[Devel] Re: Understanding CPU containers

[Dhaval Giani]

On Wed, Sep 09, 2009 at 04:00:27PM +0530, Mukesh G wrote:
> Hi,
>        I am trying to understand the behavior of CPU containers as I
> am unable to explain few things.
> - Built the latest kernel 2.6.30.5 and installed on my Intel core2Duo desktop
> 
> - Mounted the cpu subsystem using
> 
>       - mount -t cgroup -ocpu cgroup /containers/cpu/
> 
> - Created 3 sub directories under /containers/cpu
> 
>         - 512 for cpu.shares=512
> 
>         - 1024 for cpu.shares=1024
> 
>         - 2048 for cpu.shares=2048
> 
> - Created 3 bash terminals and attached each one to the individual cpu
> sub system using the /bin/echo command. This essentially allows any
> process created by the shells to be automatically added to the cpu
> subsystem to which the shell belongs
> 
> - Ran a compute intensive benchmark “openssl speed aes-256-cbc”
> benchmark on all the shells at the same time.
> 
> - Enclosing the numbers from the run…
> 
>     - Observed CPU Utilization : 99.8 , 49.9 , 49.9
> 
> Results from the run
> 
> The 'numbers' are in 1000s of bytes per second processed.
> 
> CPU shares allocation
> 
>                             16 bytes                     64 bytes
>             256 bytes             1024 bytes             8192 bytes
> 
> 512                        35459.95k                   43441.75k
>           46660.35k            46707.71k              77040.30k
> 
> 1024                      57448.63k                    44050.54k
>           46558.98k            46633.03k              47252.76k
> 
> 2048                    71186.36k                    87142.02k
>         92513.79k            93769.05k              94795.09k
> 
> 1024 vs 512          1.62                            1.01
>             0.99                     0.99                      0.61
> 
> 2048 vs 1024         1.23                           1.97
>            1.98                      2.01                     2
> 
> Observations
> 
>         - Unless the cpu resources are overcommitted, there is no
> value in the allocating shares to the containers.
>         - 2048 vs 1024 cpu containers, the scale is 2X, except for 16 bytes
>         - 512 vs 1024 cpu containers, there is no difference at all,
> except for 16 bytes
>         - 512 vs 1024 cpu containers, there is no difference for 8192
> bytes as the remaining 2 openssl runs are complete.

(word wrapping has broken your table for me. I will try to decipher it
and post observations on it later on.)

>         - For CPU bound, there has to be an over commit on the CPUs
> otherwise the share allocation does not matter

Bharata has posted some patches recently at
http://lwn.net/Articles/348578/ which allow one to throttle the amount
of CPU bandwidth available to a cgroup. Any feedback on that will be
great!

>         - One cannot dynamically assign cpus to the container, by
> default, it runs on the no of cores available.

Right. But you can use cpusets to carve out the system if you want to.

-- 
regards,
Dhaval
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