Partition placement by PR/SM

Every new machine generation of IBM Z brings with it an increase in number of physical processors and memory capacity. Some generations can also bring change in the physical configuration of the server. The z15 for example, can have from one to five drawers instead of a maximum of four on the z14. As another example, z15 has fixed two chips per node versus the two or three chips per node on z14. The logical partitions on the other hand can come in various configurations, including “Dedicated” logical partition, shared “Hiperdispatch = YES” logical partition, and shared “Hiperdispatch = NO” partition. Each of the partition types can request as many logical processors and memory as the machine generation will allow, which is usually less than the physical resources available on the machine. The optimal placement of logical partitions on the physical server, given its configuration, is an NP-hard problem. Memory access latency and cache usage play vital roles in the performance of logical partitions, and it is imperative that placement is optimal. Moreover, on z15, the integrated facility for linux processors and internal coupling facility processors can be moved from one chip to another, during reoptimization of partition placement, in addition to general-purpose and IBM Z integrated information processors that are already allowed to be moved, compounding the placement problem. This article describes the changes made to the Processor Resource/Systems Manager (PR/SM) heuristic placement algorithm for z15 and how it surmounts the problems inherent for optimal placement of logical partitions.