------------------------<snip>------------------
There's no reason you can't issue multiple EXCP's against the same
DCB before issuing any WAIT's. IOS will queue the requests in the
order they are received and if the requests are eligible for PAV
processing, IOS will take care of it for you. Much ado about nothing
here ?? Remember that even with PAV you're only allowed a single
channel program to be active on any single extent. (Or at least that
used to be true! <G>)
But it comes down to how I program my application in terms of having
multiple outstanding EXCPs. Implicit in your comment is that it is
natural to have multiple EXCPs outstanding at a time, so therefore
there's nothing to do to exploit PAV. However the "simplest" approach
to what I'm doing is to only have one outstanding EXCP, and when I get
asked to write a block WAIT on the outstanding EXCP, then set off
another one. That way the application logic is overlapped with the I/O.
The question is, is it worth going any further. How many EXCPs is it
worth having outstanding? Is two better? 20? There's no point in
implementing code that will have multiple outstanding EXCPs, greatly
increasing the complexity, if all that's going to happen is that they
are going to be executed serially anyhow. Thus my attempt to
understand EXCP queuing and PAV.
---------------------------<unsnip>------------------------
Beyond queuing up multiple requests, you have no control over PAV usage.
What if you're going to a volume with NO PAV's?? In the time it takes to
write a complete cylinder, you should be able to get the next cylinder's
data ready, given the differences between processor speed and I/O speed.
My little PDSSCAN program will essentially lock a volume to 100% busy
using only two buffers and BPAM reads, and there's still a LOT of CPU
cycles left over for other users, even after I scan a buffer byte-for-byte.
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