Re: [PATCH] sched/fair: add support to tune PELT ramp/decay timings

[Peter Zijlstra]

On Mon, Apr 09, 2018 at 05:51:34PM +0100, Patrick Bellasi wrote:
> The PELT half-life is the time [ms] required by the PELT signal to build
> up a 50% load/utilization, starting from zero. This time is currently
> hardcoded to be 32ms, a value which seems to make sense for most of the
> workloads.
> 
> However, 32ms has been verified to be too long for certain classes of
> workloads. For example, in the mobile space many tasks affecting the
> user-experience run with a 16ms or 8ms cadence, since they need to match
> the common 60Hz or 120Hz refresh rate of the graphics pipeline.
> This contributed so fare to the idea that "PELT is too slow" to properly
> track the utilization of interactive mobile workloads, especially
> compared to alternative load tracking solutions which provides a
> better representation of tasks demand in the range of 10-20ms.

Initially the 32 was chosen to more or less correspond to the effective
scheduling period (sysctl_sched_latency based). The thinking was that if
you pick a PELT window shorter than the period, the result becomes
unstable due to not all tasks getting an equal go at things.

(of course, stuffing enough tasks on a rq will break this, but at that
point you have worse problems to deal with)

Should we retain this? Esp. with the lower end (8ms) I worry we'll see
more of those effects.


> Fortunately, since the integration of the utilization estimation
> support in mainline kernel:
> 
>    commit 7f65ea42eb00 ("sched/fair: Add util_est on top of PELT")
> 
> a fast decay time is no longer an issue for tasks utilization estimation.
> Although estimated utilization does not slow down the decay of blocked
> utilization on idle CPUs, for mobile workloads this seems not to be a
> major concern compared to the benefits in interactivity responsiveness.

By picking a smaller PELT window, the util_est window shrinks
correspondingly; is that intentional or do we want to modify
UTIL_EST_WEIGHT_SHIFT to negate the PELT window changes?