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

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Date:	Fri, 16 Nov 2012 03:11:22 +0200
From:	Marcus Sundman <marcus@...ox.fi>
To:	Jan Kara <jack@...e.cz>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Debugging system freezes on filesystem writes

On 13.11.2012 15:51, Jan Kara wrote:
> On Fri 09-11-12 15:12:43, Marcus Sundman wrote:
>> On 09.11.2012 01:41, Marcus Sundman wrote:
>>> On 07.11.2012 18:17, Jan Kara wrote:
>>>> On Fri 02-11-12 04:19:24, Marcus Sundman wrote:
>>>>> 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.
>>> t=01:06 http://sundman.iki.fi/vmstat.pre-freeze.txt
>>> t=01:08 http://sundman.iki.fi/vmstat.during-freeze.txt
>>> t=01:12 http://sundman.iki.fi/vmstat.post-freeze.txt
>> Here are some more vmstats:
>> http://sundman.iki.fi/vmstats.tar.gz
>>
>> They are from running this:
>> while true; do cat /proc/vmstat > "vmstat.$(date +%FT%X).txt"; sleep
>> 10; done
>>
>> There were lots and lots of freezes for almost 20 mins from 14:37:45
>> onwards, pretty much constantly, but at 14:56:50 the freezes
>> suddenly stopped and everything went back to how it should be.
>    I was looking into the data but they didn't show anything problematic.
> The machine seems to be writing a lot but there's always some free memory,
> even direct reclaim isn't ever entered. Hum, actually you wrote iotop isn't
> showing much IO going on but vmstats show there is about 1 GB written
> during the freeze. It is not a huge amount given the time span but it
> certainly gives a few MB/s of write load.

I didn't watch iotop during this particular freeze. I'll try to keep an 
eye on iotop in the future. Is there some particular options I should 
run iotop with, or is a "nice -n -20 iotop -od3" fine?

> There's surprisingly high number of allocations going on but that may be
> due to the IO activity. So let's try something else: Can you switch to
> console and when the hang happens press Alt-Sysrq-w (or you can just do
> "echo w >/proc/sysrq-trigger" if the machine is live enough to do that).
> Then send me the output from dmesg.  Thanks!

Sure! Here are two:
http://sundman.iki.fi/dmesg-1.txt
http://sundman.iki.fi/dmesg-2.txt


Best regards,
Marcus

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