Comprehensive description of CPU terms for an AIX lpar

Question

lparstat on one of the AIX logical partitions show the following :

System configuration: type=Shared mode=Capped smt=On lcpu=4 mem=8192 psize=16 ent=0.25

How does fit in to configurations set up during the lpar creation. Such as : Min, Assigned, Max and Virtual CPU definitions of an lpar?

What does the entitled cpu ( ent=0.25 ) in above result mean?

Answer 1

First, I think that you would be more interested in lparstat -i. This format is much easier to understand when comparing lparstat data to profile data.

But, with regards to what entitled cpu means:

So on an AIX LPAR profile you have the following options regarding CPU.

Minimum Processing Units 
Desired Processing Units
Maximum Processing Units

Minimum Virtual Processors
Desired Virtual Processors
Maximum Virtual Processors

Processing Units determines your LPARs entitled capacity. This takes a little bit of understanding regarding the Power Hypervisor. The hypervisor determines how much of the frames resources each LPAR receives. The HV slices up each processor in time slices and depending on your PU you are guaranteed that much processor each second. So:

If you have .2 PU you are guaranteed 20% of 1 processor during each second.

I believe each time slice is 1/10th of a second. This is why their is a rule of ten between Entitled Capacity and Virtual Processors.

• If your EC was .2 and you have 1 Virtual CPU the HV will give that CPU 2 time slices.
• If your EC was .2 and you have 2 Virtual CPU you are fine (Each virtual cpu gets 1 time slice).
• If your EC is .2 and you have 20 Virtual CPU the HMC will not let you build the profile, the HV will not be able to allocate the resources - pending a drastic change in the laws of physics =).