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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
