开云体育

On Tue, Mar 11, 2025 at 11:30:46AM -0400, Ross Patterson via groups.io wrote:

On Tue, Mar 11, 2025 at 10:50 Alexander Huemer via groups.io <aziD5eeth9fe=
[email protected]> wrote:

You can install VM on top of VM

My knee-jerk question to my instructor back then was:

How deep can you go?

He didn't know.

Technically, there is no limit.  Diagnose code x'0000' will attempt to
return up to 5 levels of CP info.  Practically, the "top level" system gets
slower and slower as you add more levels of CP underneath it.  It was
common to run two levels for testing purposes, back in the 70s and 80s, but
not for production, due to poor performance.  With today's real and
emulated mainframes, you could easily do more than that in production.

That sounds encouraging, thanks for laying that out.
I would think too that performance shouldn't be a problem.
This is very much an academic exercise anyways. The idea is just to 
demonstrate that it is possible, not to solve any real-world problem.
 

Ever since, I have an idea in my head that comes back occasionally.
Can you install VM 'recursively'?
What I mean by that is the following:
...

Yes, but you'd have to write that installation process yourself.  Nothing
in the normal VM installation makes that simple.

Right. The problem is, I know so little about VM and its ecosystem that 
I don't even know where to start.

I am lacking the experience with VM to assess whether this is 
possible at all or if perhaps it is possible in principle but only 
with later versions of VM than VM/370 or something like that.

Its possible with every version of VM and there are as far as I know no hard coded limits limits. Performance suffers the deeper you go because many instructions need to be interpreted at each level.
At some point you will run out of real memory because in order to do certain things pages need to be locked in memory.
? ?

Successive later versions of VM have added more and more functions to
improve automation capabilites.  Stuff like the Secondary Console Image
Facility.

Sounds great, but I can't say I really know what that is.

In case anybody on this list is interested to tinker on this, please let 
me know, on of off list.

I would say even with the later facilities you can't automate something that generates an arbitrary depth of VMs.
Lets take things at a simple level. Each VM needs its own system disk that you IPL from. These need to be known in some way, by the top level VM before you start.
So if they are physical disks the top level VM needs to have a separate IO address in DMKRIO.
Yes you can use minidisks but that doesn?t help much. The number of cylinders is defined at startup...

-Alex

Dave