Unofficial Guidelines for Shared Processor Settings

Many times I’ll have conversations with new Power Systems users and administrators about how best to size their LPARs. New users often ask about the correct number of virtual processors for a partition, if they have the right number of virtual processors configured and if there’s some preferred ratio of virtual processors to desired processors. These are great questions and the answers can help guide administrators in their sizing efforts as well as have a profound effect on the overall performance of their applications.

This article offers some guidelines for processor settings on shared processor partitions and, in particular, discusses the relationships between the number of desired and virtual processors and their effects on application performance.

When creating LPARs, administrators must decide whether to create dedicated LPARs, shared dedicated LPARs or shared LPARs. Dedicated LPARs are straightforward and easy to create. Administrators assign CPU capacity to dedicated LPARs using whole numbers. Dedicated LPARs don’t share their CPU cycles. Shared dedicated LPARs also use whole numbers to define the CPU capacity of their partitions. But these partitions are allowed to share their unused CPU cycles with other partitions. Shared LPARs are assigned CPU capacity in fractions. These partitions can both donate and receive unused CPU cycles.

Mike Rede is an IBM Power Systems Client Technical Specialist.

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