Is 8gb of DDR5 equivalent in bandwidth to 12-16gb of DDR3?

[Aikouka]

And dynamically allocate between system and graphics RAM. I recall Bethesda saying it was really hard to port some of the Skyrim stuff to the PS3 because the XBox had a combined 512 MB RAM pool while the PS3 had separate CPU & GPU RAM pools even with the same 512 MB total. When they ran out of graphics RAM on the XBox they just allocated more to the GPU than the CPU (for example 320 MB GPU and 192 MB CPU) but the PS3 doesn't allow this and only could do 256 GPU and 256 CPU.

I thought I heard that developers could access the CPU's RAM on the PS3 from the GPU (it wasn't that fast though), but not the reverse.

There is also likely a cost component of their reason. Having a single memory interface and a combined CPU/GPU on a single chip is very cost friendly vs. separate CPU and GPU chips and separate memory interfaces. For example, an APU is always cheaper than a similarly low end CPU with an HD5450.

I doubt that there will be much of a cost difference between the two. It will probably cost slightly more with R&D to develop the GDDR5 solution because all existing APU solutions that I know of work with DDR3. I don't think it's a difficult transition, but it's certainly a hurdle that must be crossed. I would imagine that if they were to do it the same way as the PS3, they would just do a fairly standard two-chip implementation rather than an APU.

 

[386DX]

To answer the original question the amount of memory (8GB or 16GB) has nothing to do with the bandwidth. Since DDR3 and GDDR5 transfer the same amount of data per clock (MHz) the only thing that determines the Bandwidth is the clock speed of the memory and the bus width.

First lets discuss clock speed. This is where GDDR5 has the advantage over DDR3. 1600 MHz is a common speed for DDR3 memory with 1833 and 2133 MHz becoming popular. Typical GDDR5 run at 3600-4000 MHz. So GDDR5 typically has about twice the bandwidth of DDR3 if you're only looking at clock speed.

Now onto the second thing that affects bandwidth and that is the bus width. DDR3 typically runs on a 64 bit bus (1 DIMM) while GDDR5 on a 32 bit. As you can see while DDR3 is clocked about half as fast it's bus width is twice as wide so the bandwidth achieved is about the same as GDDR5. However it is quite easy to increase the bus width by adding more "channels". Your typical computer can run DDR3 in dual channel mode giving you effectively a 128 bit bus, while the higher end can run in quad channel mode giving you effectively a 256 bit bus. This works as well for GDDR5, if you use 4 chips in parallel you can achieve a 128 bit bus. If you use 8 chips in parallel you can achieve a 256 bit bus (like on higher end graphics cards). GDDR5 has more bandwidth at 256 bit bus then DDR3.

That isn't to say GDDR5 has no drawbacks. The latency on GDDR5 is much worst then DDR3. And GDDR5 uses much more pins (for power and grounding) which makes the trace on board more complex and shorter. This works fine on graphics cards (and consoles) because you can design so the chips close to the GPU but won't work well for motherboards because the RAM is on it's own PCB and could be far away from the CPU/GPU.

My personal opinion on the PS4 is the use of GDDR5 will help give the console a lot of graphical power but the latency is going to hurt normal CPU performance a bit because AMD CPUs where never great with their cache and the added latency to GDDR5 isn't going to help that.

 

[poohbear]

They are using 8GBs of RAM because they don't think they will have a use for more than 8, and they feel comfortable fitting game data or whatever the user will use the system for into 8GBs.

They are using GDDR5 because it's a graphics ram that offers a lot of bandwidth and a lot less latency. But they are not using a main system RAM and instead the 8GBs will be used for everything. Which is a lot of really fast RAM. Graphics require fast RAM, but today's most intense graphics don't really need more than 2 to 3 GBs of RAM at the resolutions the console will be playing at. Which means the other 5 to 6GBs is, for all intents and purposes, system RAM.

You could not run a console on 100GBs of DDR3 like you could on 8GBs of GDDR5. There's no real point in asking what the DDR3 equivalent is to 8GBs of GDDR5. You won't find one. DDR3 is too slow to do what GDDR5 does.

Yes thank you, thats the answer right there. Sorry u found it confusing, i found ur original post confusing, so was just asking for a clarification. If im not familiar with it, phrasing the question would be hard in the first place. Anyways thanks again for taking the time to answer it, didn't mean to get u riled up. U have a nice day now.