Tom Quarendon wrote:
Since I got such a good response from the last question, here's another.
My understanding of channel program execution is that there is one
queue per UCB, i.e per volume. In other words there isn't a huge
amount to be gained by executing lots of concurent channel programs to
the same file (you'd get some advantage, maybe, in that you'd be able
to construct your next EXCP invocation and have the EXCP do all the
prep work for it before actually checking that you previous one is done).
Parallel access volumes however mean that there are potentially
multiple UCBs per device and therefore multiple channel program
queues. However if I examine the list of UCB associated with the
extents of my file they are all the same. So is(are) parallel access
volume technology something I can exploit in my application? The best
I could presumably do is to allow multiple channel programs to be
scheduled by my application against different extents in the file, but
since my UCB addresses are all the same, that's not going to work. Is
this an MVS configuration issue? i.e is there something that would
need to be set up on the machine to allow this? Or is this more a case
of different jobs will get different UCB addresses for the same
volume, so they can be parallelized, but you can't exploit that within
a single app?
Searching on the web, Syncsort reckons it exploits PAV to get 30
percent improvement in elapsed time or something. I want a piece of
that if I can get it.
Using EXCP, you can simultaneously start as many channel programs as you
like -- each with its own IOB/IOBE pair.
The DEFINE EXTENT command determines the scope of serialization for
reads/writes within the PAV-capable device. Within the scope of this
serialization, concurrent read operations are allowed; write operations
cause single-threading.
If you're authorized, you can provide your own prefix to minimize
contention within an extent. If unauthorized, you will need to just live
with the system-provided serialization at the extent level.
--
Edward E Jaffe
Phoenix Software International, Inc
5200 W Century Blvd, Suite 800
Los Angeles, CA 90045
310-338-0400 x318
[EMAIL PROTECTED]
http://www.phoenixsoftware.com/
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