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SubjectRe: [ckrm-tech] [RFC 0/4] sched: Add CPU rate caps (improved)
MAEDA Naoaki wrote:
> Peter Williams wrote:
>
>> 4. Overhead Measurements. To measure the implications for overhead
>> introduced by these patches kernbench was used on a dual 500Mhz
>> Centrino SMP system. Runs were done for a kernel without these
>> patches applied, one with the patches applied but no caps being used
>> and one with the patches applied and running kernbench with a soft cap
>> of zero (which would be inherited by all its children).
>>
>> Average Optimal -j 8 Load Run:
>>
>> Vanilla Patch Applied Soft Cap 0%
>>
>> Elapsed Time 1056.1 (1.92) 1048.2 (0.62) 1064.1 (1.59)
>> User Time 1908.1 (1.09) 1895.2 (1.30) 1926.6 (1.39)
>> System Time 181.7 (0.60) 177.5 (0.74) 173.8 (1.07)
>> Percent CPU 197.6 (0.55) 197.0 (0) 197.0 (0)
>> Context Switches 49253.6 (136.31) 48881.4 (92.03) 92490.8 (163.71)
>> Sleeps 28038.8 (228.11) 28136.0 (250.65) 25769.4 (280.40)
>
> I tried to run kernbench with hard cap, and then it spent a very
> long time on "Cleaning souce tree..." phase. Because this phase
> is not CPU hog, my expectation is that it act as without cap.
>
> That can be reproduced by just running "make clean" on top of a
> kernel source tree with hard cap.
>
> % /usr/bin/time make clean
> 1.62user 0.29system 0:01.90elapsed 101%CPU (0avgtext+0avgdata
> 0maxresident)k
> 0inputs+0outputs (0major+68539minor)pagefaults 0swaps
>
> # Without cap, it returns almost immediately
>
> % ~/withcap.sh -C 900 /usr/bin/time make clean
> 1.61user 0.29system 1:26.17elapsed 2%CPU (0avgtext+0avgdata 0maxresident)k
> 0inputs+0outputs (0major+68537minor)pagefaults 0swaps
>
> # With 90% hard cap, it takes about 1.5 minutes.

This is harder capping than I would expect. I'll look into probable
causes. It could be caused by the simplification I made to the
calculation of sinbin time.

>
> % ~/withcap.sh -C 100 /usr/bin/time make clean
> 1.64user 0.34system 3:31.48elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k
> 0inputs+0outputs (0major+68538minor)pagefaults 0swaps
>
> # It became worse with 10% hard cap.

And so it should.

>
> % ~/withcap.sh -c 900 /usr/bin/time make clean
> 1.63user 0.28system 0:01.89elapsed 100%CPU (0avgtext+0avgdata
> 0maxresident)k
> 0inputs+0outputs (0major+68537minor)pagefaults 0swaps
>
> # It doesn't happen with soft cap.

That's because soft caps allow you to go over the cap if no other tasks
want the CPU.

Thanks for the feedback,
Peter
--
Peter Williams pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce
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