linux-kernel - App blocked in futex() burns 14% CPU.

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Date:	Fri, 15 Jul 2011 00:34:20 +0200 (CEST)
From:	Jesper Juhl <jj@...osbits.net>
To:	linux-kernel@...r.kernel.org
cc:	Eric Dumazet <dada1@...mosbay.com>,
	Darren Hart <dvhltc@...ibm.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: App blocked in futex() burns 14% CPU.

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@...gon ~]$ 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.

At this point I mainly want to know if I'm the only one seeing this and if 
someone has a good explanation for this, before I try to dig deeper into 
it myself. 
So, any ideas/explanations?

-- 
Jesper Juhl <jj@...osbits.net>       http://www.chaosbits.net/
Don't top-post http://www.catb.org/jargon/html/T/top-post.html
Plain text mails only, please.

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