Editing Processor affinity (section)

==Usage==
Processor affinity can effectively reduce cache problems, but it does not reduce the persistent [[Load balancing (computing)|load-balancing]] problem.<ref>[http://www.tmurgent.com/WhitePapers/ProcessorAffinity.pdf "White Paper - Processor Affinity"] - From [http://www.tmurgent.com tmurgent.com]. Accessed 2007-07-06.</ref> Also note that processor affinity becomes more complicated in systems with non-uniform architectures. For example, a system with two [[Multi-core processor|dual-core]] [[hyper-threading|hyper-threaded]] [[Central processing unit|CPUs]] presents a challenge to a scheduling algorithm.

There is complete affinity between two virtual CPUs implemented on the same core via hyper-threading, partial affinity between two cores on the same physical processor (as the cores share some, but not all, cache), and no affinity between separate physical processors. As other resources are also shared, processor affinity alone cannot be used as the basis for CPU dispatching. If a process has recently run on one virtual hyper-threaded CPU in a given core, and that virtual CPU is currently busy but its partner CPU is not, cache affinity would suggest that the process should be dispatched to the idle partner CPU. However, the two virtual CPUs compete for essentially all computing, cache, and memory resources. In this situation, it would typically be more efficient to dispatch the process to a different core or CPU, if one is available. This could incur a penalty when process repopulates the cache, but overall performance could be higher as the process would not have to compete for resources within the CPU.{{citation needed|date=June 2021}}