Summary

This White Paper describes the z/VM 6.3 HiperDispatch polarization modes and their affect on middleware performance.

The first step in this analysis was to update z/VM 6.3 to the z13 exploitation SPE (APAR VM65586 and related). While the main focus of this APAR is the support of the new Simultaneous Multithreading (SMT) feature for z/VM 6.3 on IBM® z13, the APAR also provides improvements on non-SMT z Systems® machines. It contains changes in CPU and scheduler management that also benefit zEnterprise 196 systems. The workload throughput increased by 8.6% and the CPU cost was reduced by by 3.5%, which in this environment represents almost a full IFL (0.92 IFL).

The HiperDispatch feature provided by z/VM 6.3 introduced a new approach for dispatching virtual CPUs on physical CPUs that is controlled by the SRM (System Resource Manager) polarization parameter. The new dispatch behavior is called vertical polarization, which is also the default. The previous dispatch behavior is called horizontal polarization.

The use of the HiperDispatch feature in combination with vertical polarization did achieve good cache performance due to reduced motion of logical CPUs, the grouping of logical CPUs, and so on. However, this was achieved at the cost of longer dispatch waits. As a result, the USTAT report showed a higher percentages of samples in which the user waited for a CPU. However, the throughput of our workloads increased by 3% and the CPU load was reduced by 2.5 IFLs (from 28.8 IFLs by horizontal to 26.2 with vertical). We could also show that this improvement is caused by a better cache utilization of the z196 processors.

Therefore, the observed increase of dispatch delay times with our workload pattern did not indicate a decrease in performance. On the contrary, the system's overall performance was significantly improved!