Simultaneous multithreading (SMT)

Higher workload throughput may be achieved because the z17 processor chip offers intelligently implemented 2-way simultaneous multithreading. Simultaneous multithreading (SMT) allows two active instruction streams per core, each dynamically sharing the core's execution resources. SMT is in z17 for workloads running on SAPs, Integrated Facility for Linux® (IFL), and z Integrated Information Processor (zIIP).

Each software Operating System has the ability to intelligently drive SMT in a way that is best for its unique requirements. z/OS® SMT management consistently drives the cores to high thread density to reduce SMT variability and deliver repeatable performance across varying CPU utilization. This provides more predictable SMT capacity. The z/VM® SMT management optimizes throughput by spreading a workload over the available cores until it demands the additional SMT capacity.

The operating system running in a logical partition will optionally enable that logical partition to use SMT. Some logical partitions may be running with SMT enabled while others are not. It is important to understand that logical partition management of processor resources is performed at the logical core level. The number of processors you define for use in the logical partition is the number of logical cores it has. When the partition is not enabled for SMT, this is equivalent to the number of CPUs in the logical partition. When a logical partition enables SMT, each logical core gets two CPUs (separate instruction streams) defined. The operating system then decides when one or both of the CPUs of a particular core are to be used. PR/SM then manages the dispatching of the logical cores to physical cores.