SMT Mode

Simultaneous multithreading (SMT) is a processor technology that allows multiple instruction streams (threads) to run concurrently on the same physical processor, improving overall throughput. To the operating system, each hardware thread is treated as an independent logical processor. Single-threaded (ST) execution mode is also supported.

To obtain best performance for the IBM® POWER9™ servers, most HPC applications that use matrix multiplication extensively must be run in SMT-4 mode with four threads per core on POWER9 SMT-4 servers and in SMT-8 mode with eight threads per core on POWER9 SMT-8 servers. For example, in most cases, the performance of double precision general matrix multiplication (DGEMM) obtained in SMT-4 mode that uses the ESSL Symmetric Multi-Processing (SMP) Library with four threads that are bound to each core is better than the performance obtained by using one thread that is bound to each core or by running the applications in ST mode or SMT-2 mode.

To obtain the best performance for the IBM POWER8 servers, most HPC applications that use matrix multiplication extensively should use one thread bound to each core.