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SubjectRe: BFS vs. mainline scheduler benchmarks and measurements
Ingo Molnar wrote:
> * Felix Fietkau <nbd@openwrt.org> wrote:
>
>> Ingo Molnar wrote:
>> > 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?
I did some tests with BFS v230 vs CFS on Linux 2.6.30 on a different
MIPS device (Atheros AR2317) with 180 MHz and 16 MB RAM. When running
iperf tests, I consistently get the following results when running the
transfer from the device to my laptop:

CFS: [ 5] 0.0-60.0 sec 107 MBytes 15.0 Mbits/sec
BFS: [ 5] 0.0-60.0 sec 119 MBytes 16.6 Mbits/sec

The transfer speed from my laptop to the device are the same with BFS
and CFS. I repeated the tests a few times just to be sure, and I will
check vmstat later.
The difference here cannot be flash related, as I ran a kernel image
with the whole userland contained in initramfs. No on-flash filesystem
was mounted or accessed.

- Felix


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