System information

16 Logical Partitions on System i5
The server firmware distributes the processing units evenly among all virtual processors
assigned to a logical partition. For example, if logical partition A has 1.4 processing units and
2 virtual processors assigned, then each virtual processor will be equal to 0.7 physical
processing units. These 2 virtual processors will support the logical partition workload.
The number of processing units available for each virtual processor is limited. The minimum
number of processing units for each virtual processor depends on the server model on which
the logical partitions are created. The maximum number of processing units for each virtual
processor is always 1.00.
The logical partition operations will be faster and produce better results if the number of
virtual processors assigned is very close to the number of processing units from fractional
processors assigned to that logical partition. The operating system in the logical partition will
be able to manage the workload more effectively.
In some cases, you may increase the number of virtual processors for the logical partition in
order to give more concurrent operations for the operating system. The addition of some
virtual processors may slightly improve the performance of the operating system. The
operating system will be able to shift processing powers among processes very well.
However, you have to consider the impact of adding too many virtual processors for a shared
processor logical partition, since this will degrade the performance of each virtual processor.
If there are too many virtual processors for a logical partition whose processing unit is not
increased, the number of processing units per new virtual processor is smaller than for the
previous virtual processor. Then the operating system cannot shift processing powers among
processes very well.
When you create a partition profile with shared processors, HMC will calculate the number of
minimum, desired, and maximum virtual processors for the logical partition based on the
minimum, desired, and maximum processing units assigned to this logical partition. By
default HMC will calculate these default virtual processors as follows:
򐂰 The minimum virtual processor for logical partitions is the number of minimum processing
units rounded to the next whole number.
򐂰 The desired virtual processor for logical partitions is the number of desired processing
units rounded to the next whole number.
򐂰 The maximum virtual processor for logical partitions is the number of maximum
processing units divided by minimum processing units for each virtual processor. The
result is rounded to the previous whole number.
An example of virtual processor calculation done by HMC is shown in Figure 1-12.