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SubjectRe: Linux Kernel Markers - performance characterization with large IO load on large-ish system

* Alan D. Brunelle <Alan.Brunelle@hp.com> wrote:

> o All kernels start off with Linux 2.6.23-rc6 + 2.6.23-rc6-mm1
>
> o '- bt cfg' or '+ bt cfg' means a kernel without or with blktrace
> configured respectively.
>
> o '- markers' or '+ markers' means a kernel without or with the
> 11-patch marker series respectively.
>
> 38 runs without blk traces being captured (dropped hi/lo value from 40 runs)
>
> Kernel Options Min val Avg val Max val Std Dev
> ------------------ --------- --------- --------- ---------
> - markers - bt cfg 15.349127 16.169459 16.372980 0.184417
> + markers - bt cfg 15.280382 16.202398 16.409257 0.191861
>
> - markers + bt cfg 14.464366 14.754347 16.052306 0.463665
> + markers + bt cfg 14.421765 14.644406 15.690871 0.233885

actually, the pure marker overhead seems to be a regression:

> - markers - bt cfg 15.349127 16.169459 16.372980 0.184417
> + markers - bt cfg 15.280382 16.202398 16.409257 0.191861

why isnt the marker near zero-cost as it should be? (as long as they are
enabled but are not in actual use) 2% increase is _ALOT_. That's the
whole point of good probes: they do not slow down the normal kernel.

_Worst case_ it should be at most a few instructions overhead but that
does not explain the ~2% wall-clock time regression you measured here.

So there's something wrong going on - either markers have unacceptably
high cost, or the measurement is not valid.

Ingo
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