Tuesday, May 30, 2006

Computer Science: More Threads = Less Contention?

Have you read The Next Mainstream Programming Language? The original PDF was written by Tim Sweeney, the founder of Epic, the video game company that created Unreal. He really got me thinking about the problems he was facing.

Let's suppose you have 10,000-100,000 objects. Let's suppose that at any moment any one of them might need to "touch" 50 other objects. That's the setup.

Now, let's suppose that you also have a super-lightweight threading system that can handle 100,000 threads. Yeah, sounds far-fetched, but you'd be surprised. Consider allocating a thread for each object. Along with its own thread, each object should have an input queue into which you can post new events. Naturally, this input queue should be protected by a mutex.

The interesting result is that because there are so many threads trying to access so many mutexes, the chances of any two threads trying to access the same mutex to push a new event is relatively small is you assume a random distribution of events. Practically speaking, more fine grained locking can result in reduced thread contention.

Of course, life isn't random, but it's an interesting thought.


Carl Friedrich Bolz said...

I think this is pretty much what ccp games (http://www.ccpgames.com/) is doing. I saw some cool talks about how they are using stackless python to have thousands of threads. Of course they are not exploiting several cores that way, but it shows that the technique can be useful.

Anonymous said...

check out the web server YAWS written in the programming language Erlang.

There are programming models without threads.

Shannon -jj Behrens said...

Stackless Python with coroutines is what I was hinting at.

Steve said...

Well, the minute I saw the post I thought "Stackless Python", but I am still trying to understand why you felt it necessary to hint. Why not jusst come out and say what you mean?

Shannon -jj Behrens said...

Steve, your question is reasonable. I figured lucky Python people like me weren't the only ones lucky enough to have massively scalable threading systems. I figured the rest of the world would eventually start catching up, so I didn't want to limit the scope of what I was trying to say.

Brandon L. Golm said...

this reminds me of the language I wrote "squirmy" ... it has one thread and one object and can only add two numbers. There's never any contention.

Shannon -jj Behrens said...

Squirmy, hmm, I've heard of it. It was interesting in that no one had ever managed to write a buffer overflow or memory leak in that language, right?

I bet if you wrote "Hello World" in Squirmy, it would run *really fast*!