What is the difference between busy-wait and polling?

Question

From the Wikipedia article on Polling

Polling, or polled operation, in computer science, refers to actively sampling the status of an external device by a client program as a synchronous activity. Polling is most often used in terms of input/output (I/O), and is also referred to as polled I/O or software driven I/O.

Polling is sometimes used synonymously with busy-wait polling (busy waiting). In this situation, when an I/O operation is required the computer does nothing other than check the status of the I/O device until it is ready, at which point the device is accessed. In other words the computer waits until the device is ready.
Polling also refers to the situation where a device is repeatedly checked for readiness, and if it is not the computer returns to a different task. Although not as wasteful of CPU cycles as busy-wait, this is generally not as efficient as the alternative to polling, interrupt driven I/O.

So, when a thread doesn't use the "condition variables", will it be called "polling" for the data change or "busy waiting"?

Answer 1

Suppose one has a microprocessor or microcontroller which is supposed to perform some action when it notices that a button is pushed.

A first approach is to have the program enter a loop which does nothing except look to see if the button has changed yet and, once it has, perform the required action.

A second approach in some cases would be to program the hardware to trigger an interrupt when the button is pushed, assuming the button is wired to an input that's wired so it can cause an interrupt.

A third approach is to configure a timer to interrupt the processor at some rate (say, 1000x/second) and have the handler for that interrupt check the state of the button and act upon it.

The first approach uses a busy-wait. It can offer very good response time to one particular stimulus, at the expense of totally tuning out everything else. The second approach uses event-triggered interrupt. It will often offer slightly slower response time than busy-waiting, but will allow the CPU to do other things while waiting for I/O. It may also allow the CPU to go into a low-power sleep mode until the button is pushed. The third approach will offer a response time that is far inferior to the other two, but will be usable even if the hardware would not allow an interrupt to be triggered by the button push.

In cases where rapid response is required, it will often be necessary to use either an event-triggered interrupt or a busy-wait. In many cases, however, a polled approach may be most practical. Hardware may not exist to support all the events one might be interested in, or the number of events one is interested in may substantially exceed the number of available interrupts. Further, it may be desirable for certain conditions to generate a delayed response. For example, suppose one wishes to count the number of times a switch is activated, subject to the following criteria:

1. Every legitimate switch event will consist of an interval from 0 to 900us (microseconds) during which the switch may arbitrarily close and reopen, followed by an interval of at least 1.1ms during which the switch will remain closed, followed by an interval from 0 to 900us during which the switch may arbitrarily open and reclose, followed by an interval of which at least 1.1ms during which the switch will be open.
2. Software must ignore the state of the switch for 950us after any non-ignored switch opening or closure.
3. Software is allowed to arbitrarily count or ignore switch events which occur outside the above required blanking interval, but which last less than 1.1ms.
4. The software's reported count must be valid within 1.99ms of the time the switch is stable "closed".

The easiest way to enforce this requirement is to observe the state of the switch 1,000x/second; if it is seen "closed" when the previous state was "open", increment the counter. Very simple and easy; even if the switch opens and closes in all sorts of weird ways, during the 900us preceding and following a real event, software won't care.

It would be possible to use a switch-input-triggered interrupt along with a timer to yield faster response to the switch input, while meeting the required blanking requirement. Initially, the input would be armed to trigger the next time the switch closes. Once the interrupt was triggered, software would disable it but set a timer to trigger an interrupt after 950us. Once that timer expired, it would trigger an interrupt which would arm the interrupt to fire the next time the switch is "open". That interrupt would in turn disable the switch interrupt and again set the timer for 950us, so the timer interrupt would again re-enable the switch interrupt. Sometimes this approach can be useful, but the software is a lot more complicated than the simple polled approach. When the timer-based approach will be sufficient, it is often preferable.

In systems that use a multitasking OS rather than direct interrupts, many of the same principles apply. Periodic I/O polling will waste some CPU time compared with having code which the OS won't run until certain events occur, but in many cases both the event response time and the amount of time wasted when no event occurs will be acceptable when using periodic polling. Indeed, in some buffered I/O situations, periodic polling might turn out to be quite efficient. For example, suppose one is receiving a large amount of data from a remote machine via serial port, at most 11,520 bytes will arrive per second, the device will send up to 2K of data ahead of the last acknowledged packet, and the serial port has a 4K input buffer. While one could process data using a "data received" event, it may be just as efficient to simply check the port 100x/second and process all packets received up to that point. Such polling would be a waste of time when the remote device wasn't sending data, but if incoming data was expected it may be more efficient to process it in chunks of roughly 1.15K than to process every little piece of incoming data as soon as it comes in.

Answer 2

The difference between the two is what the application does between polls.

If a program polls a device say every second, and does something else in the mean time if no data is available (including possibly just sleeping, leaving the CPU available for others), it's polling.
If the program continuously polls the device (or resource or whatever) without doing anything in between checks, it's called a busy-wait.

This isn't directly related to synchronization. A program that blocks on a condition variable (that should signal when a device or resource is available) is neither polling nor busy-waiting. That's more like event-driven/interrupt-driven I/O.
(But for example a thread that loops around a try_lock is a form of polling, and possibly busy-waiting if the loop is tight.)