Experimental Physics and
Industrial Control System

"Dalesio, Leo" <[email protected]> · Wed, 8 Jul 2009 10:29:33 -0400

Just to be brutally clear.....

Array pointers get queued - NOT the data.
If you overwrite an array record before the data is sent to the client, the array is overwritten and a second monitor is posted - that is a second pointer to the same buffer is put on the queue. So the array will be sent twice. NOT two different arrays. THe most recent array will be sent twice. So, it is important to consume these at the rate that they are produced.

There are tools for measuring IOC performance. Keep the IOC loaded under 50%. Keep the link loaded under 15%. Make sure that the client can process the data fast enough.

Bob

-----Original Message-----
From: Ralph Lange [mailto:[email protected]]
Sent: Wed 7/8/2009 10:24 AM
To: Bruno Coudoin
Cc: Dalesio, Leo; [email protected]
Subject: Re: Detecting data lost from a subscriber

Hello Bruno,

For arrays, the server side queue does not contain the values, but a 
pointer to the array.

In the case of the server queue filling up, i.e. the server generating 
more updates than CA is able to push onto the network...
...atomic data will be kept as copies in the queue, i.e. data is lost 
only if the queue gets filled up completely, in which case the server 
overwrites the last queue element each time it gets a new update.
...array data will be kept in the application (e.g. IOC database), so 
that any new update happening before the queue is empty will lead to 
data loss.

Or - in other words: A half-full server queue is a healthy situation for 
atomic data (no updates lost), and an unhealthy situation for array data 
(CA will always send the current array data).

Usually the server side queue is a good thing to handle short changes in 
network and/or client load. For arrays, as soon as you need the queue, 
you lose data.

Background: Arrays can be large. If CA would copy arrays onto the queue, 
the IOC could easily run out of memory due to a client or network 
problem, which would make the IOC far too vulnerable.

Hope this answers your question...
Ralph

On 08.07.2009 15:58 Bruno Coudoin wrote:
> Le mercredi 08 juillet 2009 à 11:08 +0200, Ralph Lange a écrit :
>   
>
>> Careful: Array data is not put into the server side queue (only 
>> references to the array), so using an array to add the sequence number 
>> would at the same time enable you to detect missing updates and create a 
>> lot of missing updates. (Probably not what you want.)
>>     
>
> I don't understand this point. In what would it make things worse, could
> you please explain me.
>   

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System