Need help understand Virtual Processors

First of all I have performed a Google search and internal search and found several descriptions but nothing I can wrap my head around and feel 100% confident about.

I feel really silly for asking this as I manage a P6 570 with 12 lpars but I have difficulity with Virtual Processors.

I can understand the Processing units section. I guess I'll let you be the judge of that.

Minimum units are the lowest amount of processing power the server with start with.

Desired units are what the server will start with if available.

Maximum units are the max I can change the Desired value to through the DLPAR process.

Also I was thinking that the Maximum units are the max the server could utilize if set in an Uncapped mode. Depending on if there are any free slices at the time it needs the extra processing power.

I understand that Virtual processors are a representation of a physical processor to the O/S. So in my env below I have a Desired processor count of .5 so at a minimum I am required to have 1 Virtual processor but I can have up to 10 x .5 or 5 Virtual processors.

If I understand what I read, if you increase the number of virtual processors, you increase the number of operations that can run concurrently. I don't get it; you just don't get something out of nothing.

I don't think I can assign half a physical processor and choose to assign 5 Virtual to the system and expect that it will just work like it has 5 physical processors.

I know I am talking too much but some folks like lots of info and some probably wanted me to shut up 5 minutes ago.

I just need to understand how the virtual processor values play a part in this and how I should rightly manage those values.

My env;

 
Processing mode = Shared
 
Processing units
Total manages system processing units: 8.00
 
Minimum = .1
Desired = .5
Max = .7
 
Virtual Processors
 
Minimum virtual = 1.0
Desired virtual = 1.0
Maximum virtual = 1.0
 
Sharing Mode = Uncapped
Weight = 128

O/S AIX 5.3, ranging from TL6 to TL9

Thank you for your time.

A Virtual CPU is the virtual device used by a Physical CPU to provide CPU resources to an LPAR. Each virtual CPU can be backed only by one physical CPU, so you are right about that.

There would be no point in having 5.0 CPU units and 1 vCPU as 4 vCPUs would go to waste.

Its easiest to deal with capped LPARs first, they will never borrow CPU above their desired setting. They use what they have and thats all - generally useful for oerformance test environments - or where licensing is controlled at a physical CPY Level (and yes I'm ignoring CPU pools).

With uncapped partitions they will borrow CPU resources up to the maximum and equal to the number of desired vCPUS. The tradeoff is that the amount of CPU the box uses isn't always fixed, as other LPARS may have call on that CPU.

so in the case you descrive above the LPAR gets the additional 4.5 CPUs only when the LPARS that own the CPU aren't using it - or if its not allocated within the server, that its the only LPAR that wants them. In short, the LPAR can only have the extra CPU if its avialble and no other LPAR is using it.

I generall set max vCPU and max CPU = number of CPUs in the box - I've never found a reason to limit myself to a lower value.

ross,

If I understand you correctly the desired vCPU is the value that I want to set if I wanted to allow this server to use up to X number of CPU's, if the resources were available.

The max CPU is only relating to how much up can up the desired CPU through DLPAR and the max vCPU is relating to the most I can up the desired vCPU through DLPAR.

Thank you for your patience.

Yes, your understanding is correct.