Re: busted CFS group load balancer?

[Peter Zijlstra]

On Tue, 2008-11-18 at 13:30 +0100, Peter Zijlstra wrote:
> On Mon, 2008-11-17 at 23:33 -0800, Ken Chen wrote:
> > On Mon, Nov 17, 2008 at 9:19 PM, Peter Zijlstra wrote:
> > > Note that with larger cpu count and/or lower group weight we'll quickly
> > > run into numerical trouble...
> > >
> > > I would recommend trying this with the minimum weight in the order of
> > > 8-16 times number of cpus on your system.
> > >
> > > There is only so much one can do with 10 bit fixed precision math :/
> > 
> > That is probably one of the many problems.  I also found that the
> > updates to the per-cpu task_group's sched_entity load weight
> > (tg->se[cpu]->load.weight) is very problematic and very erratic.
> > 
> > The total rq_weight is calculated at one beginning of tg_shares_up(),
> > 
> >         for_each_cpu_mask(i, sd->span) {
> >                 rq_weight += tg->cfs_rq[i]->load.weight;
> >                 shares += tg->cfs_rq[i]->shares;
> >         }
> > 
> > However, the scaling of per-cpu se->load.weight in function
> > __update_group_shares_cpu() takes another lookup of
> > tg->cfs_rq[cpu]->load.weight at a different time.
> > cfs_rq[cpu].load.weight aren't always consistent across these two
> > times.  Due to these inconsistency of value taken on per cpu cfs_rq,
> > I've see tg->se[cpu]->load.weight jumping all over the place.  In our
> > environment, the cpu loads are very dynamic.  Process
> > queuing/dequeuing at high rate.
> 
> Ok, if your load values are very unstable in the order of the
> load-balance interval then you're hosed too, the same is true for the
> normal smp load-balancer.
> 
> The cgroup load-balancer makes that even more problematic.
> 
> Again, there's just very little you can do about that, except increase
> the coupling between cpus and thereby increase the overhead. Try
> decreasing 
> sysctl_sched_shares_ratelimit.


Also, lower sysctl_sched_shares_thresh to 1 or 0.


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