Re: [RFC] CPU controllers?

[Sam Vilain <sam@vilain.net>]

Srivatsa Vaddagiri wrote:
> One possibility is to add a basic controller, that addresses some minimal
> requirements, to begin with and progressively enhance it capabilities. From this
> pov, both the f-series resource group controller and cpu rate-cap seem to be 
> good candidates for a minimal controller to begin with.
>
> Thoughts?
>   

Sounds like you're on the right track, but I don't know whether we can
truly be happy making the performance/guarantee trade-off decision for
the user.

You could grossly put the solutions into several camps;

1. solutions which have very low impact and provide soft assurances only
2. solutions which provide hard limits
3. solutions which provide guarantees

I think it's almost invariant that the latter solutions have more of a
performance impact, and that it's quite important that normal system
throughput does not suffer from the "scheduling namespace" solution that
we come up with.

> Salient features of various CPU controllers that have been proposed so far are
> summarized below. I have not captured OpenVZ and Vserver controller aspects
> well. Request the maintainers to fill-in!
>   [...]
> 2. Timeslice scaling (Maeda Naoaki and Kurosawa Takahiro)
>
> Features:
> 	* Provide guaranteed CPU execution rate on a per-task-group basis
> 	  Guarantee provided over an interval of 5 seconds.
> 	* Hooked to Resource Group infrastructure currently and hence 
> 	  guarantee/limit set thr' Resource Group's RCFS interface.
> 	* Achieves guaranteed execution by scaling down timeslice of tasks
> 	  who are above their guaranteed execution rate. Timeslice can be 
> 	  scaled down only to a minimum of 1 slice.
> 	* Does not scale down timeslice of interactive tasks (even if their
> 	  CPU usage is beyond what is guaranteed) and does not avoid requeue
> 	  of interactive tasks.
> 	* Patch is quite simple
>
> Limitations:
> 	* Does not support limiting task-group CPU execution rate
>
> Drawbacks:
> 	(Some of the drawbacks listed are probably being addressed currently 
> 	 with a redesign - which we are yet to see)
>
> 	* Interactive tasks (and their requeuing) can come in the way of
> 	  providing guaranteed execution rate to other tasks
> 	* SMP load balancing does not take into account guarantee provided to 
> 	  task groups.
> 	* It may not be possible to restrict CPU usage of a task group to only 
> 	  its guaranteed usage if the task-group has large number of tasks 
> 	  (each task is run for a minimum of 1 timeslice)
> 	* May not handle bursty loads
> 	
>   [...]
> 4. VServer CPU controller
>
> Features:
> 	- Token-bucket based
>   

The VServer scheduler is also timeslice scaling - it just uses the token
bucket to know how much to scale the timeslices. It doesn't care about
interactive bonuses, although it does lessen the interactivity bonus a
notch or two (to -5..+5).

This means that it's performance neutral in the general case.

> Drawbacks:
> 	- ?
>   

It fits into category 1 (or, using Herbert Poetzl's enhancements, 2), so
does not provide guarantees.

> Limitations:
> 	- ?

Doesn't deal with huge numbers of processes; but with task group ulimits
that problem goes away in practice.

Sam.