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
Unreal 3
[2] The new Unreal 3 engine is a state of the art game development framework for next-generation consoles and DirectX9 PC's, but what sparked our interest for this article was the fact that it is probably one of the first multithreaded game engines for the most popular game genre: first person shooters.
Tim Sweeney: For multithreading optimizations, we're focusing on physics, animation updates, the renderer's scene traversal loop, sound updates, and content streaming.We are not attempting to multithread systems that are highly sequential and object-oriented, such as the gameplay.
Implementing a multithreaded system requires two to three times the development and testing effort of implementing a comparable non-multithreaded system, so it's vital that developers focus on self-contained systems that offer the highest effort-to-reward ratio.
Tim Sweeney: It's too early to talk numbers, but we certainly expect Unreal Engine 3 titles to see significant gains on multi-core platforms.
AnandTech: In the past years, games have typically depended more on GPU power than on CPU power (a mid-range CPU with a high end video card was/is faster than a high end CPU with a mid-range video card even at relatively low resolutions). Is the multithreaded nature of the Unreal 3 engine a sign that CPU performance is playing again a more important role in the gaming experience?
Tim Sweeney: Unreal Engine games have always been more CPU-intensive than the norm, for two reasons. First, we're always trying to push the leading edge with physics and other CPU-based features. Second, the Unreal Engine has a much more extensive gameplay scripting interface aimed at empowering mod authors and improving developer productivity by enabling safer and higher-level gameplay development. So we're not going to have any trouble keeping up with increases in CPU power.
Multi-core will be especially valuable because CPU performance scaling due to frequency improvements has tapered off over the past few years.
Clock speed has increased slowly, and real performance hasn't increased in proportion to clocks. But two cores have approximately twice the real aggregate performance as one core, so we're about to see a nonlinear improvement.
Finally, keep in mind that the Windows XP driver model for Direct3D is quite inefficient, to such an extent that in many applications, the OS and driver overhead associated with issuing Direct3D calls approaches 50% of available CPU cycles.Hiding this overhead will be one of the major immediate uses of multi-core.
AnandTech: Did you make use of auto-parallelisation compiler technology (like the auto parallelisation found in Intel C++ compiler) to make the engine multithreaded?
Tim Sweeney: Auto-parallelization of C++ code is not a serious notion. This falls in the same category as the Intel compiler's strip-mining optimizations and other such tricks, which are designed to speed up one particular loop in one particular SpecFP benchmark. These techniques applied to C/C++ programs are completely infeasible on the scale of real applications.
AnandTech: What about OpenMP?
Tim Sweeney: There are two parts to implementing multithreading in an application. The first part is launching the threads and handing data to them; the second part is making the appropriate portions of your 500,000-line codebase thread-safe. OpenMP solves only the first problem. But that's the easy part - any idiot can launch lots of threads and hand data to them. Writing thread-safe code is the far harder engineering problem and OpenMP doesn't help with that.
AnandTech: Programming multiple threads can be complex. Wasn't it very hard to deal with the typical problems of programming multithreaded such as deadlocks, racing and synchronization?
Tim Sweeney: Yes! These are hard problems, certainly not the kind of problems every game industry programmer is going to want to tackle. This is also why it's especially important to focus multithreading efforts on the self-contained and performance-critical subsystems in an engine that offer the most potential performance gain. You definitely don't want to execute your 150,000 lines of object-oriented gameplay logic across multiple threads - the combinatorical complexity of all of the interactions is beyond what a team can economically manage. But if you're looking at handing off physics calculations or animation updates to threads, that becomes a more tractable problem.
We also see middleware as one of the major cost-saving directions for the industry as software complexity increases. It's certainly not economical for hundreds of teams to write their own multithreaded game engines and tool sets. But if a handful of company write the core engines and tools, and hundreds of developers can reuse that work, then developers can focus more of their time and money on content and design, the areas that really set games apart.
AnandTech: The current OpenGL and DirectX are - AFAIK - not very well adapted to multithreaded programming. How did you solve this problem? Or wasn't it a problem at all?
Tim Sweeney: There is only one GPU in there, and though it is highly parallel at the pixel level, its execution is still serial on the granularity of state changes and triangle submission. So it is natural that the interface to the GPU remain single-threaded, and that part of one CPU thread be dedicated to submitting rendering commands.
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NullSubroutine - Monday, March 14, 2005 - link
I think there is a few things that most people overlook when looking at multi-cpu/multi-core, almost all benchmarks that I have seen are written and tested on systems with clean installs, and have no other programs running (anti-virus, aim, msn, teamspeak, IRC, p2p software, firewall, decode human genome :b, etc). I would think that most people do leave many programs open, such as those above, when playing games.With this in mind, people will find an increase of system performance when leaving multiple programs running. It wont be an increase for performance for benchmark testbeds so much, as an increase in real world performance.
So basically it won't increase speed in these circumstances, but limit the decrease of fps while running many different programs.
fitten - Monday, March 14, 2005 - link
Article: "Be warned that Intel was already showing performance increases, which are not realistic "up to 124%"."#5, there's another explanation as well, but it's a more rare condition. Suppose you had two processors (doesn't even have to be dual core), each with 1M L2 cache. Suppose you also had a problem that has data that is 1.5M in size and is very coarse grained (very parallelizable). One processor cannot fit all the data into L2 cache so it will have to run at main memory speeds most/all of the time. With two processors, each gets 768K, which can easily fit into its L2 cache, which enables each processor to run at L2 cache speeds. This would show up as a superlinear speedup (two cores = more than 2X as fast). This is an extreme example, but one I expect to find in published marketing propaganda.
#13 " A though! I still think threads are rubbish, that processes and better schedulers are the way forward. "
Well, with threads you get shared memory for "free", if you've ever written processes that use shared memory, well, there you are. However, since a threaded kernel and a process based kernel are pretty much the same when a process has only one thread, there's little difference between the two for single-threaded executables and you can continue to use your multi-process model without any problems.
As with #17... like it or not, multi-core/multi-processor boxes are what's coming. You can choose to use what resources are available to you or you can stick to one process programming. Some groups will choose to use what resources are available and some won't. The marketplace will sort out the winners/losers based on which solution is better.
#18 The PPU is just another form of multiprocessing (just like GPUs are). It's just Asymmetric Multiprocessing (AMP) instead of Symmetric Multiprocessing (SMP). It's not new or anything. I do agree, though, that the PPU has a lot of potential and, just out of my own preferences, goes by the idea that adding specialized hardware (cheaply) usually is a bigger win than adding more generalized hardware. Just think of graphics cards today. Adding a relatively cheap graphics card will make your game run much better/prettier than adding another P4 or Opteron.
Basically, my thoughts are this: The gaming industry has already gone "multi-threaded" in an asymmetric way simply because of 3D video cards. They already have solved some problems by abstracting parts of their problem. This is simply adding more resources that they can take advantage of, or not, as they see fit. Having dual-core or dual processor systems doesn't prevent them from writing as they've done today. The main issue, for the short term, is that they will need to know whether or not they are on a dual core machine and write accordingly. The main reason that multithreaded games haven't really caught on as of yet is because 99% (or more) of the target audience has only one core. Spending the amount of time/effort to optimize for dual processors for less than 1% of your target market doesn't make sense. If 90% of the market had dual processors, then it would probably be worth the effort to plan to use the resources available. Since both major CPU houses are going dual core and it looks like that's the "way it's gonna be", there will be a rocky period for a while while dual core machines are rare, but they will get more common until the point where they are in the majority. At that time, it will make sense to consider single core machines as the degenerate case and, basically, make single cores the exception instead of the rule.
Calin - Monday, March 14, 2005 - link
#20, a multicore implementation could have shared cache, and also have very fast inter processor communications. You could write a program with small interdependent threads that wait to end both and update parts of some common data. The data used stays in the common cache, and every update is made extremely fast.Compare this to a dual processor, that must maintain its caches in synchronization. After a fraction of a millisecond (or less) or work, the processors update different portions of the common data. And there goes: invalidation of cache lines, writing of modified cache lines to memory, the processors must fight for a single FSB (the case with Intel Pentium processors), and so on. You can see that there are some cases (even if somehow artificial) when a proper implementation of dual core can be much faster than multiprocessor.
The best advantage the multicore will have over multiprocessor would be in numerical tasks like weather prediction, and other highly interdependant computation tasks
hzmonte - Tuesday, October 4, 2005 - link
"a multicore implementation could have shared cache and also have very fast inter processor communications... Compare this to a dual processor, that must maintain its caches in synchronization." Is this the real reason that multi-core multiprocessing is better than multi-chip multiprocessing (the traditional SMP)? A multi-core chip can have dedicated caches (per core) too, and that requires synchronization. And multi-chip SMP could also have shared cache and fast inter-chip communication. Well, you may argue that it is easier to make inter-core communication faster than inter-chip communication. But is this really the fundamental reason why multicore is better than multichip? Could someone explain why a processor manufacturer and a consumer would prefer making/buying a multicore than multichip processors? As far as power consumption and leakage is concerned, isn't it true that multichip is more manageable? In a paper "Planning Considerations for Multicore Processor Technology" by John Fruehe (May 2005) in dell.com/powersolutions, the author compares the effective performance level of a multicore and multichip processors. (But he does not address my question.) Without giving reason, he assume that the core-to-core scalability is 70% (that is, the second core delivers 70% of its processor power due to overhead) whereas the estimated socket-to-socket (i.e. chip-to-chip) scalability is 80% (that is, the dual processors achieve 180% of their combined processing power). That is kind of interesting. I really want to see a comparison between multi-core multiprocessing vs. multi-chip multiprocessing.ksherman - Monday, March 14, 2005 - link
at80eighty, by sexy, I mean SCARY AS ALL FREAKIN REASON!!! ;-)Calin - Monday, March 14, 2005 - link
High IPC is not the form of parallelism from the article - the focus of the article was on running a process on two (or more) different cores. The idea is that high IPC profits all the programs, no matter how written. Multi thread is different - the idea is to have parts of a program that execute simultaneously but with very few interrelations (you can have a thread to paint the interface in a game, while having another thread to paint the rest of the screen. The threads would be with almost no correlations (except for sending commands).High IPC is not a solution in x86 world because the code tends to have dependencies close to each other, so you can start executing 100 instructions at a time, but 99 of them needs to wait for the execution of one. You simply have those moments when all execution must wait for an instruction to end.
EPIC (Itanium) will help with that, as the high IPC could be guaranteed by the instructions - at every clock you can execute one instruction = equivalent to several x86 instructions. So, the performance would be the clock speed multiplied by an IPC of 3 or 4, unlike the Athlon (let's say) that have a performance generated by its larger clock speed multiplied by 1 IPC or something.
Kensei - Monday, March 14, 2005 - link
Wonderful article! I loved the "hardware meets software" focus of this piece. I've had many questions about the practicality of multi-threaded applications and this article answered many of them. Also, loved the interview with Sweeny.bob661 - Monday, March 14, 2005 - link
#9I am offended by the word "steam".
Matthew Daws - Monday, March 14, 2005 - link
#20:MarriedMan - Yes, I think so. This is actually an interesting question. As I understand it, I think both AMD and Intel are using pretty much the same technology in both, so that communication channels on the motherboard (in the dual CPU case) will be replaced by communication channels on the CPU die. I think AMD's approach is better only because HT etc. lends itself to dual-core much better than Intel's older technology. I guess the next generation of dual-core chips might be somewhat different though. Anyone else know anything?MarriedMan - Monday, March 14, 2005 - link
I assume that when a program is multi-threaded to take advantage of dual core CPUs, it will automatically take advantage of dual CPU systems as well.Is that a correct assumption? Will the Unreal 3 engine use multiple single core CPUs on an MP system?