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Processor affinity
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<p>Processor affinity, or CPU pinning or "cache affinity", enables the binding and unbinding of a <a href="/facts/Process_(computing)/Vcq7EnUV">process</a> or a <a href="/facts/Thread_(computing)/hh8WHyPd">thread</a> to a <a href="/facts/Central_processing_unit/Jxp1bqMV">central processing unit</a> (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 <a href="/facts/Scheduling_algorithm/XptmcM1a">scheduling algorithm</a> in a <a href="/facts/Symmetric_multiprocessing/T9CahUsN">symmetric multiprocessing</a> operating system. Each item in the queue has a tag indicating its kin <a href="/facts/Central_processing_unit/Jxp1bqMV">processor</a>. At the time of resource allocation, each task is allocated to its kin processor in preference to others. </p><p>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 <a href="/facts/CPU_cache/x8BYFcdv">cache memory</a>) 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 <a href="/facts/Cache_miss/gmJYN8wm">cache misses</a>, but may slow down ordinary programs because they would need to wait for that CPU to become available again. A practical example of processor affinity is executing multiple instances of a non-threaded application, such as some graphics-rendering software. </p><p>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. </p>