Re: [Xenomai-help] i386 2.4 backport performing (too?) well

[Jan Kiszka <kiszka@domain.hid>]

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Klaas Gadeyne wrote:
> With the arrival of the 2.4 i386 adeos ipipe patch for xenomai [1], I
> decided to try to compile xenomai-trunk for a 2.4 kernel. This worked
> flawlessly, and moreover, I got excellent latency results:
> 
> I used the "same" kernel config as for our 2.4.31 rtai3.0r5 kernel,
> which is based on Takis Issaris' liveCD config.
> 
> This resulted in a maximal latency of 30 usec after a run of over 100
> minutes under heavy load (tar and dd loops, compiling, keyboard
> interrupts and ping flood) [2].
> 
> For comparison, on the same hardware platform: - the RTAI lxrt-latency
> on rtai 3.0r5 (adeos oldgen r18c1
>   patch for 2.4.31 also) test reports 38 usec
> - the latency test of xenomai 2.01 running on a 2.6.14-ipipe-1.0-10
>   kernel resulted in a latency of 80 usec.
> 
> This seems too good to be true?  Can one simply compare the results of
> the former RTAI lxrt-latency test with the xenomai latency test?

Mmh, I tend to be sceptical as well, also remembering the results
Wolfgang posted about 2.6 vs. 2.4 on low-end PPC
(https://mail.gna.org/public/xenomai-core/2005-11/msg00131.html).

Well, such worst-case improvement may be real if and only if there is
less *kernel* code in hard-irq-off sections with 2.4. The complexity of
adeos/ipipe and xenomai isn't changed between both scenarios.

But I rather think that 2.4 just stresses the caches less than 2.6, thus
is may be more tricky to trigger the real worst-case path. So, what we
already saw with PPC: 2.4 and 2.6 may likely show similar RT
performances under Xenomai, but the overall system performance is much
better on low-end! However, I would be happy if this theory is too
pessimistic.

Jan


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