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?
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  • PS3 Masterbater 5 - Tuesday, January 09, 2007 - link

    I WOULD JUST LIKE TO SAY THAT IF PS3 HAD A HOLE IN IT I WOULD INSERT MY PENIS IN AND MAKE SWEET LOVE TO IT BECAUSE IT IS THE GREATEST THING EVER. NINTENDO WII CAN SUCK MY HUGE COCK BECAUSE ITS A LITTLE BITCH AND IT IS THE POOR MANS PS3. IF NINTENDO WII WAS A MAN IT WOULD HAVE A VERY SMALL PENIS AND STILL BE A VIRGIN YOU GUYS ARE SO JEALOUS THAT I HAD THE FIRST PS3 EVER AND I WILL DOMINATE ANYONE IN "RESISTANCE : FALL OF MAN"
    Reply
  • Wizzdo - Friday, January 14, 2011 - link

    Definitely a bigger head below than above! Reply
  • steveyoung123456789 - Friday, December 09, 2011 - link

    your a virgin pussy and if i ever find out where you live i will kick your ass!!!!!!!!!!!! Reply
  • steveyoung123456789 - Friday, December 09, 2011 - link

    Btw your a psycho for wanting to fuck a gaming cousel... smh.... queef!! Reply
  • SilverTrine - Friday, November 17, 2006 - link

    The GPU in the Ps3 is more than enough for what its intended for. Theres no magic in GPUs they're just specialized processors.

    In the Xbox360 the GPU carries more of the processing load. Remember the unified ram that the GPU uses in the Xbox360 is 700mhz fast.

    The GPU in the Ps3 also has 700mhz ram. However the Cell processor has access to XDR ram running at a whopping 3.2ghz! In the Ps3 system the Cell with the superfast XDR ram will do more of the grunt work and rely less on the GPU.

    Saying the GPU in the Xbox360 somehow gives the system is a mistake. What would you rather have doing processing work a GPU running relatively slow with 700mhz ram or a extremely fast Cell processor with 3.2ghz XDR ram?

    However utilizing this on the Ps3 will require more specialized programming, the Xbox360 because its fairly conventional will be able to tap more of its power sooner than the Ps3.
    Reply
  • tipoo - Wednesday, August 06, 2014 - link

    Uhh, "mhz fast" doesn't matter an iota. The bandwidth of that XDR RAM was still 25GB/s to the Cell, it just works in a different way than GDDR, needs a higher clock speed for similar bandwidth. The clock speed was no advantage. And the RSX could only get data back at 15GB/s from the Cell going to the XDR. Reply
  • theteamaqua - Tuesday, July 12, 2005 - link

    http://theconsolewars.blogspot.com/2005/05/xbox-36...
    i just wan people to know that how bias this site is, i mean this guy has no idea what he is talking about
    Reply
  • jwix - Wednesday, July 06, 2005 - link

    #77 I wouldn't say Anand's article was "full of shit." I would say it was a bit sensationlist, as stated in the Arstechnica article. What surprised me more than anything was that Anand would post such an article, then remove it so quickly. That's not his style.
    Bottom line though - these consoles will offer nothing new or innovative in the way of gameplay. I think I'll stick with my PC and Nintendo DS for now.
    Reply
  • steveyoung123456789 - Friday, December 09, 2011 - link

    get a life Reply
  • calimero - Wednesday, July 06, 2005 - link

    http://arstechnica.com/news.ars/post/20050629-5054...

    btw Anand article was "full of shit" (sorry but that is the right phrase) and it's not odd that Anand pull it. It's quite embarassing for Anand; someone already told: one thing is to write test of CPU speed and speed of graphics card in games... and another to analyse CPU architecture.
    Reply

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