Re: device support for R3.14.1 Linux IOC

[Andrew Johnson <anj@aps.anl.gov>]

Dylan Maher Loughnan wrote:

> 	I am  experimenting with EPICS base R3.14.1. running it on a 
> standard RedHat7.3 machine. I have written device support for a few NI DAQ 
> cards, which is going very well. 

Great.  If you're willing to make this code available to other sites when 
it's finished, please submit entries for those cards to the EPICS Hardware 
Support list. </commercial>

> The scan period seems to be consistently extended by an 100th 
> of a second, when using device support.  I first noticed this when 
> I toggled a binary out at 10Hz to generate a square wave.  I saw the 
> effect very clearly on the oscilloscope.

> Here is the ca-monitor with empty device support
>
>  DML:Bo0                        02/05/03 16:06:27.070044  
>  DML:Bo0                        02/05/03 16:06:27.280062  
>  DML:Bo0                        02/05/03 16:06:27.490039  
>  DML:Bo0                        02/05/03 16:06:27.700034  
>  DML:Bo0                        02/05/03 16:06:27.910037  
>  DML:Bo0                        02/05/03 16:06:28.120074  
>  DML:Bo0                        02/05/03 16:06:28.330045  
>  DML:Bo0                        02/05/03 16:06:28.540051  
>  DML:Bo0                        02/05/03 16:06:28.750047  
>  DML:Bo0                        02/05/03 16:06:28.960049  
>  
> Using soft records, there is no such problem.
>
> Has anyone else noticed this?

Yes, and I spent some time looking at it many months ago, although I can't 
remember the exact details now.  IIRC it occurs because the linux kernel 
uses a 100Hz clock tick, and doesn't do timer arithmetic to an accuracy 
greater than that tick, so depending on how long your scan task takes to 
execute, the system call that implements nanosleep() frequently rounds up 
twice, adding an extra 0.01 seconds to the delay.

This is definitely not due to an EPICS bug (I checked that at the time), 
and I don't think there's any easy way to get around it other than 
possibly explore the high resolution timer kernel patches that are 
supposed to exist - I've never tried them though.

- Andrew
--
Tongue-twister: Say "Peggy Babcock" 10 times without stumbling...