Re: Stolen and degraded time and schedulers

Jeremy Fitzhardinge
2007-03-14T04:38:04Z

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.

J