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

[Darren Hart]

On 07/14/2011 03:51 PM, Chris Friesen wrote:
> 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

Try strace with -f. If you really want to collect statistics about where
it is spending it's time, consider oprofile or perf.

>>
>> 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?
> 

Agreed.

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel
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