Re: [PATCH V2 0/3] Introduce Thermal Pressure

From: Ingo Molnar <mingo@kernel.org>
To: Thara Gopinath <thara.gopinath@linaro.org>
Cc: mingo@redhat.com, peterz@infradead.org, rui.zhang@intel.com,
	linux-kernel@vger.kernel.org, amit.kachhap@gmail.com,
	viresh.kumar@linaro.org, javi.merino@kernel.org,
	edubezval@gmail.com, daniel.lezcano@linaro.org,
	vincent.guittot@linaro.org, nicolas.dechesne@linaro.org,
	bjorn.andersson@linaro.org, dietmar.eggemann@arm.com,
	Quentin Perret <quentin.perret@arm.com>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>
Subject: Re: [PATCH V2 0/3] Introduce Thermal Pressure
Date: Wed, 17 Apr 2019 20:29:32 +0200	[thread overview]
Message-ID: <20190417182932.GB5140@gmail.com> (raw)
In-Reply-To: <5CB75FD9.3070207@linaro.org>

* Thara Gopinath <thara.gopinath@linaro.org> wrote:

> 
> On 04/17/2019 01:36 AM, Ingo Molnar wrote:
> > 
> > * Thara Gopinath <thara.gopinath@linaro.org> wrote:
> > 
> >> The test results below shows 3-5% improvement in performance when
> >> using the third solution compared to the default system today where
> >> scheduler is unware of cpu capacity limitations due to thermal events.
> > 
> > The numbers look very promising!
> 
> Hello Ingo,
> Thank you for the review.
> > 
> > I've rearranged the results to make the performance properties of the 
> > various approaches and parameters easier to see:
> > 
> >                                          (seconds, lower is better)
> > 
> > 			                 Hackbench   Aobench   Dhrystone
> >                                          =========   =======   =========
> > Vanilla kernel (No Thermal Pressure)         10.21    141.58        1.14
> > Instantaneous thermal pressure               10.16    141.63        1.15
> > Thermal Pressure Averaging:
> >       - PELT fmwk                             9.88    134.48        1.19
> >       - non-PELT Algo. Decay : 500 ms         9.94    133.62        1.09
> >       - non-PELT Algo. Decay : 250 ms         7.52    137.22        1.012
> >       - non-PELT Algo. Decay : 125 ms         9.87    137.55        1.12
> > 
> > 
> > Firstly, a couple of questions about the numbers:
> > 
> >    1)
> > 
> >       Is the 1.012 result for "non-PELT 250 msecs Dhrystone" really 1.012?
> >       You reported it as:
> > 
> >              non-PELT Algo. Decay : 250 ms   1.012                   7.02%
> 
> It is indeed 1.012. So, I ran the "non-PELT Algo 250 ms" benchmarks
> multiple time because of the anomalies noticed.  1.012 is a formatting
> error on my part when I copy pasted the results into a google sheet I am
> maintaining to capture the test results. Sorry about the confusion.

That's actually pretty good, because it suggests a 35% and 15% 
improvement over the vanilla kernel - which is very good for such 
CPU-bound workloads.

Not that 5% is bad in itself - but 15% is better ;-)

> Regarding the decay period, I agree that more testing can be done. I 
> like your suggestions below and I am going to try implementing them 
> sometime next week. Once I have some solid results, I will send them 
> out.

Thanks!

> My concern regarding getting hung up too much on decay period is that I 
> think it could vary from SoC to SoC depending on the type and number of 
> cores and thermal characteristics. So I was thinking eventually the 
> decay period should be configurable via a config option or by any other 
> means. Testing on different systems will definitely help and maybe I am 
> wrong and there is no much variation between systems.

Absolutely, so I'd not be against keeping it a SCHED_DEBUG tunable or so, 
until there's a better understanding of how the physical properties of 
the SoC map to an ideal decay period.

Assuming PeterZ & Rafael & Quentin doesn't hate the whole thermal load 
tracking approach. I suppose there's some connection of this to Energy 
Aware Scheduling? Or not ...

Thanks,

	Ingo