Advanced POWER Virtualization on IBM System p5 - Previous ...

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If you dynamically remove a virtual processor, you cannot specify an
identification for particular virtual processor to be removed. The operating
system will choose the virtual processor to be removed.
Shared processors may render AIX 5L affinity management useless. AIX 5L
will continue to utilize affinity domain information as provided by firmware to
build associations of virtual processors to memory, and it will continue to
show preference to redispatching a thread to the virtual processor that it last
ran on.
An uncapped partition with a weight of 0 has the same performance impact as
a partition that is capped. The HMC can dynamically change either the weight
or a partition from capped to uncapped.
Dispatching of virtual processors
There is additional processing associated with the maintenance of online virtual
processors, so you should carefully consider their capacity requirements before
choosing values for these attributes.
Under AIX 5L V5.3 ML3, a new feature is introduced to help manage idle virtual
processors.
Virtual processors have dispatch latency since they are scheduled. When a
virtual processor is made runnable, it is placed on a run queue by the <strong>POWER</strong>
Hypervisor, where it waits until it is dispatched. The time between these two
events is referred to as dispatch latency.
The dispatch latency of a virtual processor depends on the partition entitlement
and the number of virtual processors that are online in the partition. The capacity
entitlement is equally divided among these online virtual processors, so the
number of online virtual processors impacts the length of each virtual processor's
dispatch. The smaller the dispatch cycle, the greater the dispatch latency.
At the time of writing, the worst case virtual processor dispatch latency is 18
milliseconds, since the minimum dispatch cycle that is supported at the virtual
processor level is one millisecond. This latency is based on the minimum
partition entitlement of 1/10 of a physical processor and the 10 millisecond
rotation period of the Hypervisor's dispatch wheel. It can be easily visualized by
imagining that a virtual processor is scheduled in the first and last portions of two
10 millisecond intervals. In general, if these latencies are too great, then clients
may increase entitlement, minimize the number of online virtual processors
without reducing entitlement, or use dedicated processor partitions.
Chapter 2. <strong>Virtualization</strong> technologies on System p servers 43