Experimental Physics and
Industrial Control System

Ralph Lange <[email protected]> · Tue, 21 Jul 2009 16:28:52 -0400

This is a non-trivial and somewhat delicate issue.

Andrew is right: If a certain gateway's priority should be higher or

lower than other clients' priorities strictly depends on your system

design. For a gateway to the outside world you would want a low priority

to minimize impact on your IOCs, for a gateway to the control room, you

would want the priority to be somewhat higher.

I would also say that adding a command line option to replace the

hard-coded setting would be the next best step.

Priority by channel? I don't know. The priority sets the task priorities

for the TCP task pair on the server (IOC) side. which is only one pair

per client. So - does CA even allow different priorities between one

client and the IOC? (I don't think so, but I might be wrong.)

Even if it did, what should the gateway do if ten different clients ask

for the same channel with ten different priorities? Average? Take the

lowest? The highest? Open ten connections?

We have to find the right compromise for the gateway between

transparency, performance, and security.

Best regards from my vacation (it's a rainy day, though),
Ralph

Andrew Johnson wrote:

Hi Ernest,

On Tuesday 21 July 2009 11:29:37 Ernest L. Williams Jr. wrote:

Shouldn't connection that the PV gateway has to IOCs be attached with a
high CA Priority?

Not necessarily, that probably depends on the design of your control system.

If you only have OPI connections coming through the gateway the current

setting is probably right.

How do we change this?

The priority of any CA channel connection is controlled by the client when it

connects to the PV, so the gateway could request a higher priority for the

channels on its client-side if it was coded to do so. However both the calls

to ca_create_channel() in the gateway code currently specify the fixed value

CA_PRIORITY_DEFAULT for their priority. Note that changing the priority of

your clients can have undesirable effects within the IOCs though if you're

not careful.

One quick way to raise the priority of your gateway channels would be to

rebuild the gateway to use something like CA_PRIORITY_ARCHIVE instead,

although this would then affect all PVs on all IOCs that the gateway connects

to.

Better solutions might be to add a command-line argument to configure that

fixed value (similar to the -p <prio> option Ralph recently added to the

caget, caput, camonitor and cainfo programs in Base), or even to make it

controllable through the gateway's configuration file on a per-channel

(regexp) basis.

I believe CosyLab has experience in making changes to the gateway code if you

don't have any effort available to do that in-house.

- Andrew

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System