If running under VM, you can have (for example) 4 physical cpu's shared
with a multitude of z/OS (or other) guests. Each guest can be told how
many CPUs it can use. Those are logical/virtual CPUs. If a z/OS guest is
told it has access to 2 CPUs, then VM manages the need by assigning any
free CPU to the z/OS guest when it's needed.
From z/OS's viewpoint, it has 2 CPUs. But, in actuality, at one point
real CPU 1 may be assigned to z/OS's logical CPU 2. Several cycles
later, VM may assign real CPU 3 to z/OS's logical CPU 2. z/VM manages
all this so that z/OS does not know which real CPU it is using.
With z/VM, you can evan over-allocate CPUs. While z/VM only has 2 real
CPUs, you can tell it to give 4 virtual CPUs to z/OS. Not a good
practice, but it can be done.
Tony Thigpen
[email protected] wrote on 07/29/2017 09:53 AM:
Hi,
.
Im not an MVS SYSPROG nor a hardware person, I am a CICS SYS Prog.
.
I always thought that CPUS (engines) were physical hardware.
Is this not true anymore ?
.
The other day in a meeting I heard the term Logical/Virtual CPU.
Never Heard that term.
.
.
What exactly is a Logical/Virtual CPU ?
Does a Logical/Virtual CPU need to be backed by a Physical CPU ?
.
My understanding is a Task (TCB) in an address spaces always requires a CPU to
be assigned/associated with.
So with Logical/Virtual CPUS, can I have more TCBS, actively dispatched at the
same time
in the same Address Space, than the Physical CPUs ?
.
.
curious minds want to know
Paul
