Re: FLNK loop

[Andrew Johnson <anj@aps.anl.gov>]

Chestnut, Ronald P. wrote:
> Yes. Records are "Active" until all the related processing is done.
> Active records are not "refired".

in reply to Emmanuel Mayssat's question:
> I just wanted to have confirmation that in the case of a FLNK loop (
> 2 records FLNK to one another ), the loop stops once before
> processing an already processed record.

While Ron is right about this most all the time, there are circumstances 
where two records can FLNK each other and cause a loop; both records 
must have asynchronous device support for that to occur.

Here's the timeline with two synchronous records, showing how we break 
these loops using the record PACT field:

Request to process r1 is made.
dbProcess() checks r1.PACT, finds it false.
r1.process() called:
   r1.PACT = true
   r1 performs I/O etc according to record type,
   calls recGblFwdLink()
   r1.FLNK => r2
   dbProcess() checks r2.PACT, finds it false,
   r2.process() called:
      r2.PACT = true
      r2 performs I/O etc according to record type,
      calls recGblFwdLink()
      r2.FLNK => r1
      dbProcess() checks r1.PACT, finds it true.
      dbProcess() returns
      r2.PACT = false
   r2.process() returns
   r1.PACT = false
r1.process() returns.


However if both records have asynchronous device support, the timeline 
changes and we can get into a continuous loop, controlled by the speed 
of the device:

Request to process r1 is made.
dbProcess() checks r1.PACT, finds it false.
r1.process() called:
   r1.PACT = true
   r1 starts I/O operation
r1.process() returns.

r1.device finishes I/O operation,
r1.process() called:
   r1 completes I/O processing
   calls recGblFwdLink()
   r1.FLNK => r2
   dbProcess() checks r2.PACT, finds it false,
   r2.process() called:
      r2.PACT = true
      r2 starts I/O operation
   r2.process() returns
   r1.PACT = false
r1.process() returns.

r2 device finishes I/O operation,
r2.process() is called:
   r2 completes I/O processing
   calls recGblFwdLink()
   r2.FLNK => r1
   dbProcess() checks r1.PACT, finds it false,
   r1.process() called:
      r1.PACT = true
      r1 starts I/O operation
   r1.process() returns.
   r2.PACT = false
r2.process() returns


At this point the IOC will continue to loop between the second and third 
paragraphs of that description forever, although the time it takes for 
the I/O operation to complete will control how often it loops.

It is necessary to have two or more asynchronous device supports in a 
FLNK loop for this situation to occur - with a single asynchronous 
record processing will always come back to the original record with the 
PACT field set and thus break the loop.  Synchronous records in the loop 
have no effect on this argument except to increase the stack usage. 
Note that the infinite loop will not result in a stack overflow, since 
the two paragraphs can never nest - the lockset locking prevents the I/O 
completion from happening until after the previous paragraph has come to 
the end of its work.

- Andrew
--
There is no S in exprexxo.