Question DDR5 Question

[phillyman36]

Hey can someone explain to me how the ddr5 (cl40) latency affects gaming? Is it something I can actually notice or is it more just a value when measured? How does this latency affect something like Fortnite? Would I be able to actually feel a tangible differences?

 

[Soulkeeper]

The cas latency is one of many variables that determine the performance.
By itself, it is unlikely to be noticed assuming the frequency is high enough to make up for it.

Here is an overview of some formulas:
assuming the following:
Pretech = 8 for ddr4, 16 for ddr5
Bus width = 128bit for dual channel

Bandwidth ddr4 3200MHz:
(Pretech/2)*(Bus width)/8*3200/1000 = 204.8Gbit/s = 25.6GiB/s
Bandwidth ddr5 6400MHz:
(Pretech/2)*(Bus width)/16*6400/1000 = 409.6Gbit/s = 51.2GiB/s

Latency ddr4 3200MHz CL 20:
(read one byte) 20*2000/3200 = 12.5ns
(read 8 bytes) 20*2000/3200 + (7000/3200) = 14.69ns

Latency ddr5 6400MHz CL 40:
(read one byte) 40*2000/6400 = 12.5ns
(read 8 bytes) 40*2000/6400 + (7000/6400) = 13.59ns
(read 16 bytes) 40*2000/6400 + (15000/6400) = 14.84ns

These Latencies are essentially times to "burst" for single accesses to memory. If you do multiple (ie: larger than 1,4,8 bytes for ddr4, or over 16 bytes for ddr5) then you are limited by bandwidth and other factors. Also none of this even begins to take into account other complexities or commands, cache, etc.

I should have used 4800 cl40 ddr5 for the comparison.
The general idea being latency might be lower for some things, but the bandwidth and CPU cache is likely to make up for it in most scenarios. Once we see ~6400 with fast latencies, then the overrall latency will be comparible worst case.
We didn't get a prefetch "burst length" increase going from ddr3->ddr4, so this will be a bigger transistion than we saw then. Also the increased banks and bank groups should help a lot.
For gaming, and most apps, ddr5 with higher CL will be faster than ddr4 just due to it's other improvements.

I likely made some mistakes in here, but you should get the basic idea.

 

[Turbonium]

The cas latency is one of many variables that determine the performance.
By itself, it is unlikely to be noticed assuming the frequency is high enough to make up for it.

Here is an overview of some formulas:
assuming the following:
Pretech = 8 for ddr4, 16 for ddr5
Bus width = 128bit for dual channel

Bandwidth ddr4 3200MHz:
(Pretech/2)*(Bus width)/8*3200/1000 = 204.8Gbit/s = 25.6GiB/s
Bandwidth ddr5 6400MHz:
(Pretech/2)*(Bus width)/16*6400/1000 = 409.6Gbit/s = 51.2GiB/s

Latency ddr4 3200MHz CL 20:
(read one byte) 20*2000/3200 = 12.5ns
(read 8 bytes) 20*2000/3200 + (7000/3200) = 14.69ns

Latency ddr5 6400MHz CL 40:
(read one byte) 40*2000/6400 = 12.5ns
(read 8 bytes) 40*2000/6400 + (7000/6400) = 13.59ns
(read 16 bytes) 40*2000/6400 + (15000/6400) = 14.84ns

These Latencies are essentially times to "burst" for single accesses to memory. If you do multiple (ie: larger than 1,4,8 bytes for ddr4, or over 16 bytes for ddr5) then you are limited by bandwidth and other factors. Also none of this even begins to take into account other complexities or commands, cache, etc.

I likely made some mistakes in here, but you should get the basic idea.

As great as this response is, perhaps it would be useful to also address his question regarding gaming/Fortnite (which I would do, if I knew the answer).

 

[Soulkeeper]

Yeah, thanks.

Short answer:
For gaming, and most apps, ddr5 with higher CL should be faster than ddr4 with lower CL just due to it's other improvements.

We'll know for sure when the benchmarks and indepth reviews start coming out.

 

[phillyman36]

@Soulkeeper Thanks. I was puzzled by comments of people saying the 40cl would be horrible for gaming.

 

[biostud]

@Soulkeeper Thanks. I was puzzled by comments of people saying the 40cl would be horrible for gaming.

The obvious answer is: let's see when we have computers where we can do benchmarks.

There are so many factors determining how well a game run, besides memory latency so it really does not make a lot of sense talking about it unless you have a system where you can do some tests. Also different games are affected differently.

 

[kschendel]

@Soulkeeper Thanks. I was puzzled by comments of people saying the 40cl would be horrible for gaming.

I suspect that most of those comments were made by people who don't really understand that the CL number is clock ticks, and can't be compared with other CL numbers until you adjust for frequency.

There are a depressing number (meaning, more than zero) of youtube videos and articles purporting to compare memory performance, by people who think that they are holding latency constant, when in fact they are holding the clock tick count constant. Not the same thing at all.