Why is async programming faster

Question

I keep hearing that using async programming patterns will make my code run faster. Why is that true? Doesn't the same exact code have to run either way, whether it runs now or it runs later?

Answer 1

It's not faster, it just doesn't waste time.

Synchronous code stops processing when waiting for I/O. Which means that when you're reading a file you can't run any other code. Now, if you have nothing else to do while that file is being read then asynchronous code would not buy you anything much.

Usually the additional CPU time that you can use is useful for servers. So the question is why do asynchronous programming instead of starting up a new thread for each client?

It turns out that starting and tearing down threads is expensive. Some time back in the early 2000s a web server benchmark found that tclhttpd compared favorably to Apache for serving static image files. This is despite the fact that tclhttpd was written in tcl and Apache was written in C and tcl was known to be 50 times slower than C. Tcl managed to hold its own against Apache because tcl had an easy to use asynchronous I/O API. So tclhttpd used it.

It's not that C doesn't have asynchronous I/O API. It's just that they're rarely used. So Apache didn't use it. These days, Apache2 uses asynchronous I/O internally along with thread pools. The C code ends up looking more complicated but it's faster - lesson learned.

Which leads us to the recent obsession with asynchronous programming. Why are people obsessed with it? (most answers on Stackoverflow about javascript programming for example insist that you should never use synchronous versions of asynchronous functions).

This goes back to how you rarely see asynchronous programs in C even though it's the superior way of doing things (GUI code is an exception because UI libraries learned early on to rely on asynchronous programming and Events). There are simply too many functions in C that are synchronous. So even if you wanted to do asynchronous programming you'll end up calling a synchronous function sooner or later. The alternative is to abandon stdlib and write your own asynchronous libraries for everything - from file I/O to networking to SQL.

So, in languages like javascript where asynchronous programming ended up as the default style there is pressure from other programmers to not mess it up and accidentally introduce synchronous functions which would be hard to integrate with asynchronous code without losing a lot of performance. So in the end, like taxes, asynchronous code has become a social contract.