Valve Hardware Day 2006 - Multithreaded Edition
by Jarred Walton on November 7, 2006 6:00 AM EST- Posted in
- Trade Shows
Gaming's Future, Continued
So what can be done besides making a world look better? One of the big buzzwords in the gaming industry right now is physics. Practically every new game title seems to be touting amazing new physics effects. Perhaps modern physics are more accurate, but having moderate amounts of physics in a gaming engine is nothing new. Games over a decade ago allowed you to do such things as pick up a stone and throw it (Ultima Underworld), or shoot a bow and have the arrow drop with distance. Even if the calculations were crude, that would still count as having "physics" in a game. Of course, there's a big difference in the amount of physics present in Half-Life and those present in Half-Life 2, and most people would agree that the Half-Life 2 was a better experience due to the improved interaction with the environment.
Going forward, physics can only become more important. One of the holy grails of gaming is still the creation of a world that has a fully destructible environment. Why do you need to find a stupid key to get through a wooden door when you're carrying a rocket launcher? Why can't you blow up the side of the building and watch the entire structure fall to the ground, perhaps taking out any enemies that were inside? How about the magical fence that's four inches too tall to jump over - why not just break it down instead of going around? It's true that various games have made attempts in this direction, but it's still safe to say that no one has yet created a gaming environment that allows you to demolish everything as you could in the real world (within reason). Gameplay still needs to play a role in what is allowed, but the more the possibilities for what can be done are increased, the more likely we are to see revolutionary gameplay.
Going along with physics and game world interactions, Valve spoke about the optimizations they've made in a structure called the spatial partition. The spatial partition is essentially a representation of the game world, and it is queried constantly to determine how objects interact. From what we could gather, it is used to allow rough approximations to take place where it makes sense, and it also helps determine where more complex (and accurate) mathematical calculations should be performed. One of the problems traditionally associated with multithreaded programming has been locking access to certain data structures in order to keep the world in a consistent state. For the spatial partition, the vast majority of the accesses are read operations that can occur concurrently, and Valve was able to use lock-free and wait-free algorithms in order to greatly improve performance. A read/write log is used to make sure the return values are correct, and Valve emphasized that the lock-free algorithms were a huge design win when it came to multithreading.
Another big area that can stand to see a lot of improvement is artificial intelligence. Often times, AI has a tacked on feel in current games. You want your adversaries to behave somewhat realistically, but you don't want the game to spend so much computational power figuring out what they should do that everything crawls to a slow. It's one thing to wait a few seconds (or more) for your opponent to make a move in a chess match; it's a completely different story in an action game being rendered at 60 frames per second. Valve discussed the possibilities for having a greater number of simplistic AI routines running, along with a few more sophisticated AI routines (i.e. Alyx in Episode One).
They had some demonstrations of swarms of creatures interacting more realistically with the environment, doing things like avoiding dangerous areas, toppling furniture, swarming opponents, etc. (The action was more impressive than the above screenshots might indicate.) The number of creatures could also be increased depending on CPU power (number of cores as well as clock speed), so where a Core 2 Quad might be able to handle 500 creatures, a single core Pentium 4 could start to choke on only 80 or so creatures.
In the past, getting other creatures in the game world to behave even remotely realistically was sufficient -- "Look, he got behind a rock to get shelter!" -- but there's so much more that can be done. With more computational power available to solve AI problems, we can only hope that more companies will decide to spend the time on improving their AI routines. Certainly, without having spare processor cycles, it is difficult to imagine any action games spending as much time on artificial intelligence as they spend on graphics.
There are a few less important types of AI that could be added as well. One of these is called "Out of Band AI" -- these are AI routines that are independent of the core AI. An example that was given would be a Half-Life 2 scene where Dr. Kleiner is playing chess. They could actually have a chess algorithm running in the background using spare CPU cycles. Useful? Perhaps not that example, unless you're really into chess, but these are all tools to create a more immersive game world, and there is almost certainly someone out there that can come up with more interesting applications of such concepts.
So what can be done besides making a world look better? One of the big buzzwords in the gaming industry right now is physics. Practically every new game title seems to be touting amazing new physics effects. Perhaps modern physics are more accurate, but having moderate amounts of physics in a gaming engine is nothing new. Games over a decade ago allowed you to do such things as pick up a stone and throw it (Ultima Underworld), or shoot a bow and have the arrow drop with distance. Even if the calculations were crude, that would still count as having "physics" in a game. Of course, there's a big difference in the amount of physics present in Half-Life and those present in Half-Life 2, and most people would agree that the Half-Life 2 was a better experience due to the improved interaction with the environment.
Going forward, physics can only become more important. One of the holy grails of gaming is still the creation of a world that has a fully destructible environment. Why do you need to find a stupid key to get through a wooden door when you're carrying a rocket launcher? Why can't you blow up the side of the building and watch the entire structure fall to the ground, perhaps taking out any enemies that were inside? How about the magical fence that's four inches too tall to jump over - why not just break it down instead of going around? It's true that various games have made attempts in this direction, but it's still safe to say that no one has yet created a gaming environment that allows you to demolish everything as you could in the real world (within reason). Gameplay still needs to play a role in what is allowed, but the more the possibilities for what can be done are increased, the more likely we are to see revolutionary gameplay.
Going along with physics and game world interactions, Valve spoke about the optimizations they've made in a structure called the spatial partition. The spatial partition is essentially a representation of the game world, and it is queried constantly to determine how objects interact. From what we could gather, it is used to allow rough approximations to take place where it makes sense, and it also helps determine where more complex (and accurate) mathematical calculations should be performed. One of the problems traditionally associated with multithreaded programming has been locking access to certain data structures in order to keep the world in a consistent state. For the spatial partition, the vast majority of the accesses are read operations that can occur concurrently, and Valve was able to use lock-free and wait-free algorithms in order to greatly improve performance. A read/write log is used to make sure the return values are correct, and Valve emphasized that the lock-free algorithms were a huge design win when it came to multithreading.
Another big area that can stand to see a lot of improvement is artificial intelligence. Often times, AI has a tacked on feel in current games. You want your adversaries to behave somewhat realistically, but you don't want the game to spend so much computational power figuring out what they should do that everything crawls to a slow. It's one thing to wait a few seconds (or more) for your opponent to make a move in a chess match; it's a completely different story in an action game being rendered at 60 frames per second. Valve discussed the possibilities for having a greater number of simplistic AI routines running, along with a few more sophisticated AI routines (i.e. Alyx in Episode One).
They had some demonstrations of swarms of creatures interacting more realistically with the environment, doing things like avoiding dangerous areas, toppling furniture, swarming opponents, etc. (The action was more impressive than the above screenshots might indicate.) The number of creatures could also be increased depending on CPU power (number of cores as well as clock speed), so where a Core 2 Quad might be able to handle 500 creatures, a single core Pentium 4 could start to choke on only 80 or so creatures.
In the past, getting other creatures in the game world to behave even remotely realistically was sufficient -- "Look, he got behind a rock to get shelter!" -- but there's so much more that can be done. With more computational power available to solve AI problems, we can only hope that more companies will decide to spend the time on improving their AI routines. Certainly, without having spare processor cycles, it is difficult to imagine any action games spending as much time on artificial intelligence as they spend on graphics.
There are a few less important types of AI that could be added as well. One of these is called "Out of Band AI" -- these are AI routines that are independent of the core AI. An example that was given would be a Half-Life 2 scene where Dr. Kleiner is playing chess. They could actually have a chess algorithm running in the background using spare CPU cycles. Useful? Perhaps not that example, unless you're really into chess, but these are all tools to create a more immersive game world, and there is almost certainly someone out there that can come up with more interesting applications of such concepts.
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JarredWalton - Tuesday, November 7, 2006 - link
What's with the octal posting? Too many CPU cores running? ;)I deleted the other 7 identical posts for you. Careful with that Post Comment button!
saratoga - Tuesday, November 7, 2006 - link
Server kept timing out when I hit post, so I assumed it wasn't committing :)exdeath - Tuesday, November 7, 2006 - link
You can see my recent comments on this topic here:http://www.dailytech.com/Article.aspx?newsid=4847&...">http://www.dailytech.com/Article.aspx?newsid=4847&...
In my experience relying on atomic CPU swap operations isn't enough as it only works with a single value (32 bit word for example).
While you lock and swap a 32 bit Y value, someone else has just finished reading the newly written X value but beat you to the lock to read the old Y value before you've updated. Clearly whole data structures need to be coherent, not just small atomic values.
Also it’s unusual to modify objects observable states mid frame. Even if you avoided the above example so that the X,Y pair was always updated together, you'd still have different objects interpreting the position as a whole of that object in different places at different times. State data must be held constant to all observers throughout the context of a single frame.
exdeath - Tuesday, November 7, 2006 - link
Even if you avoided the above example so that the X,Y pair was always updated together, you'd still have different objects interpreting the position as a whole of that object in different places at different times in the same frame.JarredWalton - Tuesday, November 7, 2006 - link
I'm assuming your comment is in regards to the PS3/Cell comments on the last page? It's sort of sounds like you're arguing about the way Valve has chosen to go about doing things, or that you disagree with some of the opinions they've expressed concerning other hardware. We have only tried to provide a very high-level overview of what Valve is doing, and we hardly touched the low-level details -- Valve didn't spend a lot of time on specific implementation issues either. All they did was provide us with some information about what they are doing, and a bit of opinion on what they think of the rest of the hardware options.Preventing anything else from doing write operations to the world state during an entire frame in order to keep things coherent is a big problem with multithreading. Apparently Valve has found a way around that, or at least found a way to do it more efficiently, using lock free and wait free algorithms. No, I can't honestly say I really understand what those algorithms do, but if they say it worked better for their code base I'm willing to trust them.
As far as the PS3/Cell processor goes, Valve did say that they have various thoughts on how to properly utilize the architecture. It is simply going to be more difficult to do relative to Xbox 360 and PC. It's not impossible, and companies are definitely going to tackle this problem. As far as how they tackle it, I'm more than a bit rusty on my coding background, and other than high-level details I'm not too concerned how they improve their multithreading code on any specific platform, just that they do it.
exdeath - Tuesday, November 7, 2006 - link
The other issue is OS support.Compiler add-on's or third party APIs can only serve to hide the details or make things look cleaner. But no matter what, the final barrier between the application and the OS are the API calls provided by the OS threading model. Thus no third party implementation can be better than the OS thread model itself in terms of performance and overhead. All those can do is make it easier to use at the top by handling the OS details.
I imagine threading APIs on popular OSes will start to evolve, just like graphics APIs have, once everyone gets on the multi-core bandwagon and starts to get a feel for what's available in the OS APIs and what they'd rather have. So far, Vista's thread pool API looks good, but I still don't see an API to determine such basic things as checking if the work queue is empty and all threads are idle, etc.
Currently I find it's easier to implement my own thread pool manager which does atomic increments and decrements on a 'task count' variable as tasks are entered or completed in the queue. Checking if all tasks are done involves testing that task count against 0 and signaling an event flag that wakes any management threads sleeping until all its work tasks to complete. It also allows for more flexibility in 'before and after' housekeeping as work threads move from task to task and that kind of control isn't offered in the XP’s built in thread pool API, nor Vista’s as far as I can tell.
exdeath - Tuesday, November 7, 2006 - link
Not arguing their methods, a lot of things in this article are in line with my own opinions on multithreading, pretty much the best way to got about it. I'm just pointing out that atomic lock/swap operations in hardware are very primitive and typically operate only on CPU word size values, not entire data structures. Thus it's possible between doing two atomic operations on two variables on one core, another core can get an old version of one variable and a new version of another.core1: compute X
core2: ...
core1: lock/write x
core2: read x, get newly written version
core1: compute Y
core2: read Y, get old y before the update
core1: lock/write Y
core2: ...
The task on core2 is working with inconsistent data, the new X and the old Y. If the task on core2 only uses the data as input, i.e.: AI tracking another AI entity, it has the wrong position, and won't know about it since it has no need to perform its own lock/write (so it never gets the exception that says the value changed). Even if it did, it would have to throw out all work and redo it with the new Y, and then it could possibly change again.
Looping and retrying seems wasteful. And I’m thinking the only way to catch such a hardware error on a failed lock/write update is via exceptions, and handling a thrown exception on an attempt to write a single 32 bit value is very wasteful of CPU cycles.
In my own research I have had excellent results with double buffering any modified data. Each threaded task only updates its hidden internal working state for frame n+1 while all reads to the object are read from its external current state for frame n. At the end of the frame when all parallel tasks have completed, the current/working states are swapped, and the work queue is filled again to start the next frame.
This ensures that throughout the entire computation of frame n+1, the current frame n state will be available to all threads, and guaranteed to not be modified through the duration of current frame. So basically all threads can read anything they want and modify their own data. On PC/360 the time to swap everything is basically nothing; you just swap a few pointers, or a single pointer to an array/structure of current/working data for the frame.
On the PS3 some data copying and moving will be required, but this is mandatory due to design anyway and assisted by an extremely smart and powerful DMAC.
One place to be critical about is message passing between objects since it requires posting (writing) data to be picked up by another object. But the time to lock/post/unlock a queue is negligible compared to the time it takes to process the results leading up to the creation of the message. This is similar to the D3D notion of doing as much as you can before you lock and only do the minimal work needed inside the lock and unlock as quickly as possible.
GhandiInstinct - Tuesday, November 7, 2006 - link
Jarred Walton,My question: Will Valve's games in 2007 be released with specificaitons such as: "For minimum requirements you need a dual-core cpu, for maximum results you need a quad-core" or anything to that nature? Because I seem to be confused in what Valve is working on dual or quad or both or neither or something different, and what I should get to best utilize their games and multi-core software in general.
Thanks.
JarredWalton - Tuesday, November 7, 2006 - link
Episode Two should come out sometime in 2007, and before that happens you will get the multithreading patch affecting previous Source engine titles. Right now, it doesn't sound like anything released in the next year or so from valve is going to require dual cores. That's what I was trying to get out on the conclusion page where I mentioned that they are targeting an "equivalent experience" regardless of what sort of processor you are running.So just like you could turn down the level of detail in Half-Life 2 and run it on DX8 or even DX7 hardware, Source engine should be able to accommodate single core processors all the way up through N-core processors. The engine will spawn as many threads as you have processor cores, with one main thread serving as the controller and N - 1 helper threads. Xbox 360 for example would have 5 helper threads plus the master thread, because it has three course each capable of executing to threads simultaneously.
Patrese - Tuesday, November 7, 2006 - link
Great article, good to see dual-quad cores being used for something in games. By the way, the kitchen examples made me hungry... :)