Microsoft's Xbox 360, Sony's PS3 - A Hardware Discussion

How Many Threads?

Earlier this year we saw the beginning of a transition from very fast, single core microprocessors to slower, multi-core designs on the PC desktop.  The full transition won’t be complete for another couple of years, but just as it has begun on the desktop PC side, it also has begun in the next-generation of consoles. 

Remember that consoles must have a lifespan of around 5 years, so even if the multithreaded transition isn’t going to happen with games for another 2 years, it is necessary for these consoles to be built around multi-core processors to support the ecosystem when that transition occurs. 

The problem is that today, all games are single threaded, meaning that in the case of the Xbox 360, only one out of its three cores would be utilized when running present day game engines.  The PlayStation 3 would fair no better, as the Cell CPU has a very similar general purpose execution core to one of the Xbox 360 cores.  The reason this is a problem is because these general purpose cores that make up the Xbox 360’s Xenon CPU or the single general purpose PPE in Cell are extremely weak cores, far slower than a Pentium 4 or Athlon 64, even running at much lower clock speeds. 

Looking at the Xbox 360 and PlayStation 3, we wondered if game developers would begin their transition to multithreaded engines with consoles and eventually port them to PCs.  While the majority of the PC installed base today still runs on single-core processors, the install base for both the Xbox 360 and PS3 will be guaranteed to be multi-core, so what better platform to introduce a multithreaded game engine than the new consoles where you can guarantee that all of your users will be able to take advantage of the multithreading. 

On the other hand, looking at all of the early demos we’ve seen of Xbox 360 and PS3 games, not a single one appears to offer better physics or AI than the best single threaded games on the PC today.  At best, we’ve seen examples of ragdoll physics similar to that of Half Life 2, but nothing that is particularly amazing, earth shattering or shocking.  Definitely nothing that appears to be leveraging the power of a multicore processor. 

In fact, all of the demos we’ve seen look like nothing more than examples of what you can do on the latest generation of GPUs - not showcases of multi-core CPU power.  So we asked Microsoft, expecting to get a fluffy answer about how all developers would be exploiting the 6 hardware threads supported by Xenon, instead we got a much more down to earth answer. 

The majority of developers are doing things no differently than they have been on the PC.  A single thread is used for all game code, physics and AI and in some cases, developers have split out physics into a separate thread, but for the most part you can expect all first generation and even some second generation titles to debut as basically single threaded games.  The move to two hardware execution threads may in fact only be an attempt to bring performance up to par with what can be done on mid-range or high-end PCs today, since a single thread running on Xenon isn’t going to be very competitive performance wise, especially executing code that is particularly well suited to OoO desktop processors. 

With Microsoft themselves telling us not to expect more than one or two threads of execution to be dedicated to game code, will the remaining two cores of the Xenon go unused for the first year or two of the Xbox 360’s existence?  While the remaining cores won’t directly be used for game performance acceleration, they won’t remain idle - enter the Xbox 360’s helper threads. 

The first time we discussed helper threads on AnandTech was in reference to additional threads, generated at runtime, that could use idle execution resources to go out and prefetch data that the CPU would eventually need. 

The Xbox 360 will use a few different types of helper threads to not only make the most out of the CPU’s performance, but to also help balance the overall platform.  Keep in mind that with the 360, Microsoft has not increased the size of the media that games will be stored on.  The dual layer DVD-9 spec is still in effect, meaning that game developers shipping titles for the Xbox 360 in 2006 will have the same amount of storage space as they did back in 2001.  Given that current Xbox titles generally use around 4.5GB of space, it’s not a big deal, but by 2010 9GB may feel a bit tight. 

Thanks to idle execution power in the 3-core Xenon, developers can now perform real-time decompression of game data in order to maximize storage space.  Given that a big hunk of disc space is used by audio and video, being able to use more sophisticated compression algorithms for both types of data will also help maximize that 9GB of storage.  Or, if space isn’t as much of a concern, developers are now able to use more sophisticated encoding algorithms to encode audio/video to use the same amount of space as they are today, but achieve much higher quality audio and video.  Microsoft has already stated that in game video will essentially use the WMV HD codec.  The real time decompression of audio/video will be another use for the extra power of the system. 

Another interesting use will be digital audio encoding; in the original Xbox Microsoft used a relatively expensive DSP featured in the nForce south bridge to perform real-time Dolby Digital Encoding.  The feature allowed Microsoft to offer a single optical out on the Xbox’s HD AV pack, definitely reducing cable clutter and bringing 5.1 channel surround sound to the game console.  This time around, DD encoding can be done as a separate thread on the Xenon CPU - in real time.  It reduces the need for Microsoft to purchase a specialized DSP from another company, and greatly simplifies the South Bridge in the Xbox 360. 

But for the most part, on day 1, you shouldn’t expect Xbox 360 games to be much more than the same type of single threaded titles we’ve had on the PC.  In fact, the biggest draw to the new consoles will be the fact that for the first time, we will have the ability to run games rendered internally at 1280 x 720 on a game console.  In other words, round one of the next generation of game consoles is going to be a GPU battle. 

The importance of this fact is that Microsoft has been talking about the general purpose execution power of the Xbox 360 and how it is 3 times that of the PS3’s Cell processor.  With only 1 - 2 threads of execution being dedicated for game code, the advantage is pretty much lost at the start of the console battle. 

Sony doesn’t have the same constraints that Microsoft does, and thus there is less of a need to perform real time decompression of game content.  Keep in mind that the PS3 will ship with a Blu-ray drive, with Sony’s minimum disc spec being a hefty 23.3GB of storage for a single layer Blu-ray disc.  The PS3 will also make use of H.264 encoding for all video content, the decoding of which is perfectly suited for the Cell’s SPEs.  Audio encoding will also be done on the SPEs, once again as there is little need to use any extra hardware to perform a task that is perfectly suited for the SPEs.