[email protected] wrote: >The other day in a meeting I heard the term Logical/Virtual CPU. >Never Heard that term.
Think of it this way: Hardware defines physical CPUs. Those can be assigned to LPARs. Each LPAR has one or more logical CPUs, each of which can be a fraction of or a whole physical CPU. z/VM can run in an LPAR, and also defines virtual CPUs, which run on logical CPUs, which run on real CPUs. No matter what, at some point there are instructions executing on bare metal. But as others have noted, you could have a virtual machine with five virtual CPUs defined in an LPAR that has four logical CPUs running on a system with 20 physical CPUs. It's been a while since I played with an HMC, but I'm pretty sure you can't have more logical CPUs in an LPAR than physical CPUs, because that likely makes no sense. But you can overcommit, and PR/SM then manages the resources. So if LPARs on system with three physical CPUs could each be defined to have two logical CPUs, and (depending on capping) either one could use up to two entire CPUs, if the other one isn't busy. It might help to think of PR/SM as "VM in the hardware" (since it is: early versions actually had recognizable VM messages appearing on the console! Of course they've diverged a fair bit, but the fundamentals are the same). It's turtles all the way down. .phsiii
