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. 

Introducing the Xbox 360’s Xenon CPU Does In-Order Matter?


View All Comments

  • Darkon - Friday, June 24, 2005 - link


    WTF are you talking ?

    The Cell does general-purpose processing although not as good as 360 cpu.

    And Anand I suggest you do some more research on cell
  • Alx - Friday, June 24, 2005 - link

    Someone explain to me how Sony will support 1080p please. If developers make the games run at acceptable framerate at that resolution, most people running them at 720p and 480i will be wasting at least half of PS3's rendering power.

    On the other hand if XBOX360 game devs make their games run just fast enough at 720p, that'll give them far more resources to work with than those poor Sony game devs.
  • Shinei - Friday, June 24, 2005 - link

    That's not necessarily true, #48. The Cell processor doesn't do general-purpose processing, so it can't do decoding on its own--and as far as I know, even pressed DVDs have to be decoded by some kind of processor. (Of course, I know next to nothing about video equipment, so I could be wrong...) Reply
  • arturnow - Friday, June 24, 2005 - link

    Another difference between RSX and G70 is hardware video decoder - PureVideo, i'm sure RSX doesn't need that which saves transistors count Reply
  • freebst - Friday, June 24, 2005 - link

    Actually, in response to 31 there is no 1080p 60 frame/sec signal. the only HD signals are 1080 30p, 24p, 60i, 720 60p, 30p, 24p. Reply
  • BenSkywalker - Friday, June 24, 2005 - link

    Why the support for lower resolutions? I'm a bit confused by this- I can't see why anyone who isn't a fanatic loyalist wouldn't want to see the highest resolution possible supported by the consoles. The XBox(current) supports 1080i and despite the extreme rarity in which it is used- it IS used. Supporting 1080p x2 may seem like overkill, but think of the possibilities in terms of turn based RPGs or strategy games(particularly turn based) where 60FPS is very far removed from required.

    The most disappointing thing about the new generation of consoles is MS flipping its customers off in terms of backwards compatability. Even Nintendo came around this gen and MS comes up with some half done emulation that works on some of 'the best selling' games. Also, with their dropping production of the original XB already it appears they still have an enormous amount to learn about the console market(check out sales of the original PS after the launch of the PS2 for an example).
  • Warder45 - Friday, June 24, 2005 - link

    errr #31 not 37 Reply
  • Warder45 - Friday, June 24, 2005 - link

    #37 is right on the money. There is a good chance that there will be no HDTV that can accept a 1080p signal by the time the PS3 comes out.

    It seems less like Sony future proofing the PS3 and more like Sony saying we have bigger balls then MS. Not to say MS is exempt from doing the same.
  • IamTHEsnake - Friday, June 24, 2005 - link

    Excellent article Anand and crew.

    Thank you for the very informative read.
  • masher - Friday, June 24, 2005 - link

    > "Collision detection is a big part of what is commonly
    > referred to as “game physics.” ..."

    Sorry, collision detection is computational geometry, not physics.

    > "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... "

    Again, untrue. You walk the tree on the PPE, whereas you do the actual intersection tests on the SPs. The SPs are also ideally suited to calculating the positions of each object (read: real physics) and updating the tree accordingly.

Log in

Don't have an account? Sign up now