Edit: Some links no longer work.
Originally posted July 23, 2013 on AIXchange
Lately I’ve been getting questions about virtual processors and shared processor pools. Here are some resources on this topic that might help.
* In January Rosa Davidson of IBM delivered a great two-part presentation, “Capacity Entitlement and Virtual Processors.” The replays and slides are available here.
* Here’s an explanation of virtual processors found in the POWER6 documentation in IBM’s Information Center:
“However, when you install and run an operating system on a logical partition that uses shared processors, the operating system cannot calculate a whole number of operations from the fractional number of processing units that are assigned to the logical partition. The server firmware must therefore represent the processing power available to the operating system as a whole number of processors. This allows the operating system to calculate the number of concurrent operations that it can perform. A virtual processor is a representation of a physical processor to the operating system of a logical partition that uses shared processors. “
* Be sure to look at these IBM Systems Magazine articles on virtual processor folding and shared processor settings:
“Virtual processors are what the operating system thinks it has since it can only relate to whole numbers of processors. And the desired virtual processor value is basically the maximum number of physical processors that an uncapped shared processor partition can use if processor units are available in the shared processor pool.”
* Finally, here’s something that I wrote for IBM Systems Magazine‘s AIX EXTRA e-newsletter:
“… keep in mind you can never use more physical CPUs than virtual CPUs as defined in your LPAR. Even if you allocate one virtual processor to an LPAR and set it to be uncapped, you can’t run more than one physical processor because there would be no other virtual processors available.
“This way, you can limit the LPARs in your shared processor pools even if your LPAR is uncapped and there are 16 processors available in a shared processor pool. You still won’t be able to use more than one physical CPU because you only allocated one virtual CPU.
“A virtual processor can represent from 0.1 to 1 of a physical processor. If you have one virtual processor, the range it can physically consume will never be more than one. If you have three virtual processors, you can use from 0.3 to 3, but never more than three.
“It makes sense, as you’re basically giving your VM the illusion that it’s dealing with a physical processor. If it boots up, and sees three virtual processors, even if it’s running on 0.3 physical processors, it won’t see more than three processors. If it’s running uncapped and wanted to use four physical processors, where would they run if there are only three virtual processors?”
I’m sure more good documentation is available. Feel free to post a comment with any resources you’ve used to get up to speed with virtualization on Power.