IBM Power System POWER7 Series, Руководство по оптимизации и настройке

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Chapter 3. The POWER Hypervisor
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Entitlement also determines the number of SPLPARs that can be configured for a shared
processor pool. The sum of the entitlement of all the SPLPARs cannot exceed the number of
physical cores that are configured in a shared pool.
For example, a shared pool has eight cores and 16 SPLPARs are created, each with 0.1 core
entitlement and one virtual CPU. We configured the partitions with 0.1 core entitlement
because these partitions are not running that frequently. In this example, the sum of the
entitlement of all the 16 SPLPARs comes to 1.6 cores. The rest of the 6.4 cores and any
unused cycles from the 1.6 entitlement can be dispatched as uncapped cycles.
At the same time, keeping entitlement low when there is capacity in the shared pool is not
always a preferred practice. Unless the partitions are frequently idle or there is plan to add
more partitions, the preferred practice is that the sum of the entitlement of all the SPLPARs
configured should be close to the capacity in the shared pool. Entitlement cycles are
guaranteed, so while a partition is using its entitlement cycles, the partition is not preempted,
while a partition can be preempted when it is dispatched to use excess cycles. Following this
preferred practice allows the hypervisor to optimize the affinity of the partition’s memory and
processor cores and also reduces unnecessary preemptions of the virtual processors.
Matching entitlement of an LPAR close to its average usage for better
performance
The aggregate entitlement (minimum or wanted processor) capacity of all LPARs in a system
is a factor in the number of LPARs that can be allocated. The minimum entitlement is what is
needed to boot the LPARs, but the wanted entitlement is what an LPAR gets if there are
enough resources available in the system. The preferred practice for LPAR entitlement is to
match the entitlement capacity to average usage and let the peak be addressed by more
uncapped capacity.
When to add more virtual processors
When there is sustained need for a shared LPAR to use more resources in the system in
uncapped mode, increase the virtual processors.
How to estimate the number of virtual processors per uncapped shared
LPAR
The first step is to monitor the usage of each partition and for any partition where the average
utilization is about 100%, and then add one virtual processor, that is, use the capacity of the
configured virtual processors before you add more. Additional virtual processors run
concurrently if there are enough free processor cores available in the shared pool.
If the peak usage is below the 50% mark, then there is no need for more virtual processors. In
this case, look at the ratio of virtual processors to configured entitlement and if the ratio is
greater than 1, then consider reducing the ratio. If there are too many virtual processors that
are configured, AIX can “fold” those virtual processors so that the workload would run on
fewer virtual processors to optimize virtual processor performance.