Thursday, April 25, 2013
Sony described its upcoming PlayStation 4 as a "supercharged" PC. Powered by familiar x86 architecture manufactured by AMD, PS4 is more like a gaming PC than any previous Sony console. However, while it may use many parts found in high-end gaming PCs, PS4 system architect Mark Cerny argues that PS4 has many unique features that separate it from today's PCs.
"The 'supercharged' part, a lot of that comes from the use of the single unified pool of high-speed memory," Cerny said, pointing to the 8GB of GDDR5 RAM that's fully addressable by both the CPU and GPU. "If a PC had 8 gigabytes of memory on it, the CPU or GPU could only share about 1 percent of that memory on any given frame. That's simply a limit imposed by the speed of the PCIe. So, yes, there is substantial benefit to having a unified architecture on PS4, and it's a very straightforward benefit that you get even on your first day of coding with the system."
According to Cerny, PS4 addresses the hiccups that can come from the communication between CPU, GPU, and RAM in a traditional PC. "A typical PC GPU has two buses," Cerny told Gamasutra in a very detailed technical write-up. "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."
PS4 addresses these concerns by adding 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." The end result is that it removes synchronization issues between the CPU and GPU. "We can pass almost 20 gigabytes a second down that bus," Cerny said, pointing out that it's "larger than the PCIe on most PCs!"
"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," Cerny said. According to Cerny, the reason for the increase is that middleware will have a need to use compute as well. "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."
