Implementing asynchronous delays

Question

What would be a smart way to implement something like the following?

// Plain C function for example purposes.
void sleep_async(delay_t delay, void (* callback)(void *), void * data);

That is, a means of asynchronously executing a callback after a delay. POSIX, for example, has a few functions that do something like this, but they are mostly for asynchronous I/O (see this for what I mean). What interests me about those functions how they are executed "as if" on a new thread, according to that manual page, where an implementation may choose to spawn "a single thread...to receive all notifications". I am aware that some may nonetheless choose to spawn a whole thread for each of them, and that stuff like this may require support from the OS itself, so this is just an example.

I already have a couple of ways I could implement this (e.g. priority queue of events sorted by wake time on a timer loop, with no need to start a thread at all), but I am wondering whether there already exists smart[er] or [more] complete implementations of what I want to accomplish. For example, maybe implementations of Task.Delay() from C♯ (and coroutines like it in other language environments) do something smart in minimizing the amount of thread spawning for getting asynchronous delays.

Why am I looking for something like this? As implied by the title, I'm looking for something asynchronous. The above signature is just a simple C example to illustrate roughly what POSIX does. I am implementing some C++20 coroutines for use with co_await and friends, with thread pools and whatnot. Scheduling anything that would end up synchronously waiting on something is probably a bad idea, as it would prevent otherwise free threads from doing any work. Spawning [and potentially immediately detaching] a new thread just to add in an asynchronous delay doesn't seem like a very smart idea, either. My timer loop idea could be okay, but that implies needing a predefined timer granularity, and overhead from the priority queue.

Edit

I neglected to mention any real set of target platforms, as a commenter mentioned. I don't expect to target anything outside the "usual" desktop platforms, so the quirks of embedded development are ignored. The way I plan to use asynchronous delays themselves this way does not necessarily require threading support (everything could just be on a timer loop), but threading will nonetheless be required and used in accord (namely thread pools on which coroutines would be scheduled).

user_5481471 · Answer

The simple but inefficient way would be to spawn a thread, have it sleep for delay, and then call the callback. This can be done in just a few lines using std::async():

auto delayed_call = std::async(std::launch::async, [&]{
    std::this_thread::sleep_for(delay);
    callback(data);
});

As mentioned by Thomas Matthews, this requires support for threads. While it's fine for a one-off call, it's not efficient if you have many such delayed calls. Having a priority queue and an event loop or a dedicated thread to handle events in this queue, as you already mentioned, is probably the most efficient way to do it. If you are looking for a library that implements this, then have a look at boost::asio.

As for using C++20 coroutines, I do not think that this will make something like your sleep_async() any easier. However, an event loop could be implemented on top of it.

user_1095108 · Answer

A smart way? You mean really, really smart? That would be my own implementation, of course. You know about POSIX timers, you probably know about linux timers and the various hacks involving std::thread. But, more seriously, what you require sounds mostly to the tune of something like libeio, or libuv - both of these provide callbacks. It depends on what you can afford in binary size and whether you like the particular abstractions a library offers. The 2 libraries seem to be evolved versions of libevent and libev, libevent being the progenitor of them all.

Creating a std::thread instance involves allocating a stack frame, at the very least, which is by no means cheap.

Implementing asynchronous delays

2 Answers