Microsoft's Xbox 360, Sony's PS3 - A Hardware Discussion
by Anand Lal Shimpi & Derek Wilson on June 24, 2005 4:05 AM EST- Posted in
- GPUs
Xenon vs. Cell
The first public game demo on the PlayStation 3 was Epic Games’ Unreal Engine 3 at Sony’s PS3 press conference. Tim Sweeney, the founder and UE3 father of Epic, performed the demo and helped shed some light on how multi-threading can work on the PlayStation 3.
According to Tim, a lot of things aren’t appropriate for SPE acceleration in UE3, mainly high-level game logic, artificial intelligence and scripting. But he adds that “Fortunately these comprise a small percentage of total CPU time on a traditional single-threaded architecture, so dedicating the CPU to those tasks is appropriate, while the SPE's and GPU do their thing."
So what does Tim Sweeney see the SPEs being used for in UE3? "With UE3, our focus on SPE acceleration is on physics, animation updates, particle systems, sound; a few other areas are possible but require more experimentation."
Tim’s view on the PPE/SPE split in Cell is far more balanced than most we’ve encountered. There are many who see the SPEs as utterly useless for executing anything (we’ll get to why in a moment), while there are others who have been talking about doing far too much on SPEs where the general purpose PPE would do much better.
For the most part, the areas that UE3 uses the Cell’s SPEs for are fairly believable. For example, sound processing makes a lot of sense for the SPEs given their rather specialized architecture aimed at streaming tasks. But the one curious item is the focus on using SPEs to accelerate physics calculations, especially given how branch heavy physics calculations generally are.
Collision detection is a big part of what is commonly referred to as “game physics.” As the name implies, collision detection simply refers to the game engine determining when two objects collide. Without collision detection, bullets would never hit your opponents and your character would be able to walk through walls, cars, etc... among other things.
One method of implementing collision detection in a game is through the use of a Binary Search Partitioning (BSP) tree. BSP trees are created by organizing lists of polygons into a binary tree. The structure of the tree itself doesn’t matter to this discussion, but the important thing to keep in mind is that to traverse a BSP tree in order to test for a collision between some object and a polygon in the tree you have to perform a lot of comparisons. You first traverse the tree finding to find the polygon you want to test for a collision against. Then you have to perform a number of checks to see whether a collision has occurred between the object you’re comparing and the polygon itself. This process involves a lot of conditional branching, code which likes to be run on a high performance OoO core with a very good branch predictor.
Unfortunately, the SPEs have no branch prediction, so BSP tree traversal will tie up an SPE for quite a bit of time while not performing very well as each branch condition has to be evaluated before execution can continue. However it is possible to structure collision detection for execution on the SPEs, but it would require a different approach to the collision detection algorithms than what would be normally implemented on a PC or Xbox 360.
We’re still working on providing examples of how it is actually done, but it’s tough getting access to detailed information at this stage given that a number of NDAs are still in place involving Cell development for the PS3. Regardless of how it is done, obviously the Epic team found the SPEs to be a good match for their physics code, if structured properly, meaning that the Cell processor isn’t just one general purpose core with 7 others that go unused.
In fact, if properly structured and coded for SPE acceleration, physics code could very well run faster on the PlayStation 3 than on the Xbox 360 thanks to the more specialized nature of the SPE hardware. Not to mention that physics acceleration is particularly parallelizable, making it a perfect match for an array of 7 SPEs.
Microsoft has referred to the Cell’s array of SPEs as a bunch of DSPs useless to game developers. The fact that the next installment of the Unreal engine will be using the Cell’s SPEs for physics, animation updates, particle systems as well as audio processing means that Microsoft’s definition is a bit off. While not all developers will follow in Epic’s footsteps, those that wish to remain competitive and get good performance out of the PS3 will have to.
The bottom line is that Sony would not foolishly spend over 75% of their CPU die budget on SPEs to use them for nothing more than fancy DSPs. Architecting a game engine around Cell and optimizing for SPE acceleration will take more effort than developing for the Xbox 360 or PC, but it can be done. The question then becomes, will developers do it?
In Johan’s Quest for More Processing Power series he looked at the developmental limitations of multi-threading, especially as they applied to games. The end result is that multi-threaded game development takes between 2 and 3 times longer than conventional single-threaded game development, to add additional time in order to restructure elements of your engine to get better performance on the PS3 isn’t going to make the transition any easier on developers.
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jotch - Friday, June 24, 2005 - link
#20 well that can't be right for the whole consumer base, as I'm 24 and only know other adults that have consoles and alot of them have flashy tv's for them as well, I do. I think if you look at the market for consoles it is mainly teens and adults that have consoles - not kids. Alot of people I know started with a NES or an Atari 2500, etc and have continued to like games as they have grown up. Why is it that the best selling game has an 18 rating?? (GTA: San Andreas)The burning of the screen would be minimal unless you have a game paused for hours and the tv left on - TV technology is moving on and they often turn themselves off if a static image is displayed for an amount of time. So burning shouldn't occur.
nserra - Friday, June 24, 2005 - link
All the people that i know having consoles is kids (80%), and their parents have bought an TV just for the console, an 70€ TV.....Who is the parent that will let kids on an LCD or PLASMA (3000€) to play games (burn them).
Or there will be good 480i "compatibility" in games, or forget it....
#17 I agree.
fitten - Friday, June 24, 2005 - link
#14 There are a number of issues being discussed.For example, given the nature of current AI code, making that code parallel (as in more than one thread executing AI code working together) seems non-trivial. Data dependencies and the very branch heavy code making data dependencies less predictable probably cause headaches here. Sure, one could probably take the simple approach and say one thread for AI, one for physics, one for blah but that has already been discussed by numerous people as a possibility.
Parallel code comes in many flavors. The parallelism in the graphics card, for instance, is sometimes classified as "embarassingly parallel" which means it's trivial to do. Then there are pipelines (dataflow) which CPUs and GPUs also use. These are usually fairly easy too because the data partitioning is pretty easy. You break out a thread for each overall task that you want to do. You want to do OpA on the data, then OpB, then OpC. All OpB depends on is the output data of OpA and OpC just depends on OpB's final product. Three threads, each one doing an Op on the output of the previous.
Then there are codes that are quite a bit more complex where, for example, there are numerous threads that all execute on parts of the whole data instead of all of it at once but the solution they are solving for requires many iterations on the data and at the end of each iteration, all the threads exchange data with each other (or just their 'neighbors') so that the next iteration can be performed. These are a bit more work to develop.
Anyway, I got long-winded anyway. Basically... there are *many* kinds of parallelism and many kinds of algorithms and implementations of parallelism. Some are low hanging fruit and some are non-trivial. Since I've already read that numerous developers for each platform already see low hanging fruit (run one thread for AI, another for physics, etc.) I can only believe they are talking about things that are non-trivial, such as a multithreaded AI engine, for example (again, as opposed to just breaking out the AI engine into one thread seperate from the rest of game play).
probedb - Friday, June 24, 2005 - link
Nice article! I'll wait till they're both out and have a play before I buy either. Last console I bought was an original PlayStation :) But gotta love that hi-def loveliness at last!#3 yeah 1080i is interlaced and at such a high res and low refresh the text is really difficult to read, it'd be far better at 1080p I think since that would effectively be the same as 1920x1080 on a normal monitor. 1080i is flickery as hell for me for desktop use but fine for any video and media centre type interfaces on the PC.
A5 - Friday, June 24, 2005 - link
You know, the vast majority of the TVs these systems will be hooked up to will only do 480i (standard TV)...jotch - Friday, June 24, 2005 - link
#14 - here here!jotch - Friday, June 24, 2005 - link
#10 - sounds to me like they're way ahead of they're time, future-proofing is good as they'll need another 6 years to develop the PS4 - but the Cell and Xenon will force developers to change their ways and will prepare them for the future of developing on PC's that eventually have this kind of CPU chip design (ref intel's chip design future pic on the first page of the article), like the article says the initial round of games will be single threaded etc etc...You might get alot of mediocre games but then you should get ones that really shine bright on the PS3, noticeably Unreal 3 and I bet the Gran Turismo (polyphony) guys will put in the effort.
Pannenkoek - Friday, June 24, 2005 - link
I'm quite tired of hearing how difficult it is to develop a multithreaded game. Only pathetic programmers can not grasp the concept of parallel code execution, it's not as if the current CPU/GPU duality does not qualify as one.knitecrow - Friday, June 24, 2005 - link
you'll need HDD for online service and MMOPhow many people are going to buy a $100 HDD if they don't have to?
LanceVance - Friday, June 24, 2005 - link
"the PS3 won’t ship with a hard drive"If that's true, then will it be like:
- PS2 Memory Card; non-included but standard equipment required by all games.
- PS2 Hard Drive; non-included and considered exotic unusual equipment and used by very few games.