Re: Periodic scan rate less than 20 msec

[dmnr infn via Tech-talk <tech-talk@aps.anl.gov>]

Hi,

the version of EPICS Base involved is 7.0.3. 

I measured with the oscilloscope the interval between two retransmissions with the SCAN field and I saw that it is not possible to go below 20ms.

Increasing the baud rate I would like to be able to scan as much variable as possible in the shorted possible time.

I am new on EPICS, I don't know if this is the better way.

Thank you.

Il giorno lun 16 dic 2019 alle ore 18:35 Michael Davidsaver <mdavidsaver@gmail.com> ha scritto:

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?