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Date: Wed, 29 Feb 2012 19:40:47 +0100
From: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
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Subject: Re: [Xenomai-help] Execution time profiling
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To: Charles Lesire-Cabaniols <charles.lesire@domain.hid>
Cc: xenomai@xenomai.org

On 02/29/2012 07:03 PM, Charles Lesire-Cabaniols wrote:
> 2012/2/29 Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
> 
>> On 02/29/2012 06:52 PM, Charles Lesire-Cabaniols wrote:
>>> 2012/2/29 Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
>>>
>>>> On 02/29/2012 06:29 PM, Charles Lesire-Cabaniols wrote:
>>>>> Hi guys,
>>>>>
>>>>> I have installed a Debian+Xenomai (2.6.0) OS on my Gumstix Overo.
>>>>>
>>>>> I want to evaluate the execution time of a simple program, executed as
>> a
>>>>> real-time thread.
>>>>>
>>>>> I definitely wonder about which functions to use, as I have completely
>>>>> inconsistent measures.
>>>>> I have tried using rt_timer_read, rt_timer_tsc, clock_gettime.
>>>>> I also directly read the CNNT register with ARM instructions (which is
>>>> the
>>>>> only one I think correct) in order to have a (good?) reference.
>>>>> (...)
>>>>> What am I doing wrong?
>>>>
>>>> So, you should printf("%Lu %Lu\n", rt_timer_read(), rt_timer_tsc());
>>>>
>>>> Not printf("%lu", ...)
>>>>
>>>> --
>>>>                                            Gilles.
>>>>
>>>
>>> Effectively, that looks cleaner, thanks:
>>>
>>> ----- Xenomai rt_timer_read -----
>>> start: 49166276042 ; end: 49166432273 ; (s-e): 156231 ; CET: 156231
>>> ----- Xenomai rt_timer_tsc -----
>>> start: 639161547 ; end: 639163539 ; (s-e): 1992 ; CET: 1992
>>> ----- Xenomai clock_gettime -----
>>> [ s] start: 946684855 ; end: 946684855 ; (s-e): 0 ; CET: 0
>>> [ns] start: 275520245 ; end: 275677089 ; (s-e): 156844 ; CET: 156844
>>>
>>> My ARM instruction reads 110554.
>>>
>>> Which Xenomai function should I use?
>>> Which one is supposed to be the more accurate?
>>> Does rt_timer_read return nsecs?
>>> What is the unit of rt_timer_tsc?
>>
>> rt_timer_tsc uses whatever hardware counter is available, you need
>> rt_timer_tsc2ns or rt_timer_ns2tsc to convert between this unit to and
>> from nanoseconds.
>>
>> For more details, see:
>>
>>
>> http://www.xenomai.org/documentation/xenomai-2.6/html/api/group__native__timer.html
>>
>> Depending on how xenomai user-space was compiled, rt_timer_tsc should
>> have the lowest overhead.
>>
> 
> And are the default options the best ones?

Yes, but I am not sure the debian package uses the default one. Please
post here the disassembly of rt_timer_tsc, I will tell you if your
system is compiled for the lowest overhead.

> How can I tune the TSC performance?

You use the --enable-arm-tsc or --disable-arm-tsc option of xenomai
configure script. The default is --enable-arm-tsc=kuser and should be
the best option. If you pass --disable-arm-tsc, rt_timer_tsc will emit a
system call.

-- 
                                                                Gilles.