactly the problem
> >>is, so it could be fixed.
> >>
> >>So, whenever I write to the disk the system comes to a crawl or
> >>freezes altogether. This happens even when the writing processes are
> >>running on nice '19' and ionice 'idle'. (E.g. a 10 second compile
> >>could freeze the system for several minutes, rendering the computer
> >>pretty much unusable for anything interesting.)
> >>
> >>Here you can see a 20 second gap even in superhigh priority:
> >># nice -n -20 ionice -c1 iostat -t -m -d -x 1 > http://pastebin.com/j5qnh2VV
> >>
> >>I'm currently running 3.5.0-17-lowlatency on the ZenBook UX31E,
> >>using the NOOP I/O scheduler on the SanDisk SSD U100. The chipset
> >>seems to be Intel QS67. I've had this same problem on 3.2.0 generic
> >>and lowlatency kernels.
> >   These are Ubuntu kernels. Any chance to reproduce the issue with vanilla
> >kernels - i.e. kernels without any Ubuntu patches?
> 
> I'm afraid it's going to take a week to compile a kernel with this
> freezing going on, but I suppose I could get another computer to do
> the compiling. Or should I install some pre-compiled version? If so,
> which one?
  You can install anything precompiled. It's just that I want to rule out
some Ubuntu specific patches...

> >Also when you speak of
> >system freezing - can you e.g. type to terminal while the system is frozen?
> >Or is it just that running commands freezes?
> 
> Typing usually doesn't work very well. It works for a word or two
> and then stops working for a while and if I continue to type then
> when it resumes only the last few characters appears. Typing in the
> console is a bit better than in a terminal in X (not counting the
> several minutes it can take to switch to the console (Ctrl-Alt-F1)).
  I see. 

> Also, and this might be important, according to iotop there is
> almost no disk writing going on during the freeze. (Occasionally
> there are a few MB/s, but mostly it's 0-200 kB/s.) Well, at least
> when an iotop running on nice -20 hasn't frozen completely, which it
> does during the more severe freezes.
  OK, it seems as if your machine has some problems with memory
allocations. Can you capture /proc/vmstat before the freeze and after the
freeze and send them for comparison. Maybe it will show us what is the
system doing.

  Also you can try doing:
echo never >/sys/kernel/mm/transparent_hugepage/enabled
  and see whether it changes anything.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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