Familiar Architecture, Future-Proofed
So what does Cerny really think the console will gain from this design approach? Longevity.
Cerny is convinced that in the coming years, developers will want to use the GPU for more than pushing graphics -- and believes he has determined a flexible and powerful solution to giving that to them. "The vision is using the GPU for graphics and compute simultaneously," he said. "Our belief is that by the middle of the PlayStation 4 console lifetime, asynchronous compute is a very large and important part of games technology."
Cerny envisions "a dozen programs running simultaneously on that GPU" -- using it to "perform physics computations, to perform collision calculations, to do ray tracing for audio."
But that vision created a major challenge: "Once we have this vision of asynchronous compute in the middle of the console lifecycle, the question then becomes, 'How do we create hardware to support it?'"
One barrier to this in a traditional PC hardware environment, he said, is communication between the CPU, GPU, and RAM. The PS4 architecture is designed to address that problem.
"A typical PC GPU has two buses," said Cerny. "There’s a bus the GPU uses to access VRAM, and there is a second bus that goes over the PCI Express that the GPU uses to access system memory. But whichever bus is used, the internal caches of the GPU become a significant barrier to CPU/GPU communication -- any time the GPU wants to read information the CPU wrote, or the GPU wants to write information so that the CPU can see it, time-consuming flushes of the GPU internal caches are required."
Enabling the Vision: How Sony Modified the Hardware
The three "major modifications" Sony did to the architecture to support this vision are as follows, in Cerny's words:
- "First, we added another bus to the GPU that allows it to read directly from system memory or write directly to system memory, bypassing its own L1 and L2 caches. As a result, if the data that's being passed back and forth between CPU and GPU is small, you don't have issues with synchronization between them anymore. And by small, I just mean small in next-gen terms. We can pass almost 20 gigabytes a second down that bus. That's not very small in today’s terms -- it’s larger than the PCIe on most PCs!
- "Next, to support the case where you want to use the GPU L2 cache simultaneously for both graphics processing and asynchronous compute, we have added a bit in the tags of the cache lines, we call it the 'volatile' bit. You can then selectively mark all accesses by compute as 'volatile,' and when it's time for compute to read from system memory, it can invalidate, selectively, the lines it uses in the L2. When it comes time to write back the results, it can write back selectively the lines that it uses. This innovation allows compute to use the GPU L2 cache and perform the required operations without significantly impacting the graphics operations going on at the same time -- in other words, it radically reduces the overhead of running compute and graphics together on the GPU."
- Thirdly, said Cerny, "The original AMD GCN architecture allowed for one source of graphics commands, and two sources of compute commands. For PS4, we’ve worked with AMD to increase the limit to 64 sources of compute commands -- the idea is if you have some asynchronous compute you want to perform, you put commands in one of these 64 queues, and then there are multiple levels of arbitration in the hardware to determine what runs, how it runs, and when it runs, alongside the graphics that's in the system."
"The reason so many sources of compute work are needed is that it isn’t just game systems that will be using compute -- middleware will have a need for compute as well. And the middleware requests for work on the GPU will need to be properly blended with game requests, and then finally properly prioritized relative to the graphics on a moment-by-moment basis."
This concept grew out of the software Sony created, called SPURS, to help programmers juggle tasks on the CELL's SPUs -- but on the PS4, it's being accomplished in hardware.
The team, to put it mildly, had to think ahead. "The time frame when we were designing these features was 2009, 2010. And the timeframe in which people will use these features fully is 2015? 2017?" said Cerny.
"Our overall approach was to put in a very large number of controls about how to mix compute and graphics, and let the development community figure out which ones they want to use when they get around to the point where they're doing a lot of asynchronous compute."
Cerny expects developers to run middleware -- such as physics, for example -- on the GPU. Using the system he describes above, you can run at peak efficiency, he said.
"If you look at the portion of the GPU available to compute throughout the frame, it varies dramatically from instant to instant. For example, something like opaque shadow map rendering doesn't even use a pixel shader, it’s entirely done by vertex shaders and the rasterization hardware -- so graphics aren't using most of the 1.8 teraflops of ALU available in the CUs. Times like that during the game frame are an opportunity to say, 'Okay, all that compute you wanted to do, turn it up to 11 now.'"
Sounds great -- but how do you handle doing that? "There are some very simple controls where on the graphics side, from the graphics command buffer, you can crank up or down the compute," Cerny said. "The question becomes, looking at each phase of rendering and the load it places on the various GPU units, what amount and style of compute can be run efficiently during that phase?"
Launch and Beyond
The benefits of this powerful hardware will be seen in the PlayStation 4's launch games. But Cerny maintains that, in the future, they'll shine through in totally different ways.
"The launch lineup for PlayStation 4 -- though I unfortunately can’t give the title count -- is going to be stronger than any prior PlayStation hardware. And that's a result of that familiarity," Cerny said. But "if your timeframe is 2015, by another way of thinking, you really need to be doing that customization, because your competition will be doing that customization."
So while it takes "weeks, not months" to port a game engine from the PC to the PlayStation 4 according to Cerny, down the road, dedicated console developers can grasp the capabilities of the PlayStation 4, customize their technology, and really reap the benefits.
"There are many, many ways to control how the resources within the GPU are allocated between graphics and compute. Of course, what you can do, and what most launch titles will do, is allocate all of the resources to graphics. And that’s perfectly fine, that's great. It's just that the vision is that by the middle of the console lifecycle, that there's a bit more going on with compute."