Hi Quasar:
Back in the very beginning (OS/360 MVT in 1971), TSO was introduced. At
that time, it consisted of a "monitor" program which used time-slicing
to distribute the CPU time it was given among the TSO users that were
logged on.
With the introduction of the System Resource Manager (SRM) in MVS
(1974), things changed. From that point on, "time-sharing" was
accomplished by SRM. In MVS, a TSO user ran in its own address space and
became part of a mix of work units whose CPU usage was controlled by
SRM. Any address space was eligible to be dispatched on a CPU when it
was in a "ready" state, the opposite state can be generalized as a
"wait" state. Except for select address spaces (those marked
"non-swappable"), an address space in a wait state was eligible for
swap-out. Entering a wait state could be announced (long wait) or
discovered (detected wait). A TSO user that was inactive (in between
commands or thinking what to do next), was usually in a terminal-input
wait, as a read I/O operation was usually issued to the terminal when
the current command had finished. Thus, the address space became a
candidate for swap-out.
Because of the unpredictability of the user's actions (how soon after
the swap-out decision was made that they would hit a key and end the I/O
wait), the concept of "think time" and logical swapping was introduced.
This was intended to reduce swap-in I/O activity and the resultant CPU
needed to complete the swap-in. SRM permitted an externally controlled
parameter which represented think-time in seconds, making it possible to
allow the TSO user to remain swapped in for at least that long a period.
Once think-time passed, however, the TSO user could be "logically swapped".
In the logically swapped state, the pages belonging to the TSO user's
address space would be written to disk or expanded storage (when that
was supported), preparing for physical swapping, but would remain in
main storage until the storage was actually needed to resolve paging
demands of other address spaces. At that point, the TSO address soace
would be physically swapped and it's pages would be made available to
the rest of the system. If the used became ready (ended the wait) prior
to it's pages being needed, it would be marked swapped in and would
retain use of its existing pages in main storage. This saved the I/O and
CPU time needed to perform the actual swap in.
In today's version (z/OS) this action still occurs, although we are
inclined to use the component name WLM (WorkLoad Manager) when
describing the functions I have attributed to SRM in the description above.
Hope this helps.
Mike Myers
Mentor Services Corporation
On 11/17/2012 05:30 AM, Quasar Chunawala wrote:
Hi everybody,
I hope this finds you in the pink of health. I am Quasar, and I hail from
Mumbai, India. I own a blog on the internet, parked at
http://www.mainframes360.com. I am an application developer by profession.
I intend to write an article on TSO/E on my blog. I have been reading
matter on time-sharing and its origins on the Internet. I learnt about the
history of Time Sharing systems and how they evolved over a period of time.
I have also read, Bob Bemer’s article "*How to Consider a Computer*",
published in the Automatic Control Magazine, in March 1957, by .
I would like you to throw some light on the technical underpinnings of
how TSO really accomplishes the feat of time-sharing. I know that, there is
a TSO address-space for every active user logged on to the system. It is my
understanding that, time is sliced by the scheduler between all the TSO
jobs, other user-jobs, STARTed tasks etc. But, it occurs to me, why should
a time-slot be given to a TSO user, who hasn't pressed an AID key(like
Enter)? Maybe, he's just staring at a dataset. Isn't this a waste of
processor-time? Or am I missing out something.
Thanks and look forward to receiving a reply from you soon,
Quasar Chunawala
Sent from Windows Mail
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