Adam Jensen said:
I already explained this to you. Latency isn't an issue when your entire system is using the same type of memory. The timings are reduced by eliminating the transfer latency. It's good stuff. Timing makes very little difference in gaming performance anyway. Memory bandwidth however makes a huge difference. And PS4's memory bandwidth is 176Gb/s.
But memory has an inherent latency when combined with a memory controller.
http://en.wikipedia.org/wiki/CAS_latency
APUs still use a memory controller. I don't see how latency would be reduced (This is not something i've heard of before in the way you're describing it.), since a memory controller is always present with all memory and a memory controller only ever handles one type of memory anyway. GDDR5 simply does have higher inherent latency.
Also, it's impossible to eliminate latency, because, well, electrons still need to move 'n' ting. Unless Sony have developed FTL communications AND moved away from CMOS in one go. If so, i salute them and i also wonder why the PS4 isn't more powerful in that case seeing how it must have fantastical tech inside it. Also, it means AMD have been holding out on us, the gits.
Although it's all a bit of a strange thing to be having too much of a discussion about, the CPU in the PS4 is at a slow clock speed anyway (it's also 'slow' in the traditional sense, which is important for why GDDR5 is a good idea for PS4), it's hardly going to need really low latency memory to do well. It's only running at ~1.6GHz with an average IPC (instructions per clock) of <1, so if it missed a few cycles waiting for memory it's hardly going to be a big deal. It's why CPUs back in the past could deal with having very large latency memory. Whereas High latency memory on a chip running at 4.5GHz with a higher IPC like ivy bridge would have a larger impact on performance as the CPU will potentially waste more cycles waiting for the memory and each of those cycles can do much more than something like Jaguar can. The higher the performance of the processor, the more relevant memory latency is.
As a result, Sony made a good move by using GDDR5, since it benefits the GPU greatly as it will utilize the bandwidth, but wont disadvantage the CPU much, as it's not fast enough to appreciate the lower latency of DDR3. PCs separate the two because both aspects of PCs are 'high performance', so a compromise results in large performance degradation of one component.
To put it in the form of an analogy: low latency memory in the PS4 would be like putting really, really expensive tires on a bog standard fiat punto. It's nice for the punto to have, but it wont improve handling much more than some 'good' tires. Whereas if you slapped those amazeballs tires on a ferrari, you'd notice it.
