Asynchronous I/O

In computer science, asynchronous I/O (also non-sequential I/O) is a form of <a href="/facts/Input%2foutput/WQTE8Jas">input/output</a> processing that permits other <a href="/facts/Process_(computing)/Vcq7EnUV">processing</a> to continue before the I/O operation has finished. A name used for asynchronous I/O in the Windows API is <a href="/facts/Overlapped_I%2fO/nxF9SaUN">overlapped I/O</a>.
<a href="/facts/Input_and_output/WQTE8Jas">Input and output</a> (I/O) operations on a computer can be extremely slow compared to the processing of data. An I/O device can incorporate mechanical devices that must physically move, such as a hard drive seeking a track to read or write; this is often <a href="/facts/Orders_of_magnitude/cWzRP2dM">orders of magnitude</a> slower than the switching of electric current. For example, during a disk operation that takes ten milliseconds to perform, a processor that is clocked at one <a href="/facts/Gigahertz/6Wshj5qm">gigahertz</a> 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, called synchronous I/O or blocking I/O, would block the progress of a program while the communication is in progress, leaving <a href="/facts/System_resources/bX958SEx">system resources</a> idle. When a program makes many I/O operations (such as a program mainly or largely dependent on <a href="/facts/User_input/WQTE8Jas">user input</a>), this means that the processor can spend almost all of its time idle waiting for I/O operations to complete.
Alternatively, it is possible to start the communication and then perform processing that does not require that the I/O be completed. This approach is called asynchronous input/output. Any task that depends on the I/O having completed (this includes both using the input values and critical operations that claim to assure that a write operation has been completed) still needs to wait for the I/O operation to complete, and thus is still blocked, but other processing that does not have a dependency on the I/O operation can continue.
Many operating system functions exist to implement asynchronous I/O at many levels. In fact, one of the main functions of all but the most rudimentary of <a href="/facts/Operating_systems/8XTuvMh2">operating systems</a> is to perform at least some form of basic asynchronous I/O, though this may not be particularly apparent to the user or the programmer. In the simplest software solution, the hardware device status is <a href="/facts/Polling_(computer_science)/WVa88JIg">polled</a> at intervals to detect whether the device is ready for its next operation. (For example, the <a href="/facts/CP%2fM/n5rTgXdC">CP/M</a> operating system was built this way. Its <a href="/facts/System_call/NW44AFJr">system call</a> semantics did not require any more elaborate I/O structure than this, though most implementations were more complex, and thereby more efficient.) <a href="/facts/Direct_memory_access/DWuVgclV">Direct memory access</a> (DMA) can greatly increase the efficiency of a polling-based system, and <a href="/facts/Hardware_interrupt/Mm6C4rpc">hardware interrupts</a> can eliminate the need for polling entirely. <a href="/facts/Computer_multitasking/Q4acPNMv">Multitasking</a> operating systems can exploit the functionality provided by hardware interrupts, whilst hiding the complexity of interrupt handling from the user. <a href="/facts/Spooling/fPbanPPu">Spooling</a> was one of the first forms of multitasking designed to exploit asynchronous I/O. Finally, <a href="/facts/Multithreading_(computer_architecture)/ojEKsSNP">multithreading</a> and explicit asynchronous I/O <a href="/facts/API/GMlN4vUr">APIs</a> within user processes can exploit asynchronous I/O further, at the cost of extra software complexity.
Asynchronous I/O is used to improve energy efficiency, and in some cases, throughput. However, it can have negative effects on latency and throughput in some cases.

Asynchronous I/O open-in-new

Asynchronous I/O