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SubjectRe: BFS vs. mainline scheduler benchmarks and measurements

* Felix Fietkau <nbd@openwrt.org> wrote:

> Ingo Molnar wrote:
> > * Michael Buesch <mb@bu3sch.de> wrote:
> >
> >> On Tuesday 08 September 2009 09:48:25 Ingo Molnar wrote:
> >> > Mind poking on this one to figure out whether it's all repeatable
> >> > and why that slowdown happens?
> >>
> >> I repeated the test several times, because I couldn't really believe
> >> that there's such a big difference for me, but the results were the
> >> same. I don't really know what's going on nor how to find out what's
> >> going on.
> >
> > Well that's a really memory constrained MIPS device with like 16 MB of
> > RAM or so? So having effects from small things like changing details in
> > a kernel image is entirely plausible.
>
> Normally changing small details doesn't have much of an effect. While
> 16 MB is indeed not that much, we do usually have around 8 MB free
> with a full user space running. Changes to other subsystems normally
> produce consistent and repeatable differences that seem entirely
> unrelated to memory use, so any measurable difference related to
> scheduler changes is unlikely to be related to the low amount of RAM.
> By the way, we do frequently also test the same software with devices
> that have more RAM, e.g. 32 or 64 MB and it usually behaves in a very
> similar way.

Well, Michael Buesch posted vmstat results, and they show what i have
found with my x86 simulated reproducer as well (these are Michael's
numbers):

procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa
1 0 0 15892 1684 5868 0 0 0 0 268 6 31 69 0 0
1 0 0 15892 1684 5868 0 0 0 0 266 2 34 66 0 0
1 0 0 15892 1684 5868 0 0 0 0 266 6 33 67 0 0
1 0 0 15892 1684 5868 0 0 0 0 267 4 37 63 0 0
1 0 0 15892 1684 5868 0 0 0 0 267 6 34 66 0 0

on average 4 context switches _per second_. The scheduler is not a
factor on this box.

Furthermore:

| I'm currently unable to test BFS, because the device throws strange
| flash errors. Maybe the flash is broken :(

So maybe those flash errors somehow impacted the measurements as well?

Ingo


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