Re: [RFC] CPU hard limits

[Srivatsa Vaddagiri <vatsa@in.ibm.com>]

On Sun, Jun 07, 2009 at 09:05:23PM +0530, Balbir Singh wrote:
> > On further thinking, this is not as simple as that. In above example of
> > 5 tasks on 4 CPUs, we could cap each task at a hard limit of 80%
> > (4 CPUs/5 tasks), which is still not sufficient to ensure that each
> > task gets the perfect fairness of 80%! Not just that, hard-limit
> > for a group (on each CPU) will have to be adjusted based on its task
> > distribution. For ex: a group that has a hard-limit of 25% on a 4-cpu
> > system and that has a single task, is entitled to claim a whole CPU. So
> > the per-cpu hard-limit for the group should be 100% on whatever CPU the
> > task is running. This adjustment of per-cpu hard-limit should happen
> > whenever the task distribution of the group across CPUs change - which
> > in theory would require you to monitor every task exit/migration
> > event and readjust limits, making it very complex and high-overhead.
> >
> 
> We already do that for shares right? I mean instead of 25% hard limit,
> if the group had 25% of the shares the same thing would apply - no?

yes and no. we do rebalance shares based on task distribution, but not
upon every task fork/exit/wakeup/migration event. Its done once in a while,
frequent enough to give "decent" fairness!

> > Balbir,
> >        I dont think guarantee can be met easily thr' hard-limits in
> > case of CPU resource. Atleast its not as straightforward as in case of
> > memory!
> 
> OK, based on the discussion - leaving implementation issues out,
> speaking of whether it is possible to implement guarantees using
> shares? My answer would be
> 
> 1. Yes - but then the hard limits will prevent you and can cause idle
> times, some of those can be handled in the implementation. There might
> be other fairness and SMP concerns about the accuracy of the fairness,
> thank you for that data.
> 2. We'll update the RFC (second version) with the findings and send it
> out, so that the expectations are clearer
> 3. From what I've read and seen there seems to be no strong objection
> to hard limits, but some reservations (based on 1) about using them
> for guarantees and our RFC will reflect that.
> 
> Do you agree?

Well yes, guarantee is not a good argument for providing hard limits.
Pay-per-use kind of usage would be a better argument IMHO.

- vatsa