Is PS4 hampered by its memory system?

I think by giving CPUs a fixed budget, no matter what solution is picked, it should be fairly easy to avoid contention ...

 

I remember there has been a discussion here about DDR3 vs GDDR5 and the general consensus and data reveals that there is no significant difference of latencies. IIRC GDDR5 has relatively higher clock latency but GDDR5 also runs at a much higher clocks, in the 5.5~7Ghz range while current DDR3s run in the 1.6~2.3 Ghz range. If you factor in the higher clock latencies with the actual clocks themselves together they cancel each other out. If you measure the time itself I think they are generally the same.

Here's the info.

http://www.hynix.com/datasheet/pdf/dram/H5TQ1G4(8_6)3AFP(Rev0.1).pdf
http://www.hynix.com/datasheet/pdf/graphics/H5GQ1H24AFR(Rev1.0).pdf

Examples here are
GDDR5 timings as provided by Hynix datasheet:
CAS = 10.6ns

tRCD = 12ns

tRP = 12ns

tRAS = 28 ns
tRC = 40ns


DDR3 timings for Corsair 2133@11-11-11-28

CAS = 10.3ns

tRCD = 10.3ns

tRP = 10.3ns

tRAS = 26.2ns

tRC = 36.5ns

So latency wise you probably shouldn't see a disadvantage of GDDR5 as they're in the same ballpark.

 

All of this applies equally to the contention between the 8 CPU cores. You have 9 entities trying to access the memory at the same time, but 8 of them have priority over the GPU. I think contention between cores should logically dwarf any impact from the GPU, and it still doesn't seem to be an issue. The memory arbitration is probably designed to deal with this.

 

I have said something different: I have said to group threads that access similar data in the same cluster, in order to maximize the potential good effects of shared L2.
The dev should, on the other hand, optimize the code on the opt stage to keep a coherent access pattern (easier cache reuse among threads in same cluster?).

It might actually reduce average accesses to RAM from CPUs, due to average better local data coherency.

 

Why does contention induce additional latency just for PS4?

 

Perhaps there are some oddities with gddr5 that were not expected by AMD or Sony. Why else is AMD backing off its gddr5 based Kaveri PC apu?

There is that too, but I'm just looking at skyrim with 4k texture mods which can run @ 30fps on a 7850 and I ask why do so many ps4 game have not impressive textures? We all know very very high textures res on PCs only has a moderate impact on gpus.

 

Instead of looking at it on a bandwidth basis, look at it from a commandclock/cycles basis. How many memory cycles (ballpark range) would something that saturates a 7850 bandwidth (like Skyrim with 4k texture mods) take?
Are the remaining comandclock/ memory cycles sufficient to feed the latency sensitive cpu with a tiny cache? If not they'd have to compromise on feeding the gpu in a cycles basis.

 

PS4 has low res textures because of CPU latency created by UMA memory contention?

 

We are entering conspiracy theory land.
I would have said it's FUD land already, but no, it's a bit further up ahead...

 

It means you've given CPU a budget to work with. This then means you can think about priority, latency and what not. Every 'frame' (clock cycles equivalent) you can say: prioritise up to x cycles for CPU, then service GPU. It's probably a huge oversimplification, but it seems ... Unlikely to say the least that there wasn't a clear design decision that was made here.

And incidentally, I thought the days where a memory bus does 'only one thing' were long behind us, but I am certainly no low level expert.

 

There are dozens upon dozens of research papers on this subject that presents dozens upon dozens of solutions to tackle this issue. None I have read have stated this is a difficult problem to overcome just that giving simple preference to the cpu or gpu when accessing memory isn't a robust solution.

That being said, its going to be hard to determine the effect of memory contention on the PS4, if you don't look at the fact that PS4 isn't a discrete cpu/gpu set up but probably a HSA design which minimizes data copying between different pools of memory normally employed by a discrete system. Thereby reducing bandwidth pressure, memory contention as well as latency.

 

There are dozens upon dozens of research papers on this subject that presents dozens upon dozens of solution to tackle this issue. None I have read have stated this is a difficult problem to overcome just that simply giving preference to the cpu or gpu when accessing memory isn't a robust solution.

That being said it going to hard to determine the effect of memory contention on the PS4, if you don't look at the fact that PS4 isn't a discrete cpu/gpu set up but probably a HSA design which minimizes data copying between system and video ram needed by discrete system. Thereby reducing bandwidth pressure as well as latency.

 

What is this thread, internet warriors have discovered what game devs and Sony/AMD engineers have not? Maybe Sony found some holes they can access to reduce contention?

 

Why are you saying this? I don't get it. We are the last to discover it on earth.
We are discovering it long after the Sony/AMD engineers have complete knowledge and understanding of it. Long after game studios have discovered it. We are the last on earth to discover it, and its just a result of all the gameplay videos we've seen from all the games.