Scaling memory and number of processors

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The test results are inspected for selected z/VM® configurations. The focus of the results is on overall z/VM CPU load and on z/VM paging behavior.For z/VM configuration details, see Test configurations. For the test procedure, see Test procedure.

z/VM CPU overcommitment

Figure 1 shows that a breakup of the z/VM CPU loads was observed when the combined workload set was processed within several constrained z/VM configurations.

Figure 1. Breakup of z/VM CPU usage when real CPU and real memory are constrained

In order to emphasize the distribution of CPU loads for each z/VM configuration, in Figure 1, all results are scaled, such that all columns extend to 100 %.

Observations:

The fractions of CP system CPU load, emulation CPU load, and CP attributed to guest CPU load stay rather constant while the number of real CPUs is reduced to 10, and the amount of real memory is reduced to 96 GiB. Below these limits, the fraction of the CP system CPU load starts to rise.

Conclusions:

As long as real resource constraints stayed within tolerable limits, z/VM is quite well capable to process virtual system workload without increased system expenses. If resources are further constrained, apparently additional system expenses result. For example, we observed that system paging activity rises significantly as real memory is limited to values below 96 GiB. Recall that during the reference execution, the combined instantiated memory, that is, the sum of the virtual memory actual in use by all virtual systems, was found to be 133 GiB. Thus, when real memory was 96 GiB or below, significant z/VM real memory pressure did result. Nevertheless, the used z/VM 6.3 test environment remained capable to process the workload even in that situation, albeit with performance degradations, but without workload failures.