On NUMA systems with a lot of CPUs, it is common to assign parts of the
workload to different subsets of the available processors. This
partitioning can improve performance while reducing the ability of jobs to
interfere with each other. The partitioning mechanisms available on
current kernels might just do too good a job in some situations, though,
leaving some CPUs idle while others are overutilized. The soft
affinity patch set from Subhra Mazumdar is an attempt to improve
performance by making that partitioning more porous.

Source: LWN.net – [$] Soft CPU affinity