6.3. Cores versus vCPUs and hyperthreading

Making a determination about whether or not a particular system consumes one or more cores is currently dependent on whether or not that system has hyperthreading available. Hyperthreading is only a feature of Intel CPUs. Visit the Red Hat Customer Portal to determine whether a particular system supports hyperthreading.

Virtualized OpenShift nodes using logical CPU threads, also known as simultaneous multithreading (SMT) for AMD EPYC CPUs or hyperthreading with Intel CPUs, calculate their core utilization for OpenShift subscriptions based on the number of cores/CPUs assigned to the node, however each subscription covers 4 vCPUs or cores when logical CPU threads are used. Red Hat’s subscription management tools assume logical CPU threads are enabled by default on all systems.

For systems where hyperthreading is enabled and where one hyperthread equates to one visible system core, the calculation of cores is a ratio of 2 cores to 4 vCPUs. Therefore, a 2-core subscription covers 4 vCPUs in a hyperthreaded system. A large virtual machine (VM) might have 8 vCPUs, equating to 4 subscription cores. As subscriptions come in 2-core units, you will need two 2-core subscriptions to cover these 4 cores or 8 vCPUs.

Where hyperthreading is not enabled, and where each visible system core correlates directly to an underlying physical core, the calculation of cores is a ratio of 2 cores to 2 vCPUs.