Re: Wrong/misleading SoftIRQ statistics in account_system_time()?

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On Sat, Sep 13, 2008 at 12:52 AM, Elad Lahav <elahav@xxxxxxxxxxxx> wrote:
> I hacked the kernel so that account_system_time() can differentiate between
> "true" soft
> IRQs and code running under local_bh_disable(). (The hack itself is quite
> ugly, but does
> the job). An experiment that sends UDP packets as quickly as possible, now
> shows that over
> 25% of the time is taken by the latter. Here's a snapshot of the /proc/stat
> values,
> normalised and displayed as percentage of the total time (similar to
> mpstat):
>
> CPU   %user   %nice    %sys %iowait    %irq   %soft  %steal  %guest %bh_dis
>   %idle
>  0    2.01    0.00   46.73    0.00    3.02   23.62    0.00    0.00   24.62
>  0.00
>  0    2.49    0.00   44.28    0.00    2.99   21.89    0.00    0.00   28.36
>  0.00
>  0    1.52    0.00   44.95    0.00    3.03   22.22    0.00    0.00   28.28
>  0.00
>  0    1.49    0.00   48.51    0.00    2.97   21.29    0.00    0.00   25.74
>  0.00
>  0    3.50    0.00   48.50    0.00    3.50   23.50    0.00    0.00   21.00
>  0.00
>  0    2.51    0.00   44.72    0.00    3.02   23.12    0.00    0.00   26.63
>  0.00
>  0    3.98    0.00   47.76    0.00    2.49   23.38    0.00    0.00   22.39
>  0.00
>  0    2.00    0.00   45.00    0.00    2.50   24.00    0.00    0.00   26.50
>  0.00
>  0    1.51    0.00   49.25    0.00    2.51   23.62    0.00    0.00   23.12
>  0.00
>
> The bh_dis field is the one I added. Clearly, this is not a negligible
> amount of processor
> time.

Interesting data you brought, Elad. So clearly my theory is wrong. I
encourage you to send your patch to LKML to get wider attention and
feedback.

To me, yes, I admit personally I have a feeling that not everybody
follow the golden rule: disable bottom halves as fast as possible. So
since everybody will likely agree that Linux kernel need to improve
responsiveness and scalability, this kind of statistic improvement
will be a great help.

Thanks for sharing Elad..

regards,

Mulyadi.

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