CPU assignment to NUMA nodes

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The Linux® scheduler requires a stable mapping of CPUs to NUMA nodes. Therefore, cores are pinned to NUMA nodes when one of their CPUs is set online for the first time.

As a consequence, a CPU that is set offline is always assigned to its previous NUMA node when it is set back online. With multithreading enabled, a CPU is equivalent to a thread.

Pinned cores are distributed evenly across the NUMA nodes. You can distort this initial balance by setting a disproportionate number of CPUs from a particular NUMA node offline. New CPUs are assigned according to the number of pinned cores, not according to the number of online CPUs.

For example, assume a node A that has two cores and with one of four CPUs (threads) online. Further, assume a node B that has one core but two CPUs online. Because node B has fewer cores than node A, a newly configured CPU that is set online is assigned to node B, and the corresponding core is pinned to node B.

Note: Do not use NUMA emulation with cpuplugd. The cpuplugd daemon can distort the balance of CPU assignment to NUMA nodes. Issue the following command to find out if cpuplugd is running:

# service cpuplugd status