Experimental Physics and
Industrial Control System

Michael Davidsaver via Tech-talk <[email protected]> · Mon, 16 Dec 2019 09:35:11 -0800

On 12/16/19 1:08 AM, dmnr infn via Tech-talk wrote:
> Hello everybody,
> I want to scan a variable with a polling time less than 20 msec (for example 10 msec) and I added a row inside the file menuScan.dbd but I received this error when I run my IOC:
> 
> initPeriodic: Scan rate '.01 second' is not achievable.
> 
> It seems that the max scan rate in my system with CentOS 7 is 20 msec. What can I do to reach a lower scan rate?

This message is in actuality a warning that a scan rate _may_ not be achievable.
It is still attempted.

https://github.com/epics-base/epics-base/blob/370b38e4c99057b579db94123071eaf69ac52445/modules/database/src/ioc/db/dbScan.c#L903-L907

A different message is seen when a scan thread actually isn't keeping up
for some reason.

https://github.com/epics-base/epics-base/blob/370b38e4c99057b579db94123071eaf69ac52445/modules/database/src/ioc/db/dbScan.c#L820-L825

The warning exists because software delays for such small intervals are not always
reliable.  Especially on a general purpose OS like Linux.

Can you talk about your motivating use case?
Also which version of EPICS Base is involved?

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System