Asynchronous I/O, or non-blocking I/O, is a form of input/output processing that permits other processing to continue before the transmission has finished. Input and output operations on a computer can be extremely slow compared to the processing of data. An I/O device can incorporate mechanical devices which must physically move, such as a hard drive seeking a track to read or write; this is extremely slow compared to merely moving electrons. For example, during a disk operation that takes ten milliseconds to perform, a processor that is clocked at one gigahertz could have performed ten million instruction-processing cycles. A simple approach to I/O would be to start the access and then wait for it to complete. But such an approach would block the progress of a program while the communications is in progress, leaving system resources idle. When a program makes many I/O operations, this means that the processor can spend almost all of its time idle waiting for I/O operations to complete.

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