Editing Processor affinity (section)

{{Short description|Assignment of a task to a given core of a CPU}}
{{more sources|date=June 2021}}
In [[computer science]], '''processor affinity''', also called '''CPU pinning''' or '''cache affinity''', enables the binding and unbinding of a [[Process (computing)|process]] or a [[Thread (computing)|thread]] to a [[central processing unit]] (CPU) or a range of CPUs, so that the process or thread will execute only on the designated CPU or CPUs rather than any CPU. This can be viewed as a modification of the native central queue [[scheduling algorithm]] in a [[symmetric multiprocessing]] operating system. Each item in the queue has a tag indicating its kin [[central processing unit|processor]]. At the time of resource allocation, each task is allocated to its kin processor in preference to others.

Processor affinity takes advantage of the fact that remnants of a process that was run on a given processor may remain in that processor's state (for example, data in the [[CPU cache|cache memory]]) after another process was run on that processor. Scheduling a CPU-intensive process that has few interrupts to execute on the same processor may improve its performance by reducing degrading events such as [[cache miss]]es, but may slow down ordinary programs because they would need to wait for that CPU to become available again.<ref>{{cite web|url=https://www.ibm.com/docs/en/aix/7.2?topic=architecture-processor-affinity-binding|title=Processor affinity and binding|website=[[IBM]] |access-date=2021-06-08}}</ref> A practical example of processor affinity is executing multiple instances of a non-threaded application, such as some graphics-rendering software. {{citation needed|date=June 2021}}

Scheduling-algorithm implementations vary in adherence to processor affinity. Under certain circumstances, some implementations will allow a task to change to another processor if it results in higher efficiency. For example, when two processor-intensive tasks (A and B) have affinity to one processor while another processor remains unused, many schedulers will shift task B to the second processor in order to maximize processor use. Task B will then acquire affinity with the second processor, while task A will continue to have affinity with the original processor.{{citation needed|date=June 2021}}