Messages in this thread | | | Subject | Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in scale_rt_capacity | From | Luca Abeni <> | Date | Tue, 15 Dec 2015 14:39:44 +0100 |
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On 12/15/2015 01:43 PM, Vincent Guittot wrote: [...] >>>>> I agree that if the WCET is far from reality, we will underestimate >>>>> available capacity for CFS. Have you got some use case in mind which >>>>> overestimates the WCET ? >>>>> If we can't rely on this parameters to evaluate the amount of capacity >>>>> used by deadline scheduler on a core, this will imply that we can't >>>>> also use it for requesting capacity to cpufreq and we should fallback >>>>> on a monitoring mechanism which reacts to a change instead of >>>>> anticipating it. >>>> >>>> I think a more "theoretically sound" approach would be to track the >>>> _active_ utilisation (informally speaking, the sum of the utilisations >>>> of the tasks that are actually active on a core - the exact definition >>>> of "active" is the trick here). >>> >>> >>> The point is that we probably need 2 definitions of "active" tasks. >> >> Ok; thanks for clarifying. I do not know much about the remaining capacity >> used by CFS; however, from what you write I guess CFS really need an >> "average" >> utilisation (while frequency scaling needs the active utilisation). > > yes. this patch is only about the "average" utilization Ok; so, I think that the approach of this patch is too pessimistic (it uses the "worst-case" utilisation as an estimation of the average one).
>>> This one >>> should be updated quite often with the wake up and the sleep of tasks >>> as well as the throttling. >> >> Strictly speaking, the active utilisation must be updated when a task >> wakes up and when a task sleeps/terminates (but when a task >> sleeps/terminates >> you cannot decrease the active utilisation immediately: you have to wait >> some time because the task might already have used part of its "future >> utilisation"). >> The active utilisation must not be updated when a task is throttled: a >> task is throttled when its current runtime is 0, so it already used all >> of its utilisation for the current period (think about two tasks with >> runtime=50ms and period 100ms: they consume 100% of the time on a CPU, >> and when the first task consumed all of its runtime, you cannot decrease >> the active utilisation). > > I haven't read the paper you pointed in the previous email but it's > on my todo list. Does the GRUB-PA take into account the frequency > transition when selecting the best frequency ? I do not know... As far as I understand, the GRUB-PA approach is simple: if the active utilisation of SCHED_DEADLINE tasks is Ua, then the CPU frequency can be reduced to the maximum possible frequency multiplied by Ua (of course, this must be adjusted a little bit, because the original GRUB-PA paper only considered real-time/SCHED_DEADLINE tasks... To leave some CPU time for other tasks, you have to increase Ua a little bit).
Some time ago one of the authors of the GRUB-PA paper told me that they evaluated the performance of the algorithm by simulating the behaviour of a real CPU, but I do not know the details, and I do not know if they took the frequency transition into account.
Luca
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