Context switch.

also: context switches · context switching

The OS saving one thread or process's CPU state and loading another's; frequent switches waste CPU time and caches.

the two-minute refresher

The kernel saves registers, program counter and memory info for one task and restores another's.
Direct cost is microseconds; the indirect cost is cold CPU caches and TLB.
Thousands of busy threads mean constant switching: thread-per-connection does not scale far.
Event loops and I/O multiplexing serve many connections with few threads.

also: context switches · context switching

The OS saving one thread or process's CPU state and loading another's; frequent switches waste CPU time and caches.

The kernel saves registers, program counter and memory info for one task and restores another's.
Direct cost is microseconds; the indirect cost is cold CPU caches and TLB.
Thousands of busy threads mean constant switching: thread-per-connection does not scale far.
Event loops and I/O multiplexing serve many connections with few threads.