Using the ResUsageSAWT table to Monitor AMP Worker Tasks - comment by carrie

Vasudev,
 
The flowcontrolled field is a snapshot of the state of the AMP (either it is in flow control, or not) at the end of the logging period.  It does not tell you if the AMP was in flow control once or many times within the logging interval.  Because the metric only reflects a single sampling, it can easily miss flow control that is happening within the logging interval, but just not at the end of the logging interval.  Many flow control episodes are short-lived, just a few milliseconds.  
 
Flow control is the state that the AMP enters when it has exhausted all of its AMP worker tasks, and the queue of messages that are waiting for an available AMP worker task has reached a maximum length.   When an AMP is in flow control all work messages for that AMP are sent back to their source and retried.  Flow control is an indication of congestion on the AMP.
 
So if you are seeing the flowcontrolled metric report positive for flow control, you can assume that the AMP was at least in the flow control state some of the time, maybe even all of the time.  I believe that is an indication that the AMP is be, for that interval.  It is an indication that the AMP is being overworked.   The flow control condition should be managed.  Throttles are a good way to do that because they limit the level of work that is entering the database and take the pressure off of AMP worker tasks.    Reducing concurrency often helps high priority queries that are missing their SLAs perform better.
 
For more information about flow control read my blog posting
 
http://developer.teradata.com/blog/carrie/2009/11/controlling-the-flow-of-work-in-teradata
 
Iinformation in that posting explains why being in flow control is usually an indication of the AMP being congested, and why it can cause short queries to miss their SLA.
 
Thanks, - Carrie