Messages in this thread | | | Date | Tue, 13 Mar 2007 21:37:59 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: Stolen and degraded time and schedulers |
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Dan Hecht wrote: > With your previous definition of work time, would it be that: > > monotonic_time == work_time + stolen_time ??
(By monotonic time, I presume you mean monotonic real time.) Yes, I suppose you could, but I don't think that's terribly useful. I think work_time is probably most naturally measured in cpu clock cycles rather than an actual time unit. You could convert it to ns, but I don't see the point.
I know its a term in general use, but I don't think the term "stolen time" is all that useful, particularly when we're talking about a more general notion of cpu work contributing to the progress of process execution. In the cpufreq case, time isn't "stolen" per se.
(I guess I don't like the term stolen time because you don't refer to time spent on other processes as being stolen from your process: its just processor time being distributed.)
> i.e. would you be defining stolen_time to include the time lost to > processes due to the cpu running at a lower frequency? How does this > play into the other potential users, besides sched_clock(), of stolen > time? We should make sure that the abstraction introduced here makes > sense in those places too.
Be specific. What other uses are there?
> For example, the stuff that happens in update_process_times(). I > think we'd want to account the stolen time to cpustat->steal.
I guess we could do something for that. Would we account non-full-speed cpus to it? Maybe?
How is cpustat->steal used? How does it get out to usermode?
> Also we'd probably want account for stolen time with regards to > task_running_tick(). (Though, in the latter case, maybe we first have > to move the scheduler away from assuming HZ rate decrementing of > p->time_slice to get this right. i.e. remove the tick based assumption > from the scheduler, and then maybe stolen time falls in more naturally > when accounting time slices).
I think the important part is that sched_clock() be used to actually compute how much time each process gets. The fact that a time quantum gets stolen is less important. Or do you mean something else?
> I guess taking your cpufreq as an example of work_time progressing > slower than monotonic_time (and assuming that the remaining time is > what you would call stolen), then e.g. top would report 50% of your > cpu stolen when you cpu is running at 1/2 max rate.
Yes. In the same way that clock modulation gates the cpu clock, the hypervisor effectively gates the clock by giving time to other vcpus.
> And p->time_slice would decrement at 1/2 the rate it normally did when > running at 1/2 speed. Is this the right thing to do? If so, then I > agree it makes sense to model hypervisor stolen time in terms of your > "work time".
Yes, that's my thought.
> But, if not, then maybe the amount of work you can get done during a > period of time that is not stolen and the stolen time itself are > really two different notions, and shouldn't be confused. I can see > arguments both ways.
It seems to me like a nice opportunity to solve two problems with one mechanism.
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