PCSX2 Documentation/Thread Synchronization
Originally written by Jake Stine
It's the year 2009, and it's almost over at that; and as anyone reading this blog well knows, multithreaded applications are the here-and-now and future of desktop computing. It's the only way we can take advantage of multicore CPUs. But multithreaded programming offers more than just improved multicore performance. Using threaded programming is actually very important to developing software that behaves nicely. By that I mean software that refreshes its window contents quickly, responds to your mouse clicks, and lets you cancel stuff.
For that the best approach is usually threading, with the alternative being something called "Cooperative multitasking" where by a program is written such that it splits all tasks into neat little chunks. For example, the two possible ways to implement loading an image (let's say a png image):
- Load the image one scanline at a time, and then after each scanline manually check for keyboard, mouse, or other input, and refresh the screen.
- Load the image using a thread, and let the usual "global" windows message handler dispatch keyboard, mouse, and refresh messages as usual.
The second approach has several advantages. For one, it needs fewer temporary heap allocations (which are typically slow and fragment memory). It is more responsive: windows messages will be handled in parallel to the image loading, so you don't even need to "wait" until the end of a scanline for user input to have its effect. It's also more scalable: while the first system is able to load one image at a time only in co-operative fashion (extending it to support multiple is possible, but very difficult), the threaded approach can be scaled to load dozens of images at once with no additional complications.
The drawback is that thread synchronization and especially structured error handling across threads tends to be much more complicated than that of the linear cooperative model. If you don't have errors to handle, or don't really care about handling errors, then threaded tasking isn't so bad.
Enter PCSX2, where everything ends up being damn complicated. Being a perfectionist, I figured I'd design the new GUI completely on the threaded model, doing away with cooperative design almost completely. Such a design should help avoid any deadlocking scenarios and allow the emu to recover from almost any error gracefully. Problem: The emulator has a lot of inter-dependent parts and pieces that need to be interlocked and synchronized, and all of them can throw out a variety of errors -- which too I'd like to handle smartly; requesting extra user input when appropriate (and not just throwing out annoying or vague message boxes).
Interlocking dependencies can be a nightmare. For example, if you start a thread that loads an image, and then block on that thread until it completes, you're worse off than if you wrote yourself a cooperative image loader because now the whole program stalls waiting for the thread to complete anyway. And like everything else, there are two ways to handle this:
- Use a "friendly" blocking mechanism that periodically polls the user input and updates display. This is no better than cooperative single-thread designs though, as it has slow response times and doesn't scale well to multiple threads.
- Build your entire GUI around "messages" and "callbacks" (sometimes also called "signals"). This is the most flexible and user-friendly option but can add a lot of "framework" to any codebase.
I tried to use the first approach initially, because I was in a hurry to get things working. But it's been problematic since day 1, so now I'm redoing most things to use the second method instead.
The second one is in fact the recommended design by Microsoft, and one they've been using for almost everything in Windows ever since Win95. It's one of the reasons the Win32 API feels "heavy" to a lot of programmers, but as it turns out, it's not without good reason.