Re: [RFC v4 0/8] TurboSched: A scheduler for sustaining Turbo Frequencies for longer durations

[Pavel Machek <pavel@ucw.cz>]

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Hi!

> >> Abstract
> >> ========
> >>
> >> The modern servers allows multiple cores to run at range of frequencies
> >> higher than rated range of frequencies. But the power budget of the system
> >> inhibits sustaining these higher frequencies for longer durations.
> > 
> > Thermal budget?
> 
> Right, it is a good point, and there can be possibility of Thermal throttling
> which is not covered here.
> But the thermal throttling is less often seen in the servers than the throttling
> due to the Power budget constraints. Also one can change the power cap which leads
> to increase in the throttling and task packing can handle in such
> cases.

Ok. I thought you are doing this due to thermals. If I understand
things correctly, you can go over thermal limits for a few seconds
before the silicon heats up. What is the timescale for power budget?

> BTW, Task packing allows few more cores to remain idle for longer time, so
> shouldn't this decrease thermal throttles upto certain extent?

I guess so, yes.

> > >> These numbers are w.r.t. `turbo_bench.c` multi-threaded test benchmark
> >> which can create two kinds of tasks: CPU bound (High Utilization) and
> >> Jitters (Low Utilization). N in X-axis represents N-CPU bound and N-Jitter
> >> tasks spawned.
> > 
> > Ok, so you have description how it causes 13% improvements. Do you also have metrics how
> > it harms performance.. how much delay is added to unimportant tasks etc...?
> > 
> 
> Yes, if we try to pack the tasks despite of no frequency throttling, we see a regression
> around 5%. For instance, in the synthetic benchmark I used to show performance benefit,
> for lower count of CPU intensive threads (N=2) there is -5% performance drop.
> 
> Talking about the delay added to an unimportant tasks, the result can be lower throughput
> or higher latency for such tasks.

Thanks. I believe it would be good to mention disadvantages in the
documentation, too.

Best regards,
							Pavel
							
-- 
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