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This document describes the algorithms used to calculate the size of a virtual processor in a shared processor environment using the Power5 architecture. The IBM documentation does not fully explain this concept and this document attempts to clarify this issue.

When defining an LPAR through the HMC for the Power5 architecture, the type of processors assigned to the LPAR must be defined. The possible choices for this are: Dedicated and Shared. If "Shared" is selected, the following input fields are presented:

Processing mode
Dedicated
Shared


Processing units

Total Managed System processing units: 16
Minimum processing units:
Desired processing units:
Maximum processing units:


Virtual processors

Minimum processing units: 0.10
Minimum virtual processors:
Desired virtual processors:
Maximum virtual processors:

When entering "shared" mode processors, the "Processing units" input fields define the total amount of processing units that will be allocated to all virtual processors. This translates to the following algorithm:

Algorithm:

Vs = Pu / Vn

Rules:

1.00 Pu = 1 full power5 physical processor
Pu < Pt
Vn <= 64

Variable Definitions:

Vs = Virtual processor size
Pu = Physical processing units ( number of physical processors )
Vn = Number of virtual processors assigned to LPAR
Pt = Total number of physical processors in frame


As an example of using this algorithm:

Processing mode
Dedicated
Shared


Processing units

Total Managed System processing units: 16
Minimum processing units:
Desired processing units:
Maximum processing units:


Virtual processors

Minimum processing units: 0.10
Minimum virtual processors:
Desired virtual processors:
Maximum virtual processors:

These values would allocate "0.5" physical processing units to the LPAR and "2" virtual processors. The size of each virtual processor would be "0.25" physical processing units.

Algorithm:

Vs = Pu / Vn
Vs = 0.5 / 2
Vs = 0.25

Variable Definitions:

Vs = Virtual processor size
Pu = Physical processing units ( number of physical processors )
Vn = Number of virtual processors assigned to LPAR


Another example using this algorithm:

Processing mode
Dedicated
Shared


Processing units

Total Managed System processing units: 16
Minimum processing units:
Desired processing units:
Maximum processing units:


Virtual processors

Minimum processing units: 0.10
Minimum virtual processors:
Desired virtual processors:
Maximum virtual processors:

These values would allocate "2.5" physical processing units to the LPAR and "5" virtual processors. The size of each virtual processor would be "0.50" physical processing units.

Algorithm:

Vs = Pu / Vn
Vs = 2.5 / 5
Vs = 0.50

Variable Definitions:

Vs = Virtual processor size
Pu = Physical processing units ( number of physical processors )
Vn = Number of virtual processors assigned to LPAR


A final example illustrating how the EGATE Proof of Concept LPAR's were configured:

Processing mode
Dedicated
Shared


Processing units

Total Managed System processing units: 16
Minimum processing units:
Desired processing units:
Maximum processing units:


Virtual processors

Minimum processing units: 0.10
Minimum virtual processors:
Desired virtual processors:
Maximum virtual processors:

In this example, if the desired number of physical processing units was allocated to the LPAR, "3.0" physical processing units would be allocated to the LPAR and "6" virtual processors. The size of each virtual processor would be "0.50" physical processing units.

Algorithm:

Vs = Pu / Vn
Vs = 3.0 / 6
Vs = 0.50

Variable Definitions:

Vs = Virtual processor size
Pu = Physical processing units ( number of physical processors )
Vn = Number of virtual processors assigned to LPAR

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Virtual Processor Size
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