Re: Debugging system freezes on filesystem writes

[Jan Kara]

On Tue 27-11-12 18:14:42, Marcus Sundman wrote:
> On 22.11.2012 01:30, Jan Kara wrote:
> >On Fri 16-11-12 03:11:22, Marcus Sundman wrote:
> >>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?
> >   I'm not really familiar with iotop :). Usually I use iostat...
> 
> OK, which options for iostat should I use then? :)
  I'm back from vacation. Sorry for the delay. You can use
iostat -x 1

> >>>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
> >   Thanks for those and sorry for the delay (I was busy with other stuff).
> >I had a look into those traces and I have to say I'm not much wiser. In the
> >first dump there is just kswapd waiting for IO. In the second dump there
> >are more processes waiting for IO (mostly for reads - nautilus,
> >thunderbird, opera, ...) but nothing really surprising. So I'm lost what
> >could cause the hangs you observe.
> 
> Yes, mostly it's difficult to trigger the sysrq thingy, because by
> the time I manage to switch to the console or running that echo to
> proc in a terminal the worst is already over.
  I see. Maybe you could have something like
while true; do echo w >/proc/sysrq-trigger; sleep 10; done
  running in the background?
  
> >Recalling you wrote even simple programs
> >like top hang, maybe it is some CPU scheduling issue? Can you boot with
> >noautogroup kernel option?
> 
> Sure. I've been running with noautogroup for almost a week now, but
> no big change one way or the other. (E.g., it's still impossible to
> listen to music, because the songs will start skipping/looping
> several times during each song even if there isn't any big "hang"
> happening. And uncompressing a 100 MB archive (with nice '19' and
> ionice 'idle') is still, after a while, followed by a couple of
> minutes of superhigh I/O wait causing everything to become really
> slow.)
  Hum, I'm starting to wander what's so special about your system that you
see these hangs while noone else seems to be hitting them. Your kernel is a
standard one from Ubuntu so tons of people run it. Your HW doesn't seem to
be too special either.

BTW the fact that you ionice 'tar' doesn't change anything because all the
writes are done in the context of kernel flusher thread (tar just writes
data into cache). But still it shouldn't lock the machine up. What might be
interesting test though is running:
  dd if=/dev/zero of=file bs=1M count=200 oflags=direct

Does this trigger any hangs?

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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