PS4 boasts an APU with features that current PC’s do not have access to. This will change in future, but since every PS4 will have a fixed set of features, those features on PS4 is more likely to be utilized.
What is an APU? It is a processor with integrated CPU and GPU, where the CPU and GPU share the same die. They are part of AMD’s vision for Heterogeneous Systems Architecture (HSA), formerly known as Fusion. The tools to make the most out of HSA systems is maturing, and it will take time before developers can get to grips with this.
How will the APU benefit the PS4?
It will provide high performance per watt which means a cooler and quieter console with lower electric bills. It will also allow a unified RAM path, where the CPU and GPU can “hammer” the same memory space.
Xbox360 enjoyed the benefits of unified ram and its GPU had direct access to CPU’s L2 cache, a special streaming of data called Xbox procedural synthesis (XPS) where the CPU could easily hand off data to the GPU with extremely low latencies from the L2 cache of the CPU.
On the PS4, the unified ram principle is taken to a whole new level, where CPU and GPU have access to a very high bandwidth memory pool. Don’t confuse this with shared memory on cheap laptops. On those, memory reserved for the GPU was not directly accessible by the CPU, and they almost always had a very low bandwidth memory solution, along with a very weak integrated GPU with low bandwidth connection between the CPU and GPU.
PS4’s APU is more advanced than AMD’s 2nd generation APU’s, which can share the ram but GPU and CPU still have access to different portions of the reserved RAM, which requires fast interconnects within the APU betwen GPU and CPU to let them deal with the same memory areas. PS4’s APU changes this, both GPU and CPU have access to the same memory space. This means that CPU can hand off work to the GPU and vice versa without the need to copy information.
Having to copy introduces massive latencies and stalls and requires developers to work around this using creative methods, or they choose to keep things only on the CPU side even though the process could be lightning fast on the GPU, simply because transferring all the data back and forth would take more time, thus nullifying any benefit that the GPU could provide.
On PS4’s APU where the data in memory can be manipulated either by the GPU or CPU or in sequence without any performance hits due to data transfer, CPU can pre-process data using branch-heavy algorithms and hand it off to the GPU, or the GPU could pre-process data for the CPU. It’s like a super-efficient tag team. Mark Cerny, in an interview with the Japanese press, mentions that these features aren’t put to use in the launch titles, but will be something that opens up new possibilities later on. It is easy to understand, since current devkits have APU’s that do not have the latest features provided by the PS4 APU.
Whereas PS4 can be treated and coded for like an ordinary PC which will result in a performance comparable to mid-to-high end PC’s, in the long run, developers will tap into these resources allowed by the unique features of hardware, which will provide extra performance that will allow PS4 to prosper in the later years of its life-cycle. PS4 is not designed to win the brute force race (a race that is already lost to the PC), but it will offer so much more from its fixed set of hardware along the course of its life.
In the same interview mentioned earlier, Mark Cerny lays out a road-map for PS4’s longevity, along with meaty details about PS4’s architecture. The mid-term goals for increasing performance is opening the shader API’s for deeper access to hardware. So you won’t get these in the launch titles. The long term prospect is mature HSA API’s to get the most out the architecture. Mr. Cerny states that [implications] will be “huge” and that these will happen in the long run.
There’s every reason to be excited about the possibilities provided by the PS4, and you can be sure Sony’s first party developers will produce outstanding visuals with the hardware. They will also play a key role providing other studios with optimized libraries which will leverage the quality on 3rd party studios. Personally, I can’t wait to see what Sony Santa Monica and Naughty Dog are up to.
