Re: App blocked in futex() burns 14% CPU.

[Chris Friesen <chris.friesen@genband.com>]

On 07/14/2011 04:34 PM, Jesper Juhl wrote:
> So, I've recently started enabling the KDE desktop search on my box and it
> runs some indexing services (naturally) to keep track of changes to files.
> One of the apps it starts is a process named "virtuoso-t". This process
> burns a *lot* of CPU when it's just started, but once it has caught up
> with recent changes it quiets down. It doesn't quite quiet down to the
> level I'd expect though. I see it constantly using 12-14% CPU time in
> 'top' even when there is *nothing* going on on the machine :
>
> top - 00:25:09 up  2:38,  2 users,  load average: 0.01, 0.04, 0.05
> Tasks: 155 total,   1 running, 153 sleeping,   0 stopped,   1 zombie
> Cpu(s):  0.7%us,  0.8%sy,  3.7%ni, 94.6%id,  0.1%wa,  0.0%hi,  0.0%si,  0.0%st
> Mem:   3853028k total,  2154696k used,  1698332k free,   184280k buffers
> Swap:  4200992k total,        0k used,  4200992k free,   996824k cached
>
>    PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
>   1952 jj        39  19  362m 263m 6544 S   14  7.0  25:09.54 virtuoso-t
>   1811 jj        20   0  635m  27m  17m S    2  0.7   2:49.78 knotify4
>   1928 jj        39  19  595m  26m  18m S    1  0.7   2:58.40 nepomukservices
>
> This box has a dual core Intel core i5-560M CPU with hyperthreading, so it
> is 12-14% of a fairly capable CPU - that's rather a lot more than I'd
> expect when the box is idle and there's nothing for the process to do.
>
> So, I tried strace'ing the process and it seems to just be blocked in
> futex() most of the time (like 99%) :
>
> [jj@dragon ~]$ strace -p 1952
> Process 1952 attached - interrupt to quit
> futex(0x3086424, FUTEX_WAIT_PRIVATE, 503, NULL
>
> So it's just sitting there doing nothing, which lead me to think that this
> is probably not a bug in the application - if it was doing some silly
> polling loop or similar I would not just see it blocked in a futex() call.
> So I'm guessing something must be happening in kernel space that's burning
> a lot of CPU.. I could be completely wrong of course, and if I am, please
> feel free to enlighten me.

You don't have enough kernel activity to account for 14% of a cpu.  Have 
you checked whether there's another thread within the process that's 
doing something in userspace?

Chris

-- 
Chris Friesen
Software Developer
GENBAND
chris.friesen@genband.com
www.genband.com