[evla-sw-discuss] alerts - reliable delivery

[Pete Whiteis]

Barry Clark wrote:

>I'm disturbed that we do see missing alert packets.  This is a few orders
>of magnitude greater loss than we were questimating on the basis of no
>data, and, if the number of missing packets grows as the square of the
>data rates, as theory suggests it should, when we complete the EVLA
>conversion, we will have 50 times the traffic and 2500 times the missing
>alert packets.  Sounds like we might be in trouble.
>
>  
>
    If I'm not mistaken, the original reason we thought we might be 
missing packets was that we were only seeing one alert from the L302 Out 
of Lock condition.   Turns out that the out of lock indicator was setup 
to alert on low to high transition (wrong way), and therefore would only 
produce a single, bogus, in-alert message.    
    Now that the L302 Alert condition has been fixed, we should rig 
something up on the bench to force alerts and characterize packet loss.  
I'd say this should preceed any re-design decisions we might make.  
  

>Because TCP often takes a little vacation to think about things, in a
>real-time system we cannot have the connect-and-send in the same thread
>as something needed to actually control the system.  We would need to
>put it off on a thread of its own, with a message queue to feed it the
>things it needs to send, which raises the question of how much room we
>should allocate for the message queue, and what to do when it fills up
>(eg, when the Flagger is off the air).  
>
>  
>
We currently have the alert transmitter running from its own thread.  
When the real-time system detects an alert, it notifies the transmitter 
thread via message queue mechanism.   If worse comes to worse and the 
transmitter cannot fufill its role, then the message queue FIFO (max 
depth: 50 messages) will eventually get overwritten with most current 
data.  

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