[Rtai] Timer resolution

[Paolo Mantegazza]

peak peak wrote:
> Hello,
> 
> I use periodic mode before calling start_rt_timer.
> 

I missed you wanted a hard tick at 1 MHz. It is a wishful hope, that 
will just cause an immediate crash, due to a stack overflow.
Take into account that, despite RTAI grabs the timer interrupt directly 
at the idt table level, a user task that does just a wait period at 200 
KHz consumes almost 40% of the CPU on my PIII 1 GHz, crashing at 300 KHz.
Do not think that a more powerful CPU will be much better. Jitter at the 
interrupt level can be even more that 5 us, and it can get even worse on 
most recent cpus. The most I've been able to get for an application has 
been with a periodic timer at 100 KHz, and to have it reliably I found 
it better to stay in kernel space.

paolo

> ------------------------------------------------------------------------
> *From:* Paolo Mantegazza <mantegazza at aero.polimi.it>
> *To:* peak peak <peakpeak05 at yahoo.com>
> *Cc:* Rtai at rtai.org
> *Sent:* Fri, March 4, 2011 8:29:47 AM
> *Subject:* Re: [Rtai] Timer resolution
> 
> peak peak wrote:
>  > Hello,
>  >
>  > - I am wondering about timer resolution.
>  > In the configuration when compiling the kernel, it is set at the 
> maximum of 1KHz
>  > On the 
> https://www.rtai.org/documentation/kilauea/html/group__timer.html, "The 
> resolution is that of the 8254 (1193180 Hz) on a UP machine".
>  > Does this relate to each  actual CPU's clock speed?
>  >
>  > - If I use start_rt_timer(nano2count(1000)). Does it mean the 
> resolution is 1000 nanoseconds? Increasing the resolution helps reduce 
> PC's load (i.e. it runs smoother), doesn't it?
>  >
> For an answer, you have to state what mode you set before calling 
> start_rt_timer.
> 
> paolo
> 
>  > Thank you in advance!
>  > 
>  >
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