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Difference between physical cpu(processor) and virtual cpu in aix lpar profile

Posted on 2014-10-08
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When creating an AIX lpar profile on a IBM machine through HMC, there is an option of processors and virtual processors.
What is the difference between processors and virtual processors?
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Question by:assistunix
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by:John Pope
ID: 40370148
Hi

It's a tricky one to understand, even when explaining it.  Certainly there are some good resources around to help.  Also, the terminology can make things more complex if one is not familiar with it. Here goes;

You can assign a minimum value of 0.1 of a virtual processor to 1 physical processor (to a maximum of 1). The number of virtual processors specified for an LPAR represents the maximum number of physical processors the LPAR can access. So, if your processor pool has 32 processors in it, but your LPAR only has four virtual CPUs and it’s uncapped, the most it’ll consume will be four CPUs.

Basically the difference is that as you are sharing the resources (in this case CPU) of your p-series, the VP is using CPU cycles provided by the physical CPU, rather than using a physical CPU.

HTH

Cheers, JP
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by:John Pope
ID: 40383817
Hi assistunix,

Any clearer?! :)

Cheers, JP
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by:John Pope
ID: 40459061
Hi assistance

Has my answer proved helpful on this one?

Cheers, JP.
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Author Comment

by:assistunix
ID: 40460152
Hi JP,

Here is my understanding, please correct me and clarify if i am wrong.

So lets assume i have these numbers in my LPAR and lets assume that there are 50 free Processing Units available on the machine(frame).

Entitled Capacity(Processing Unit)
Min-1
Des-2
Max-3

Virtual Processor
Min-2
Des-4
Max-8

So in shared mode, we have two sub options, capped and uncapped.

In uncapped, the LPAR will automatically increase its cpu to 4 Processing Unit if the system load requires it.
And than when the system load is no longer high, the cpu will automatically decrease back to 2 Processing unit.

Whereas in Capped mode, we will enter the Virtual Processor information as well in the LPAR but that is kind of pointless, since in Capped mode, the LPAR will only use its Entitled Physical Processor and will not increase automatically to the Virtual Processor number.

Am i correct in that understanding?
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John Pope earned 2000 total points
ID: 40461619
Hi assistunix

Yes that is right.

When an LPAR is capped, the maximum amount of physical processor resources that it can access is limited by the desired Processing Unit setting, regardless of the desired Virtual Processor setting.

Cheers, JP.
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