Coupling Facility Async / Sync Thresholds - They Are A'Changin'
MartinPacker 11000094DH Comments (2) Visits (13152)
APAR OA21635 is one of a rare breed: A change to the thresholds for z/OS's automatic CF Request conversion. (I'm told this has only happened once before.)
If you recall z/OS Release 2 (in 2001) introduced a very nice function that automatically converts Coupling Facility requests from Sync to Async, based on thresholds. The purpose of this is to minimise the CPU cost for a coupled z/OS system, while still providing reasonable request response times:
When I say response time I mean CF request response time, which may have little to do with actual application response times. But the two aren't totally divorced.
(Requests that were originally Async don't get converted to Sync, by the way.)
The big change came, as I say, in z/OS Release 2 where XES introduced a new algorithm, measuring response times and, based on thresholds, deciding whether to convert Sync requests to Async. This measurement is not done for every request, but rather once in a while. So we don't have "nervous kitten" syndrome here. :-) I like algorithms that are responsive. I don't like algorithms that are overly jumpy.
So, back to the thresholds:
Technology changes, so it's appropriate to revisit the thresholds from time to time. APAR OA21635 is a result of this. To quote Development:
What you'll see when you install this PTF varies, depending on your situation:
An interesting game I like to play is to compare request types to "local" and "remote" CFs, particularly with Duplexing. And, obviously, the response times seen. The simple case is without duplexing where "local" requesters get Sync requests in the main - at perhaps less than 50ms. And "remote" requesters get essentially Async requests, with a much higher response time, especially at distance.
Distance discussions are well informed by such analysis. As are Duplexing discussions. Which is, perhaps, the essential "take home" message of this post.
When implementing the threshold changes I would form a view of such things before and after applying the PTF. And I'd be interested to know how well it worked for you.
And remember there's nothing essentially good or bad about Sync or Async. It all depends on whether the behaviour is appropriate for your scenario.
But I'm pleased that, once in a while, the thresholds are revisited. So I think this is a good APAR.