power software i perfmgmt processor lpar.pdf

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STG Cross Platform Systems Performance

The Virtual Processors
The Processor: The hardware entity to which tasks are assigned, executing their programs.
System processors are such a basic feature of your computer system that we tend to forget that each
processor, even within a partition, is already virtualized. In a partition with many jobs, processes, and
threads, you do not need to know when or which processor is executing your task. All your program
needs to do is make a task dispatchable and you can be sure that it will get its opportunity to execute
somewhere and soon, even if all processors are busy.
Consider a partition with multiple processor cores. Even if you did know when your task was executing,
have you really ever wanted to know or control which core is being used? The partition provides
compute capacity and handles the rest for you. Providing more cores to a partition just means more
compute capacity; this provides more opportunity to concurrently execute multiple tasks, and to minimize
any task’s wait time. Still more compute capacity comes from the fact that most modern processor cores
are individually capable of concurrently executing multiple tasks via SMT (Simultaneous MultiThreading); POWER7 cores can concurrently execute up to four tasks per core. Not only could your
task be executing on any core, but it could be executing with three additional tasks on that core.
The point here is that even within a partition, the operating system’s Task Dispatcher is virtualizing
processors, hiding the details of the processor cores. Although task dispatching is actually quite complex,
you need only think of the Task Dispatcher – as in the following figure - as being a hopper of
dispatchable tasks, spreading tasks in some fair and performance optimized manner over the available
“processors” of its partition.

Figure 1 – Task Dispatcher

The innovation that we call “virtualization” is that multiple operating system instances – Partitions - can
reside on the same Symmetric multiprocessing (SMP) processors and memory previously only used by
one. With processor virtualization, each dedicated-processor partition uses just a subset of particular
cores in the system. And even though they all reside within the same SMP, as far as a dedicatedprocessor Partition A is concerned, its cores are the whole of this system; Partition A has no visibility
outside of that.

POWER7 Logical Partitions