IBM Power7 Optimization And Tuning Manual page 74

If a partition has multiple virtual processors, they might or might not be scheduled to run
simultaneously on the physical processor cores.
Partition entitlement is the guaranteed resource available to a partition. A partition that is
defined as capped can consume only the processors units that are explicitly assigned as its
entitled capacity. An uncapped partition can consume more than its entitlement, but is limited
by many factors:
Uncapped partitions can exceed their entitlement if there is unused capacity in the shared
pool, dedicated partitions that share their physical processor cores while active or inactive,
unassigned physical processors, and Capacity on Demand (CoD) utility processors.
If the partition is assigned to a virtual shared processor pool, the capacity for all of the
partitions in the virtual shared processor pool might be limited.
The number of virtual processors in an uncapped partition is throttled depending on how
much CPU it can consume. For example:
– An uncapped partition with one virtual CPU can consume only one physical processor
core of CPU resources under any circumstances.
– An uncapped partition with four virtual CPUs can consume only four physical processor
cores of CPU.
Virtual processors can be added or removed from a partition using HMC actions.
Sizing and configuring virtual processors
The number of virtual processors in each LPAR in the system ought not to
of cores available in the system (central electronic complex (CEC)/framework). Or, if the
partition is defined to run in a specific virtual shared processor pool, the number of virtual
processors ought not to exceed the maximum that is defined for the specific virtual shared
processor pool. Having more virtual processors that are configured than can be running at a
single point in time does not provide any additional performance benefit and can actually
cause more context switches of the virtual processors, which reduces performance.
If there are sustained periods during which there is sufficient demand for all the shared
processing resources in the system or a virtual shared processor pool, it is prudent to
configure the number of virtual processors to match the capacity of the system or virtual
shared processor pool.
A single virtual processor can consume a whole physical core under two conditions:
1. SPLPAR has an entitlement of 1.0 or more processors.
2. The partition is uncapped and there is idle capacity in the system.
Therefore, there is no need to configure more than one virtual processor to get one
physical core.
For example, a shared pool is configured with 16 physical cores. Four SPLPARs are
configured, each with entitlement 4.0 cores. To configure virtual processors, consider the
sustained peak demand capacity of the workload. If two of the four SPLPARs would peak to
use 16 cores (the maximum available in the pool), then those two SPLPARs would need 16
virtual CPUs. If the other two SPLPARs peak only up to eight cores, those two would be
configured with eight virtual CPUs.
Entitlement versus virtual processors
Entitlement is the capacity that an SPLPAR is ensured to get as its share from the shared
pool. Uncapped mode allows a partition to receive excess cycles when there are free
(unused) cycles in the system.
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exceed
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