Re: [RFC] CPU controllers?
- From: Sam Vilain <sam@xxxxxxxxxx>
- Date: Fri, 16 Jun 2006 09:52:16 +1200
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.
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- From: Srivatsa Vaddagiri
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