On 10/16/2018 03:33 AM, Ingo Molnar wrote: > > * Thara Gopinath <thara.gopin...@linaro.org> wrote: > >>>> Regarding testing, basic build, boot and sanity testing have been >>>> performed on hikey960 mainline kernel with debian file system. >>>> Further aobench (An occlusion renderer for benchmarking realworld >>>> floating point performance) showed the following results on hikey960 >>>> with debain. >>>> >>>> Result Standard >>>> Standard >>>> (Time secs) Error >>>> Deviation >>>> Hikey 960 - no thermal pressure applied 138.67 6.52 >>>> 11.52% >>>> Hikey 960 - thermal pressure applied 122.37 5.78 >>>> 11.57% >>> >>> Wow, +13% speedup, impressive! We definitely want this outcome. >>> >>> I'm wondering what happens if we do not track and decay the thermal >>> load at all at the PELT level, but instantaneously decrease/increase >>> effective CPU capacity in reaction to thermal events we receive from >>> the CPU. >> >> The problem with instantaneous update is that sometimes thermal events >> happen at a much faster pace than cpu_capacity is updated in the >> scheduler. This means that at the moment when scheduler uses the >> value, it might not be correct anymore. > > Let me offer a different interpretation: if we average throttling events > then we create a 'smooth' average of 'true CPU capacity' that doesn't > fluctuate much. This allows more stable yet asymmetric task placement if > the thermal characteristics of the different cores is different > (asymmetric). This, compared to instantaneous updates, would reduce > unnecessary task migrations between cores. > > Is that accurate?
Yes. I think it is accurate. I will also add that if we don't average throttling events, we will miss the events that occur in between load balancing(LB) period. > > If the thermal characteristics of the cores is roughly symmetric and the > measured CPU-intense load itself is symmetric as well, then I have > trouble seeing why reacting to thermal events should make any difference > at all. In this scenario, i agree that scheduler reaction to thermal events should not make any difference in fact we should not observe any improvement or degradation in performance. > > Are there any inherent asymmetries in the thermal properties of the > cores, or in the benchmarked workload itself? The benchmarked workload , meaning aobench? I don't think there arre any asymmetries there. On Hikey960, there are two clusters with different frequency domains. So yes I will say there is asymmetry there. Asides, IMHO, any other tasks running on the system can create an inherent asymmetry as cpu utilizations can vary. Regards Thara > > Thanks, > > Ingo > -- Regards Thara
