linux-kernel - Re: apache threads not balanced over all CPU cores, schedular problem or apache?

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Date:	Fri, 14 Nov 2008 16:08:06 +0000
From:	Rahul Pydimukkala <prahul@....iitk.ac.in>
To:	Andy Lutomirski <luto@...ealbox.com>
Cc:	kyle <kylewong@...tha.com>, linux-kernel@...r.kernel.org
Subject: Re: apache threads not balanced over all CPU cores, schedular problem or apache?

On 22:48 Thu 13 Nov     , Andy Lutomirski wrote:
> kyle wrote:

> >top - 12:44:42 up 8 days, 15:45,  7 users,  load average: 2.07, 1.86, 2.02
> >Tasks: 251 total,   1 running, 249 sleeping,   0 stopped,   1 zombie
> >Cpu0  :  0.5%us,  0.5%sy,  0.0%ni, 99.0%id,  0.0%wa,  0.0%hi,  0.0%si,
> >0.0%st
> >Cpu1  : 14.6%us,  4.4%sy,  0.0%ni, 62.6%id, 17.0%wa,  0.0%hi,  1.5%si,
> >0.0%st
> >Cpu2  :  4.3%us,  1.4%sy,  0.0%ni, 93.8%id,  0.0%wa,  0.5%hi,  0.0%si,
> >0.0%st
> >Cpu3  :  0.0%us,  0.0%sy,  0.0%ni, 98.0%id,  2.0%wa,  0.0%hi,  0.0%si,
> >0.0%st
> >Mem:   8117172k total,  8060936k used,    56236k free,   357488k buffers
> >Swap:   208824k total,     8436k used,   200388k free,  5765168k cached
> 
> Looks like you're mostly idle or waiting for I/O.  From this 
> information, it looks like the kernel is probably doing the right thing: 
> keeping a lot of processes that share the same memory and run for very 
> short periods of time on the same cores to minimize cacheline bouncing.
> 
> >
> >All httpd are running on cpu1 and cpu2 only for several hours, after that,
> >all httpd may run on cpu1 and cpu3, or cpu2 and cpu3, etc. again for 
> >several
> >hours.
> >
> >Since the apache is in prefork mode, every httpd instance should be quite
> >independant, I suppose the CPU schedular should balance the processes over
> >the avaiable cores. However never seen httpd processes evenly distributed 
> >to
> >3 cores, or 4 cores.
> 
> Is there any reason to expect it to run faster if it were balanced evenly?

Totally convincing. Thanks Andy for making this clear. I/O-bound processes 
reap no advantage from processor-level parallelism. 

-
Rahul



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