Messages in this thread | | | Subject | Re: [RFC PATCH v2] sched: Limit idle_balance() | From | Jason Low <> | Date | Tue, 23 Jul 2013 21:24:08 -0700 |
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On Tue, 2013-07-23 at 16:36 +0530, Srikar Dronamraju wrote: > > > > A potential issue I have found with avg_idle is that it may sometimes be > > not quite as accurate for the purposes of this patch, because it is > > always given a max value (default is 1000000 ns). For example, a CPU > > could have remained idle for 1 second and avg_idle would be set to 1 > > millisecond. Another question I have is whether we can update avg_idle > > at all times without putting a maximum value on avg_idle, or increase > > the maximum value of avg_idle by a lot. > > May be the current max value is a limiting factor, but I think there > should be a limit to the maximum value. Peter and Ingo may help us > understand why they limited to the 1ms. But I dont think we should > introduce a new variable just for this.
You're right. As Peter recently mentioned, avg_idle is only used for idle_balance() anyway, so we should just use the existing avg_idle estimator.
> > > > > Should we take the consideration of whether a idle_balance was > > > successful or not? > > > > I recently ran fserver on the 8 socket machine with HT-enabled and found > > that load balance was succeeding at a higher than average rate, but idle > > balance was still lowering performance of that workload by a lot. > > However, it makes sense to allow idle balance to run longer/more often > > when it has a higher success rate. > > > > If idle balance did succeed, then it means that the system was indeed > imbalanced. So idle balance was the right thing to do. May be we chose > the wrong task to pull. May be after numa balancing enhancements go in, > we pick a better task to pull atleast across nodes. And there could be > other opportunities/strategies to select a right task to pull. > > Again, schedstats during the application run should give us hints here. > > > > I am not sure whats a reasonable value for n can be, but may be we could > > > try with n=3. > > > > Based on some of the data I collected, n = 10 to 20 provides much better > > performance increases. > > > > I was saying it the other way. > your suggestion is to run idle balance once in n runs .. where n is 10 > to 20. > My thinking was to not run idle balance once in n unsuccessful runs.
When I suggested N is 20, that means that if the average idle time of a CPU is 1,000,000 ns, then we stop idle balancing if the average cost to idle balance within a domain would come up to be greater than (1,000,000 ns / 20) = 50,000 ns. In the v2 patch, N helps determine the maximum duration in which we allow each idle_balance() to run.
Thanks, Jason
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