CPU planning considerations

You must plan for the number of virtual CPUs to allocate to each zCX instance. Each virtual CPU is a z/OS dispatchable task within the zCX address space.

zCX virtual processors can be dispatched on zIIPs or general purpose CPUs. This allows z/OS to dispatch the zCX virtual CPUs on the available z/OS processors, as with any other z/OS workload based on Workload Management policies and priorities. For more details, refer Workload management considerations.

For the best performance, IBM recommends that the number of virtual CPUs across all zCX instances expecting heavy and simultaneous usage should be no greater than the number of zIIPs or general purpose processors available to the z/OS system.

Simultaneous Multi-Threading

When Simultaneous Multi-Threading (SMT) is enabled, zCX can exploit the additional zIIP CPU threads.

CPU Starvation

If the CPU resources allocated to the z/OS system are insufficient for the zCX instance(s), this can lead to a situation called CPU starvation, where a zCX instance's virtual CPUs are not dispatched in a timely fashion from the z/OS system.  The Linux guest is dependent on the virtual CPUs to be dispatched in a timely fashion, otherwise Linux process may timeout and abnormally end.

Message AZIF0146E indicates that a Linux program has abnormally ended and may indicate a CPU starvation condition. Refer to AZIF0146E in z/OS MVS System Messages, Vol 3 (ASB-BPX) for more details.