Cores versus vCPUs and simultaneous multithreading (SMT) for IBM Power

Making a determination about whether or not a particular system consumes one or more cores is currently dependent on the level of simultaneous multithreading configured (SMT).

IBM Power provides simultaneous multithreading levels of 1, 2, 4 or 8 for each core which correspond to the number of vCPUs as in Table 1.

Table 1. Different SMT levels and their corresponding vCPUs
SMT level	SMT=1	SMT=2	SMT=4	SMT=8
1 Core	# vCPUs=1	# vCPUs=2	# vCPUs=4	# vCPUs=8
2 Cores	# vCPUs=2	# vCPUs=4	# vCPUs=8	# vCPUs=16
4 Cores	# vCPUs=4	# vCPUs=8	# vCPUs=16	# vCPUs=32

For systems where SMT is configured the calculation for the number of cores required for subscription purposes depends on the SMT level. Therefore, a 2-core subscription corresponds to 2 vCPUs on SMT level of 1, and to 4 vCPUs on SMT level of 2, and to 8 vCPUs on SMT level of 4 and to 16 vCPUs on SMT level of 8 as seen in the table above. A large virtual machine (VM) might have 16 vCPUs, which at a SMT level 8 will require a 2 core subscription based on dividing the # of vCPUs by the SMT level (16 vCPUs / 8 for SMT-8 = 2). As subscriptions come in 2-core units, you will need one 2-core subscription to cover these 2 cores or 16 vCPUs.