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Forums - Sony Discussion - Mark Cerny Explains How the PS4's 8 GB GDRR5 RAM and Bus Work and Why They Were Chosen

Pemalite on 30 June 2013
ethomaz said:

The latency comparison is not true.

Like I explained before the latencies is defined by the memory controller and I give even a example where the GDDR5 have lower latency than DDR3 (GTX 680 vs A10-5800k).

You don't know what the latency of GDDR5 on PS4 and what the latency of DDR3 on Xbone... so we can't make any assumption here.

And 10ns vs 12ns theoretical (in pratical the latency is ~40ns) won't give you noticeable difference in CPU taks.


The latency issue is true, I'll repost what I posted several days ago as it's still relevent:

However, since datarates on Ram have been increasing over the years so has the latency in clock cycles, but the absolute latency has been relatively static.

Grab some DDR3 1600mhz memory, that's 800mhz IO, which has a typical CAS latency of 8, that means it has a latency of 10ns.
Grab some DDR2 800mhz memory, that's 400mhz IO, which has a typical CAS latency of 4, this is also 10ns.

Now with GDDR5 the data rates are 4x faster than the IO clock instead of 2x, I.E. 5ghz GDDR5 is 1.25ghz x4 and would have a CAS Latency of 15.
15/(1.25 GHz) = 12 ns

So the latency of GDDR5 is only 20% higher than DDR3.

That's despite the latency in terms of clock rate being almost double of DDR3. (And Note: You can get GDDR5 that has a CAS Latency higher than 15, so what I posted is a best case scenario.)

You are right that information on this is fairly light, but remember that the Xbox One and PS4 is essentially a poor mans PC, it's basically mid-range off-the-shelf stuff, so you can see what trends have been occuring and summise what's in the consoles.
And latency can have a reasonable effect on CPU performance in the PC space, especially in synthetics.



--::{PC Gaming Master Race}::--

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brendude13 on 30 June 2013

Man, I feel like I can trust every word that comes out of his mouth.



haxxiy on 30 June 2013
Pemalite said:

The latency issue is true, I'll repost what I posted several days ago as it's still relevent:

However, since datarates on Ram have been increasing over the years so has the latency in clock cycles, but the absolute latency has been relatively static.

Grab some DDR3 1600mhz memory, that's 800mhz IO, which has a typical CAS latency of 8, that means it has a latency of 10ns.
Grab some DDR2 800mhz memory, that's 400mhz IO, which has a typical CAS latency of 4, this is also 10ns.

Now with GDDR5 the data rates are 4x faster than the IO clock instead of 2x, I.E. 5ghz GDDR5 is 1.25ghz x4 and would have a CAS Latency of 15.
15/(1.25 GHz) = 12 ns

So the latency of GDDR5 is only 20% higher than DDR3.

Did Rambus ever released their XDR2 DRAM? That must have been the epithome of high latency but super fast bandwidth, probably as good as you can get without stacking the RAM on top of the chip itself. Data rates could go up to 16x IO clock if I recall correctly.