Re: [Devel] Re: Write Error: No space left on device

[Dhaval Giani <dhaval-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>]

On Tue, Oct 28, 2008 at 12:58:16PM +0530, Mukesh G wrote:
> Hi,
> 
> I need your help in clarifying some doubts...
> 
> I have setup a container for cpu. Within the container with a cpu.shares of
> 1024 (default), which is made up of 2 cpu containers (C2048; cpu.shares
> 2048) and (C1024: cpu.shares 1024).
> 
> Enclosing a cpubusy script that keeps the cpu busy performing some dumb
> processing. 2 instances of this script is executed on a dual core Intel x86
> systems (PIDS: 21804, 21805)
> 
> #!/usr/bin/perl
> 
> $goal = 2181818;
> 
> while (TRUE) {
>         for ($i=0; $i<=$goal; $i++) {
>                 $x = 0.000001;
>                 $y = sin($x);
>                 $y = $y + 0.00001;
> 
>         }
> 
>         next;
>         $y = $y + 0.01;
> }
> 
> By default, I have observed the running process becomes part of the tasks of
> the main container system. The cpu's are 100% utilized, which is correct as
> both the processes are in the same container (i.e. the parent cpu
> container).
> 
> I now add a running cpubusy script process (PID: 21804) to the C2048
> container.
> 
> Behavior 1: I noticed that the cpu utilization is still 100%, even though I
> was expecting a change in the cpu utilization as there is some enforced
> isolation. Am I reading this right?
> 

Hi Mukesh,

Not really. You would not see any change in CPU utlization. The group
scheduler is a propotional scheduler, that is it divides the CPU
bandwidth as per the propotion of the cpu shares. Now, you have only 1
process running, so it will get all the bandwidth available.

> I now add the second cpubusy script process (PID: 21805) to the C1024
> container
> 
> Behavior 2: I noticed that the cpu utilization still at 100%. This time I
> was expecting a change in cpu utilization. Not sure where I am going wrong?
>

Right, I notice you are on a dual core machine. That means both the
processes will be on different CPUs, and so you get to see the 100%
utilization for both the processes. (since a process can utilize more
than one CPU )
 
> The cpubusy script is inherently single threaded so created 2 additional
> cpubusy scripts to simulate more processes (PID: 21872, 21873). Now, I see
> the utilization change.
> 
> PID 21804: 66%
> PID 21805: 50%
> PID 21872: 50%
> PID 21873: 33%
> 
> Behavior 3: I was expecting PIDs 21872 and 21873 as they belong to the
> parent cpu container to have higher cpu utilization. The results are
> surpursing as shown above.
> 

Not really. What happens is that the CPU bandwidth available to a
control group is divided between the tasks and task groups as per their
weight. Assuming that you were running just nice 0 tasks, I would expect
C2048:C1024:PID21872:PID21873 to share CPU bandwidth in the ratio
2:1:1:1. Since there are 4 tasks, they would be split 2 on each core,
with the division of bandwidth being as per their weights. (So ideally
it should have been 80%, 40%, 40%, 40%, but is what you see).

> Now, I add the process with PID: 21872 to container system C1024 and process
> with PID: 21873
> 
> PID 21804: 66%
> PID 21873: 66%
> PID 21805: 33%
> PID 21873: 33%
> 
> Behavior 4: This is the expected behavior.
> 

Yes, absolutely true :).

Hope that helped

Thanks,
-- 
regards,
Dhaval