News from the European gaming convention Gamescom implies another PR win for Sony. According to AMD's senior product marketing manager, Marc Diana, the PS4 is the only console that will support the company's next-generation heterogeneous unified memory architecture (hUMA). When AMD announced hUMA earlier this year, it emphasized that it views the technology as essential to the future of high performance computing -- and now it seems that the capability will debut on just one console.
Edit: 8/27/2013. It now appears that the Xbox One will support HSA, based on the disclosures made by Microsoft at Hot Chips.This difference in capability is being touted as one reason why the PS4 is reportedly seen as far ahead of the Xbox One in terms of its raw performance. German IT magazine C't, which spoke with Diana(Opens in a new window), also states that numerous developers believe the PS4 is far ahead of the Xbox One on raw compute power. That's particularly interesting given John Carmack's comments on this topic a few weeks back. During his three-hour dissertation on the universe of gaming, Carmack talked about the gap (or lack thereof) between the consoles, saying: "It's interesting -- it's almost amazing -- how close they are in capabilities, how common they are, and how the capabilities they give are essentially the same... the bottom line being that they're both multicore processor with AMD graphics."
It's actually the very first topic he touched on in his speech, and in doing so he seems to throw cold water on the idea that the PS4 is going to be the breakout winner come November. Granted, John Carmack is not the God of gaming -- but he's at least a major power. How do we square these seemingly contradictory statements? By taking a closer look at what hUMA and HSA can and can't do -- and putting a bit of context around Carmack's remarks.
HSA's uncertain ramp-up - and long-term promise
The first thing to understand about hUMA/HSA is that adapting these technologies to any use-case is non-trivial. The end goal of allowing CPU and GPU to share main memory pointers and data directly sounds simple. AMD's HSA whitepaper makes it clear that making that happen is anything but. The platform is designed to support multiple languages, all of which are translated into HSAIL (HSA Intermediate Language) for code that runs on the GPU. The CPU side of the equation remains in native code. There's a specific block of hardware that's used to manage the unified memory, dubbed the HMMU, and shown below.
Here's the really important bit. AMD's description of HSAIL(Opens in a new window) emphasizes that HSAIL is "focused purely on compute and does not expose graphics-specific instructions." In other words, most of the publicly available data on expected HSA implementations doesn't apply to gaming in particular. Nevertheless, the whitepaper makes it clear that in order to function effectively, moving data and workloads between CPU and GPU has to be an efficient, low-latency operation. The CPU and GPU need to be able to schedule jobs for the other to complete, which means you need a task scheduler that's capable of understanding how to prioritize a job for the GPU vs. the CPU.
This kind of optimization work is non-trivial. A non-HSA program won't take inherent advantage of HSA or hUMA -- it'll have to be optimized for the task. Most of the papers reporting benefits of HSA optimization have been focused on workstation applications or tools like Mathematica, where the ability to leverage the GPU more efficiently is seen as a huge potential boost for its compute aspects, not its gaming focus.
There's one other thing to keep in mind when it comes to considering the benefits of HSA for gaming. Ever since the dawn of 3D acceleration, gaming has been a combined effort between the CPU and GPU. Game developers have a great deal of experience when it comes to minimizing latency and the impact of CPU-GPU data sharing on conventional architectures, which means taking advantage of HSA to deliver improvements on the standard way of doing things is going to take time.
With multiple studios rushing to finish games, it's unlikely that HSA/hUMA will serve as the distinguishing technological features on display come launch day. Instead, I suspect that the early titles will rely on the PS4's higher-end GPU for any attempt to one-up the Xbox One. That's more likely to be true for the studios working on bringing up game versions for each console; studios that have committed to exclusives may have more time to spend on the topic. Sony is likely betting on HSA/hUMA in much the same way that Microsoft is betting on cloud offloading -- as a long-term differentiation mechanism that could offer gamers unique benefits that the other console can't duplicate.
But back to Carmack's comments: Is the PS4 more powerful than the Xbox One or not?
Next page: Console convergenceConsole convergence
Carmack's comments on similarity make a great deal more sense if we consider how consoles have evolved in the past 20 years. In the PS1/N64 era, Sony and Nintendo pursued opposite storage capabilities and built consoles with very different strengths. The PS2 implemented a hybridized version of the MIPS III/IV architecture with a lot of custom work, the GameCube was based on a PowerPC processor, and the original Xbox was an x86 chip. Disc-based games were one of the few common components of the three consoles.
Last generation, the Xbox 360 and PS3 were both derived from a common CPU heritage (the Xbox 360 used three of Cell's Power Processor Elements, or PPEs), but had very different GPUs, different requirements for local storage, different storage mediums (dual-layer DVD vs. Blu-ray), and different amounts of RAM available to the CPU. The PS3 implements two blocks of 256MB of RAM for the CPU and GPU individually, while the Xbox 360 dynamically allocates memory.
Compared to the consoles that have come before them, the PS4 and Xbox One are far more alike than different. They both rely on AMD processors and GCN (Graphics Core Next)-based GPUs. They both use Blu-ray, they both have similar video decode capabilities, operate at similar clock speeds, and use the same underlying CPU instruction set. Both are far closer to being conventional PCs than any console has ever been, which means programming tools and developer software should be far easier to leverage. At a low level, the Xbox One and PS4 are more alike than any two consoles have ever been, and I suspect that fundamental similarity is what drove Carmack's comments on their relative position.
Even granting that Microsoft has been incredibly sensitive to the consumer response on the Xbox One, a fundamental hardware change at this late date would be truly unprecedented. That means the PS4 is going to ship with more raw power under the hood. Whether that translates into a meaningful game benefit that also translates into higher sales is anyone's guess; Microsoft has its own vision for the Xbox One and customers don't always prefer the system that's capable of pounding out the highest-quality graphics. Still, Sony has done a better job positioning itself for the upcoming generation by keeping the conversation focused on gaming, while Microsoft has been stuck cleaning up an enormous mess. As we head into the home stretch, it's hard to argue that the PS4 isn't leading the race.
Update (8/22/2013) AMD's corporate vice-president of marketing, John Taylor, has since contacted us regarding the comments Marc Diana made to C't. AMD's official response is as follows: "During a recent Gamescom 2013 interview, an AMD spokesperson made inaccurate statements regarding the details of our semi-custom APU architectures. AMD will not comment on the Microsoft Xbox One and Sony PS4 memory architectures and will not speak for Microsoft, Sony or other AMD customers."Our follow-up discussion with Taylor indicates that Diana's comments were either misunderstood or mistaken to some degree, but AMD cannot contractually make disclosures on Sony or Microsoft products.
After further review of available evidence, as suggested by some readers, I think it's clear that the PS4 has some degree of HSA-like functionality. It may or may not implement HSA in all the particulars as defined by AMD, but many of the specific capabilities that HSA allows have been confirmed as present on the PS4. The degree to which the PS4 will leverage these capacities for gaming is still unknown, but I continue to believe it will take awhile for such capabilities to emerge.
As others have pointed out, it's possible that the Xbox One contains certain hUMA/HSA capabilities. This remains uncertain. HSA is more than a model for ensuring cache coherency or I/O coherency between CPU and GPU. Microsoft has yet to clarify which HSA capabilities the Xbox One does and does not implement in particular.
