Re: Debugging system freezes on filesystem writes
From: Marcus Sundman
Date: Tue Nov 27 2012 - 11:14:40 EST
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? :)
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.
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.)
- Marcus
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