Re: [SocketCan] Problem of accuracy

[Wolfgang Grandegger <wg@grandegger.com>]

On 07/09/2012 05:39 PM, Mohamed HAMZAOUI wrote:
>> I compared itimer with clock_nanosleep on an ARM board and noticed the signal
>> overhead of itimer is much bigger, resulting in a higher CPU usage, than
>> clock_nanosleep upon hrtimer.
>> So I ended up using clock_nanosleep adding a constant to the last timeout.
> 
> but my problem is that when i specify 1 ms of period i have 1.3ms and

So far, you did not specify 1 ms of period. You used a delay of 1 ms
(using nanosleep).

> the problem is how to eliminate the 0.3ms. If i substitute 0.3 ms from
> all payload, what's the assurance that it does not change with CPU
> load  change ?

To understand the real-time capabilities of your system run cyclictest
again, but this time with real system load and running for more than an
hour:

  # cyclictest -t1 -p80 -n -i1000

What do you achieve?

Modify your CAN test program using clock_nanosleep() handling the period
properly, e.g. as suggested by Pavel. Here is another example:


http://www.rts.uni-hannover.de/xenomai/lxr/source/examples/rtdm/profiles/can/rtcan_rtt.c#092

What do you get with such a program?

Still, the 300us for the asynchronous write is bogus. But maybe your
system is that slow.

Wolfgang.