The example cluster that is shown in Figure 7-4 contains two nodes (Power AC922 systems),  each of which has two sockets. One nonuniform memory access (NUMA) node corresponds  to one socket and has 20 physical POWER9 cores inside it. The systems are configured with  simultaneous multithreading (SMT)-4, which means that a physical core can split a job  between four logical CPUs (threads).  IBM Parallel Environment MPI provides run options or environment variables that help to  simulate runs with different SMT modes. The following cases describe two ways where a job  uses only one logical CPU from each POWER9 core (SMT-1 mode) and fully uses all 176  logical CPUs by using the code in Example 7-15 on page 200 with IBM Parallel ESSL calls: MP_TASK_AFFINITY=core:1 MP_RESD=poe MP_PROCS=44 ./test_pdgemm MP_TASK_AFFINITY=core:1 directs the IBM Parallel Environment MPI that each MPI task  can use only one POWER9 core. Overall, the full cluster has 88 POWER9 cores (each  node contains 44 cores), so a maximum of 88 MPI ( MP_PROCS=88 ) tasks can be used for  such run. Another possible solution to simulate SMT-1 mode is to take 20 MPI tasks with two  POWER8 cores for each of them by running the following command: MP_TASK_AFFINITY=core:2 MP_RESD=poe MP_PROCS=44 ./test_pdgemm MP_TASK_AFFINITY=cpu:16 MP_RESD=poe MP_PROCS=22 ./test_pdgemm MP_TASK_AFFINITY=cpu:16 means when you use the IBM Parallel Environment MPI that  each MPI task can use only 16 logical CPUs. Each node in the cluster contains 176 logical  CPUs, so 11 MPI tasks per node and 22 for the overall cluster can be used in this run. GLYPH<SM590000> The following command shows how to use the cluster and all 176 logical CPUs per node: GLYPH<SM590000> To configure the job to use only one logical CPU per POWER9 core, run the following  command: Figure 7-4 Structure of a node in a cluster 214  IBM High-Performance Computing Insights with IBM Power System AC922 Clustered Solution  CPU 0 CPU 3 CPU 1 CPU 2 Core 0 CPU 84 CPU 87 CPU 85 CPU 86 Core 21 NUMA Node 0 Socket 0 CPU 88 CPU 92 CPU 89 CPU 90 Core 22 CPU 172 CPU 175 CPU 173 CPU 174 Core 43 NUMA Node 8 Socket 8 44 POWER9 Physical Cores 176 logical CPUs Simulating different SMT modes