Re: [ckrm-tech] [RFC 0/4] sched: Add CPU rate caps (improved)

[MAEDA Naoaki]

Peter Williams wrote:
> I've done some informal testing with smaller values of CAP_STATS_OFFSET 
> and there is only a minor improvement.
>
> However, something that does improve behaviour for short lived tasks is 
> to increase the value of HZ.  This is because the basic unit of CPU
> allocation by the scheduler is 1/HZ and this is also the minimum time 
> (and granularity) with which sinbinning and other capping measures can 
> be implemented.  This is the fundamental limiting factor for the 
> accuracy of capping i.e. if everything worked perfectly the best 
> granularity that can be expected from capping of short lived tasks is 
> 1000 / (HZ * duration) where duration is in seconds.

I already defines CONFIG_HZ=1000. Do you suggest increasing more?

> For longer living tasks, once the initial phase has passed the half life 
> of the Kalman filters takes over from "HZ * duration" in the above 
> expression.  Reducing CAP_STATS_OFFSET will shorten the half life of the 
> filters and this in turn will make capping coarser.  On the other hand, 
> if the half lives are too big then capping will be too slow in reacting 
> to changes in a task's CPU usage patterns.  So there's a sweet spot in 
> there somewhere.  There's also an upper limit imposed by the likelihood 
> of arithmetic overflow during the calculations and this has to consider 
> the fact that the average cycle length (one of the metrics) can be quite 
> long.  The current values was based on these considerations.
>
> Peter

Thanks,
MAEDA Naoaki

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