power software i perfmgmt processor lpar.pdf


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STG Cross Platform Systems Performance
there, the Virtual Processor ceases its association with a core. And this is just as you would want it; any
waiting active Virtual Processor can now use the freed core.
Each POWER7 Virtual Processor should also be thought of as representing up to four dispatched tasks,
because an SMT4 core supports up to four tasks. The Virtual Processor is considered active (for potential
use of a core) if there are one through four tasks assigned there; even just one task makes it active. When
the last task ceases execution there, the Virtual Processor becomes inactive again.
You can now see again that a Virtual Processor is really just an abstraction; it represents the notion of a
processor core – with all of its SMT hardware threads (4 in the following figure) – to the partition itself,
or more specifically, to a partition’s Task Dispatcher. The Virtual Processor effectively provides the
means by which a partition need not know the physical location of the cores on which its tasks are
executing, or, for that matter, when they really are executing.

Figure 2 – Virtual Processors

Dispatching of Virtual Processors
The difference in Virtual Processor persistence between dedicated and shared-processor partitions results
in some interesting differences in performance behavior as well.
For a POWER7 processor, a dedicated-processor partition is executing at maximum compute capacity
only when all of the partition’s cores are executing four (SMT4) tasks. When there are SMT hardware
threads available – because there are fewer tasks executing – any newly dispatchable task can begin
executing immediately. There is no queuing delay. Any additional dispatchable tasks above four tasks
per core wait for a while; often this wait is until an executing task stops, freeing up a processor. These
tasks are going to perceive a queuing delay as they wait their turn for a processor. The wait period is
dependent on both the individual task’s priority and the number of dispatchable tasks.
This wait before getting to execute is what you normally think of as a “CPU Queuing” delay. Like waits
due to I/O and lock conflicts, you know that CPU Queuing delays are a component of the response time

POWER7 Logical Partitions

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