System information

14 Logical Partitions on System i5
The shared processors pool is created from the processors left over after the dedicated
processors are assigned to logical partitions that use dedicated processors. This will typically
be spread over multiple nodes. These processors are shared among all the partitions that use
shared processors. You can allocate at least 0.1 of a shared processor or up to the total
number of processors in the system.
One single physical processor is equal to 1.00 processing units, two physical processors are
equal to 2.00 processing units, and so on. The number of processing units is equal to the
number of physical processors available in the shared processors pool. A processor must be
in the shared processors pool to become processing units.
Every logical partition that uses shared processors will receive some processing units from
the shared processors pool. These processing units will appear to the operating system as a
number of processors called virtual processors. The Power Hypervisor is responsible to
communicate the virtual processors’ information to the operating system. The number of
virtual processors for each logical partition that uses the shared processor is configured in the
logical partition profile.
In partitions using the shared processors pool, if a partition is ended and restarted, the Power
Hypervisor will attempt to re-assign the same processor(s). This is to attempt to increase
cache hits for the workload on that partition.
1.3.5 Shared uncapped processor
By default, logical partitions that use the shared processors pool are uncapped logical
partitions, which means that they can use unused processing units from the shared
processors pool. The distribution of unused processing units to uncapped partitions will be
based on the weight assigned to each partition if more than one uncapped partitions demand
additional processing units at the same time.
Uncapped weight is a number in the range of 0 through 255 that you set for each capped
partition in the shared processors pool. By setting the uncapped weight, any available unused
capacity is distributed to contending logical partitions in proportion to the established value of
the uncapped weight. The default value for uncapped weight is 128.
For example, if there are three processors available in the shared processors pool, and if two
uncapped logical partitions demand more processing units at the same time, if logical
partition A has an uncapped weight of 80 and logical partition B has an uncapped weight of
160, then logical partition A will receive 1 processing unit and logical partition B will receive 2
processing units from the shared processors pool. The uncapped weight is set in the partition
profile as shown in Figure 1-9. You can set this uncapped weight when the partition profile is
being created.
Figure 1-9 Uncapped weight setting in partition profile