Re: brocken devfreq simple_ondemand for Odroid XU3/4?

From: Willy Wolff
Date: Wed Jun 24 2020 - 04:52:23 EST

On 2020-06-24-10-14-38, Krzysztof Kozlowski wrote:
> On Wed, Jun 24, 2020 at 10:01:17AM +0200, Willy Wolff wrote:
> > Hi Krzysztof,
> > Thanks to look at it.
> >
> > mem_gov is /sys/class/devfreq/10c20000.memory-controller/governor
> >
> > Here some numbers after increasing the running time:
> >
> > Running using simple_ondemand:
> > Before:
> > From : To
> > : 165000000 206000000 275000000 413000000 543000000 633000000 728000000 825000000 time(ms)
> > * 165000000: 0 0 0 0 0 0 0 4 4528600
> > 206000000: 5 0 0 0 0 0 0 0 57780
> > 275000000: 0 5 0 0 0 0 0 0 50060
> > 413000000: 0 0 5 0 0 0 0 0 46240
> > 543000000: 0 0 0 5 0 0 0 0 48970
> > 633000000: 0 0 0 0 5 0 0 0 47330
> > 728000000: 0 0 0 0 0 0 0 0 0
> > 825000000: 0 0 0 0 0 5 0 0 331300
> > Total transition : 34
> >
> >
> > After:
> > From : To
> > : 165000000 206000000 275000000 413000000 543000000 633000000 728000000 825000000 time(ms)
> > * 165000000: 0 0 0 0 0 0 0 4 5098890
> > 206000000: 5 0 0 0 0 0 0 0 57780
> > 275000000: 0 5 0 0 0 0 0 0 50060
> > 413000000: 0 0 5 0 0 0 0 0 46240
> > 543000000: 0 0 0 5 0 0 0 0 48970
> > 633000000: 0 0 0 0 5 0 0 0 47330
> > 728000000: 0 0 0 0 0 0 0 0 0
> > 825000000: 0 0 0 0 0 5 0 0 331300
> > Total transition : 34
> >
> > With a running time of:
> > LITTLE => 283.699 s (680.877 c per mem access)
> > big => 284.47 s (975.327 c per mem access)
>
> I see there were no transitions during your memory test.
>
> >
> > And when I set to the performance governor:
> > Before:
> > From : To
> > : 165000000 206000000 275000000 413000000 543000000 633000000 728000000 825000000 time(ms)
> > 165000000: 0 0 0 0 0 0 0 5 5099040
> > 206000000: 5 0 0 0 0 0 0 0 57780
> > 275000000: 0 5 0 0 0 0 0 0 50060
> > 413000000: 0 0 5 0 0 0 0 0 46240
> > 543000000: 0 0 0 5 0 0 0 0 48970
> > 633000000: 0 0 0 0 5 0 0 0 47330
> > 728000000: 0 0 0 0 0 0 0 0 0
> > * 825000000: 0 0 0 0 0 5 0 0 331350
> > Total transition : 35
> >
> > After:
> > From : To
> > : 165000000 206000000 275000000 413000000 543000000 633000000 728000000 825000000 time(ms)
> > 165000000: 0 0 0 0 0 0 0 5 5099040
> > 206000000: 5 0 0 0 0 0 0 0 57780
> > 275000000: 0 5 0 0 0 0 0 0 50060
> > 413000000: 0 0 5 0 0 0 0 0 46240
> > 543000000: 0 0 0 5 0 0 0 0 48970
> > 633000000: 0 0 0 0 5 0 0 0 47330
> > 728000000: 0 0 0 0 0 0 0 0 0
> > * 825000000: 0 0 0 0 0 5 0 0 472980
> > Total transition : 35
> >
> > With a running time of:
> > LITTLE: 68.8428 s (165.223 c per mem access)
> > big: 71.3268 s (244.549 c per mem access)
> >
> >
> > I see some transition, but not occuring during the benchmark.
> > I haven't dive into the code, but maybe it is the heuristic behind that is not
> > well defined? If you know how it's working that would be helpfull before I dive
> > in it.
>
> Sorry, don't know that much. It seems it counts time between overflow of
> DMC perf events and based on this bumps up the frequency.
>
> Maybe your test does not fit well in current formula? Maybe the formula
> has some drawbacks...

OK, I will read the code then.

>
> >
> > I run your test as well, and indeed, it seems to work for large bunch of memory,
> > and there is some delay before making a transition (seems to be around 10s).
> > When you kill memtester, it reduces the freq stepwisely every ~10s.
> >
> > Note that the timing shown above account for the critical path, and the code is
> > looping on reading only, there is no write in the critical path.
> > Maybe memtester is doing writes and devfreq heuristic uses only write info?
> >
> You mentioned that you want to cut the prefetcher to have direct access
> to RAM. But prefetcher also accesses the RAM. He does not get the
> contents from the air. Although this is unrelated to the problem
> because your pattern should kick ondemand as well.

Yes obvisouly. I was just describing a bit the microbenchmark and the memory pattern
access. I was suggesting that a random pattern will break the effectiveness of the
prefetcher, and as such we have a worst case situation on the memory bus.

>
> Best regards,
> Krzysztof