Re: [RFC v2 PATCH 0/8] CFS Hard limits - v2

[Pavel Emelyanov <xemul@openvz.org>]

Srivatsa Vaddagiri wrote:
> On Wed, Sep 30, 2009 at 07:55:37PM +0530, Bharata B Rao wrote:
>> On Wed, Sep 30, 2009 at 05:36:29PM +0400, Pavel Emelyanov wrote:
>>> Bharata B Rao wrote:
>>>> Hi,
>>>>
>>>> Here is the v2 post of hard limits feature for CFS group scheduler. This
>>>> RFC post mainly adds runtime borrowing feature and has a new locking scheme
>>>> to protect CFS runtime related fields.
>>>>
>>>> It would be nice to have some comments on this set!
>>> I have a question I'd like to ask before diving into the code.
>>> Consider I'm a user, that has a 4CPUs box 2GHz each and I'd like
>>> to create a container with 2CPUs 1GHz each. Can I achieve this
>>> after your patches?
>> I am not sure if I understand the GHz specification you mention here.
>> Are you saying that you want run a container with 2 CPUS with each of
>> them running at half their (frequency)capacity ?
>>
>> This hard limits scheme is about time based rate limiting where you can
>> specify a runtime(=hard limit) and a period for the container and the
>> container will not be allowed to consume more than the specified CPU time
>> within a given period.
> 
> IMO Pavel's requirement can be met with a hard limit of 25%
> 
> 	2 CPU of 1GHz = (1/2 x 4) (1/2 x 2) GHz CPUs
> 		      = 1/4 x 4 2GHz CPU
> 		      = 25% of (4 2GHz CPU)
> 
> IOW by hard-limiting a container thread to run just 0.5sec every sec on a 2GHz 
> cpu, it is effectively making progress at the rate of 1GHz?

4CPUS 25% each is not the same as 2CPUS 50% each.
OTOH making 2CPU container and setting 50% for both would work, but please
look at the problem from the end user point of view. He wants to set 50% of
the CPU power. Which setup is better 0.5/1, 1/2 or 0.25/0.5?

If you look at how tc works it proposes the user to select bandwidth in
human readable values like kbps or mbps.

> - vatsa
>