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

[Philippe Gerum <rpm@xenomai.org>]

Wolfgang Grandegger wrote:
> Jan Kiszka wrote:
> 
>> 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.
> 
> 
> In the meantime I have measured latencies a few PowerPC systems with 
> Xenomai 2.1 under Linux 2.4 and 2.6 and RTAI 3.0r5 (using RTHAL) under 
> 2.4. The latency figures for Xenomai compared with RTAI 3.0r5 are 
> roughly a factor of two worse but I have not realized a significant 
> difference between 2.4 and 2.6 (with Xenmomai, of course).

We still need to compare pure kernel-space executions since RTAI does 
not support user-space for PPC, but v2.1 does not provide the kernel 
space test yet, due to the build system refactoring. Still pondering how 
to introduce this back into the codebase.

> 
> Wolfgang.
> 
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-- 

Philippe.