Re: Stolen and degraded time and schedulers

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
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