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

[Wolfgang Grandegger <wg@domain.hid>]

Philippe Gerum wrote:
> 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.

Yes, I know. That's why I have said approximately and why I have not yet 
posted figures for the MPC 5200, for example. Nevertheless, Xenomai is
pointing into the right direction and support for PowerPC is already 
excellent (on the level with X86 :-). So far, building and running 
Xenomai worked fine without any real problems. Thanks a lot.

Wolfgang.