Re: PREEMPT_RT patch vs RTAI/Xenomai

[Armin Steinhoff <armin@steinhoff.de>]

Robert Schwebel wrote:
> On Thu, May 13, 2010 at 10:01:12AM +0200, Armin Steinhoff wrote:
>   
>> I did a test with user space based CAN driver.
>>     
> The Linux CAN interface is SocketCAN. Do you see a usecase where this
> doesn't fit?
>   

IMHO, the SocketCAN interface is simply an overkill for the handling of
small CAN messages.
I estimate that the  amount of executed code for handling of a single
CAN frame is much bigger as the frame itself :)
Every read and write action creates context switches ...

This is not the case with the user space based driver.  Same story with
our PROFIBUS-DP drivers ...

>> Already the standard distribution of SUSE 11.2 (non RT) was able to
>> handle 1000 frames per seconds sent by a QNX6 machine !!
>>     
>
> Realtime != fast.
>   

But a small response time is a technological requirement in order to
meet deadlines.
The standard kernel is a _good base_ in order to implement predictive
behavior ... this would not the case if the response time would be in
the range of 100us.
OK ... you can have real-time behavior with a response time of 100us ..
but this would be useless for most real-time applications.

>> The latency test of PREEMPT_RT shows a latency of ~10us for a
>> dual-core box at 1.8GHz.
>>     
>
> It depends on the load.
>   

It depends on the load and the used priorities.

--Armin