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SubjectRe: apache threads not balanced over all CPU cores, schedular problem or apache?
kyle wrote:
> Hi,
>
> Please cc me if possible.
>
> I have a machine with Intel Q9300 4-core CPU, 8GB RAM. It runs apache 2.0.63
> with traditional prefork MPM.
> It's a high traffic site, most of the time there's over 100 httpd processes
> running and at peak hours around 200.
>
> I found that most of the time, all httpd processes are running at 2 CPU
> cores only, sometime it's cpu0 and cpu1, sometime it's cpu1 and cpu3,
> sometime it's cpu0 and cpu2 ... etc. It may take several hours the processes
> "migrate" from 2 cores to another 2. To make it clear:
>
> 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?

--Andy


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