Ronald Tatum | 2 Jun 04:59 2005

Re: what's the meaning MVT ?

Well, it seems to be getting confusing - I look in GC28-6628-7, IBM System
Operating Systems, System Control Blocks, date page missing, but the pub
corresponds to Release 20.1,  and it says the control program configurations
are shown as:

"PCP - The primary control program of the System/360 Operating System.
MFT - The *multiprogramming* with a fixed number of tasks configuration of
the System/360 Operating System.
MVT - the *multiprogramming* with a variable number of tasks configuration
of the System/360 Operating System."

All of which makes me wonder if even IBM's doc writers had a clear idea of
just what the words they used were supposed to mean ... At one time, the MFT
version was said to be "single-threaded", I suppose in the sense that were a
fixed number of partitions of memory in which various initiators lived and
which were "activated" by jobs' JCL, etc. Apparently if a program, e.g.,
called for I/O, the task (program) could specify waiting for the I/O to
complete before continuing its execution or immediately come back from the
request - in a sense, single threading itself by waiting or spawning (in
some strange sense) another "sub-thread" as it were. I'm not sure about time
slicing in MFT, but I was unfortunately exposed to an OS/VS1 shop that,

Seems the sysprog (an ex-DOS CE type) didn't think that users should be
concerned with work getting done; he had the idiot system time-slicing every
second, walking the chain of currently "executing" jobs, and giving control
to each in turn in a round-robin fashion if the particular job could do some
work. It was said, with some truth, that he really wanted to time-slice
every 200 milliseconds and do the same thing...
He did have *every* accounting exit he could think of turned on and even
bought a package from Eastman Kodak that did as much busywork with the exits
as it could. Far as I could tell, nobody every looked at much of the rubbish
he had the system generating.

Ron Tatum
----- Original Message -----
From: "Carter Browne" <cbrowne@...>
To: <hercules-390@...>
Sent: Wednesday, June 01, 2005 2:36 PM
Subject: Re: [hercules-390] what's the meaning MVT ?

> MFT - multiprocessing with a fixed number of tasks
> MVT - multiprocessing with a variable number of tasks
> There are two distinct meanings associated with the word
> "multiprocessing".  The older, and now less used one, means running
> multiple processes - what is now much more often called multi-tasking,
> although, then, as now, a process (program) could have multiple tasks
> (threads) runnable at the same time.  The newer meaning of
> multiprocessing is a system with multiple processors.
> I don't know of any clear terminology even today to distinguish between
> the multiple jobs with multiple tasks running on a system with multiple
> processors.
> Carter
> Ronald Tatum wrote:
> >Yes, the "M" in MVT and MFT stood for multi-programing - multi-processing
> >that era appears only to have referred to the 360/65 with the
> >multi-processor (two processors max) option.
> >
> >Regarding VM and MVS, there's a possibly apocryphal story that IBM, much
> >the chagrin of the enemies of VM, had to develop MVS under VM because the
> >original "builds" of MVS had a "mean-time-between-failure" on the order
of a
> >few hundred microseconds. I can't vouch for that, but a very competent
> >sysprog at Rice relayed that story to me back in the mid-70s, and I tend
> >believe him.
> >
> >It may be that after MVS finally was capable of sort of running, they
> >"fixed" a bunch of stuff that may or may not have made for problems
> >MVS under VM, but I can't imagine what or why IBM's developers would have
> >done such a foolish thing.
> >
> >Regards,
> >Ron Tatum
> >----- Original Message -----
> >From: <peterh5322@...>
> >To: <hercules-390@...>
> >Sent: Wednesday, June 01, 2005 10:11 AM
> >Subject: Re: Re: [hercules-390] what's the meaning MVT ?
> >
> >
> >
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