Simultaneous multithreading

6.6 LPAR mode

Linux® in LPAR mode can use the simultaneous multithreading technology on mainframes.

IBM® z13® introduced the simultaneous multithreading technology to the mainframe. In Linux terminology, simultaneous multithreading is also known as SMT or Hyper-Threading.

With multithreading enabled, a single core on the hardware is mapped to multiple logical CPUs on Linux. Thus, multiple threads can issue instructions to a core simultaneously during each cycle.

To find out whether multithreading is enabled for a particular Linux instance, compare the number of cores with the number of threads that are available in the LPAR. You can use the hyptop command to obtain this information.

Simultaneous multithreading is designed to enhance performance. Whether this goal is achieved strongly depends on the available resources, the workload, and the applications that run on a particular Linux instance. Depending on these conditions, it might be advantageous to not make full use of mutithreading or to disable it completely. Use the hyptop command to obtain utilization data for threads while Linux runs with multithreading enabled.

You can use the smt= and nosmt kernel parameters to control multithreading. By default, Linux in LPAR mode uses multithreading if it is provided by the hardware.