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