Re: [SocketCan] Problem of accuracy

[Wolfgang Grandegger <wg@grandegger.com>]

On 07/07/2012 12:02 PM, Mohamed HAMZAOUI wrote:
> Hi,
> 
> I recompiled the Kernel with activate preemption. I test cangen and
> tst-bcm-cycle but i have the same result around accuracy. I tried to
> change the application priority with nice command to high priority but
> the problem persist  !

This problem is not CAN related. Anyway, you can read/try/check:

- are high-resolution timers enabled in your kernel?
- use cyclictest [1] to measure user-space timing accuracy.
- use clock_nanosleep for periodic tasks.
- try to start cangen with "chrt -f 10 cangen can0".
- read "man sched_setscheduler" for a description of the Linux
  scheduling scheme.

[1] https://rt.wiki.kernel.org/index.php/Cyclictest

Wolfgang.

> Le jeudi 5 juillet 2012, Oliver Hartkopp a écrit :
>>
>> On 05.07.2012 10:12, Mohamed HAMZAOUI wrote:
>>>
>>> Hi,
>>>
>>> i'm a new user of socketcan and, i cross compile can-utils for my craneboard
>>> card (a card with CAN).
>>> When i run cangen with 1 ms period, the period is not respected (1,407, 1.399,
>>> ...) !
>>>
>>> How can i fix this problem ?
>>>
>>> my can version in /proc/net/can/version is "rev 20090105 abi 8" and i'm on
>>> : Linux am3517-crane 2.6.32 #1 Mon Apr 23 00:13:28 CEST 2012 armv7l GNU/Linux
>>>
>>
>> Hello Mohamed,
>>
>> first please move to the new CAN mailing list (see CC) - and don't use HTML-mails (text-only is better) ;-)
>>
>> Regarding your problem:
>>
>> Usually i get smaller derivations on my system - but your Linux box is probably not very fast, so that you can see these scheduling influences with cangen ...
>>
>> cangen is a userpace app and therefore it is usually not that precise as hrtimers inside the Linux kernel (if you don't use any realtime Linux).
>>
>> But you may try the Broadcast-Manager to place a 'cyclic sending job' inside the Linux kernel. See an example here:
>>
>> http://svn.berlios.de/wsvn/socketcan/trunk/test/tst-bcm-cycle.c
>>
>> Regards,
>> Oliver
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