The Quest for More Processing Power, Part Two: "Multi-core and multi-threaded gaming"
by Johan De Gelas on March 14, 2005 12:05 AM EST- Posted in
- CPUs
Threads & Gaming
Tim gave us some extremely interesting information. Yes, the extra computing power of multi-cores is welcome in the gaming industry. Better game physics, animation and intensive and accurate sound effects are made possible with more than one core.
The tools, which Intel advertises in almost any multi-core presentation, are next to useless for the problems that the developers face, as Tim explained. Auto parallelisation is a nice trick to increase the spec FP score, but it is next to useless for a real world application. The good news for Intel, AMD and others is that the CPU will play a much more important role again. Physics, Artificial Intelligence and animation can be improved significantly by being parallelised and using the extra capabilities in dual core CPUs. But there are limits to Thread Level Parallelism. While increased ILP (Instruction Level Parallelism, IPC) might require exponentional increasing efforts of the manufacturer, using more and more threads, or increased TLP (Thread Level Parallelism), requires exponentional efforts from the developers. Tim clearly emphasizes that only parts of the application can be economically parallelized. Increasing parallelisation, using more threads, is simply not feasible. There is a pretty hard economic limit to TLP.
Tim Sweeney resumes:
"You can expect games to take advantage of multi-core pretty thoroughly in late 2006 as games and engines also targeting next-generation consoles start making their way onto the PC.
Writing multithreaded software is very hard; it's about as unnatural to support multithreading in C++ as it was to write object-oriented software in assembly language. The whole industry is starting to do it now, but it's pretty clear that a new programming model is needed if we're going to scale to ever more parallel architectures. I have been doing a lot of R&D along these lines, but it's going slowly."
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hzmonte - Tuesday, October 4, 2005 - link
For everyone's convenience, here is Part 3: http://www.anandtech.com/cpuchipsets/showdoc.aspx?...">http://www.anandtech.com/cpuchipsets/showdoc.aspx?...bmayer - Friday, March 25, 2005 - link
About the automatic parallization:It can be fairly easy to do. I work on some Cray X1 and X1es. A little bit about the X1. The Processors (called Multi-Streaming Processors (MSP)) are made up of 4 Single Streaming Processors (SSP). They are vector units with a lenght of 64 or 32 (can't remember, the point is they are good sized vector units). The processors are clocked at 800MHz for the X1 or 1.3GHz if you have an X1e.
Ok so what do we see? These CPUs *suck* if your code is not vectorized and running in parallel.Guess what the Cray compiler does? Automatically vectorizes, streams (takes advantage of a full MSP instead of a single SSP), and parallizes.
They lay out very clearly what the conditions are where the compiler can NOT optimize, and give you directives where you can force it to do so. You can also get a listing of why it did not do a given optimization for any given line. Actually it gives you all information by default which combined with grep is nice.
OK so there are different types of parallelizm, and the one I have just talked about is different then what they are trying to do. This has been talking about speeding up the execution of some inner loop, which is very different from doing two different things at the same time (AI module and sound module running at the same time). BUT this can still be used for great effect. When the inner loops execute for half the time as on a single core/CPU machine we now have more time to do other things, and thus see a speed improvement.
I have thought that Sony/IBM should get in touch with Cray to supply compiler tech for the Cell processor. If the Cell is as easy to write parallel code for as the X1 is we will have some very awesome games, and clusters of PS3s.
If you want to see a very nice overview of processor history and some of the crazy things people are proposing to do with the multi-cores check out ftp://ftp.cs.wisc.edu/sohi/talks/2003/pact.pdf
I agree that parallel C++ is just not happening very well. There are languages like UPC which are starting to gain hold in the HPC market, which *could* find some use in the game market. But as the state of the art stands it is Fortran which is really great for automatically generating parallel code. But who could serriously say that someone outside of engineering writes a code in Fortran?
Great article, lots to think about!
blckgrffn - Friday, March 18, 2005 - link
Loved it. All of it. Especially the interview with Sweeney - it is always nice to hear where the future *will* lie with regards to at least one major application/game. Now, just get an interview with Carmack, and I will be happy for a long time... :DNat
Caleb Jasin - Friday, March 18, 2005 - link
#41Sorry for the late reply.
And yeah pthreads are not threads really. They are processes. When you call the pthread_create() function you create a new proccess ;)
ravedave - Wednesday, March 16, 2005 - link
Sorry to double post. You could easily make a benchmark that saw 1000000000% speed increase. Take one application give it high priority and have it loop for 5 days. It would lock everything else up. Throw in a second processor and you no longer have that problem, hence a huge speedup in the other processes. I dont trust any numbers from any manufacturer.ravedave - Wednesday, March 16, 2005 - link
Excellent article. Extremely excllent. I like the fact that you mentioned GUI updates, most people forget that almost all applications are multi-thread as far as GUI/core go. I really think that Microsoft is on the right track with .NET though. I belive .NET 2 or 2.5 will really take multithreading to the next level.RockHydra11 - Tuesday, March 15, 2005 - link
My fear is that instead of creating new architctures for their processors to increase performance, they will just shove more cores on it and pass it off on people.Verdant - Tuesday, March 15, 2005 - link
#40i do see a shift to something like C# but anything that brings a "performance" hit, is likely to scare away developers, especially since on non-windows platforms the hit is pretty huge atm.
you are right that a compiler probably isn't an answer, i was merely stating that if the industry was dedicated to creating a "deserializing" compiler it would be possible, extremely complex, and probably technically more than a "compiler" but still possible...
also you are thinking of UNIX, the linux kernel has supported ever since i can remember, take a look at the pthreads and linuxthreads (glibc2) libraries
Caleb Jasin - Tuesday, March 15, 2005 - link
#35Yeah agreed, from my own experience C# threadding is much easier than threadding in C. And I would say it is the same for most code. Developing in C# is generally much faster than in C or C++. And the tests I have seen shows about a 10% performance hit between optimal C# and C++ code. So I think it is just a question of time before we see games coded primarily in C#. In the end, the time saved could be used to write more optimal code I guess, so maybe the performance hit would be negligable.
However, I don't think that we will see compilers that are smart enough to multithread code any time soon. I wrote a very simple compiler for a very simple language in university and coding compilers is extremely complicated. As Tim says, they aren't threadding gamelogics and that wouldn't make much sense either because there are too many dependencies. And even though threadding takes alot of time, there are alot of relatively easily paralizeable code in games.
Btw, there is a small error in the article. It says that Linux has thread support. It really doesn't. A thread in Linux is a process. There is no diffrence at kernel level between starting a new thread and forking a new process.
melgross - Tuesday, March 15, 2005 - link
Don't forget that the idea behind these game engines is the reusability of the code. What I mean is that they will first tackle the problems that Sweeney thought most important, and easier, and then, one by one, the harder problems will be resolved. This might take years, but performance increases are always going to be appreciated. Competing products are always going to put pressure on on each other.Ten years from now the discussion will be about how they accomplished all of this.
While dual-cored GPUs have never been used, since that is just now becoming a viable technology, dual and quad GPUs have been used for many years now on the high end boards. Not the gamer boards that we see for $500 and below.