Help choosing memory speeds vs timing

[Gun Jam]

Im looking at ddr5 memory and had questions about how the primary Speed say 6400 and its timings say 32,39,39 would compare to say 7800 at 36,46,46

if I wanted compare speed how do we go about this to get something close to real world performance? we have something listed at 6400 with faster timings vs something faster like 7800 with slower timings.

Is there a single value we can look at to see how they would compare I doubt that speed (6400) alone would be a good judge of speed a 7800 might sound fast but how bad does it get beat up by sloppier timings?

Is all 6400 going to deliver more real world performance than 6000?

Within a speed range say 7000 is there a timing value that is more important than the other 2? could we say that 6800 32,45,45 would beat 6800 34,45,45?

 

[LucShep]

5 hours ago, Gun Jam said:

Im looking at ddr5 memory and had questions about how the primary Speed say 6400 and its timings say 32,39,39 would compare to say 7800 at 36,46,46

if I wanted compare speed how do we go about this to get something close to real world performance? we have something listed at 6400 with faster timings vs something faster like 7800 with slower timings.

Is there a single value we can look at to see how they would compare I doubt that speed (6400) alone would be a good judge of speed a 7800 might sound fast but how bad does it get beat up by sloppier timings?

Is all 6400 going to deliver more real world performance than 6000?

Within a speed range say 7000 is there a timing value that is more important than the other 2? could we say that 6800 32,45,45 would beat 6800 34,45,45?

 

I'm presuming you're on about this for Intel 12th or 13th gen, and not AMD Ryzen 7000 series (DDR5-6000 is the sweet-spot for those).

I think if you're interested in RAM, maybe spend some time understanding how it all works, Speed vs Latency, as well as the main timings and other parameters, such as Command Rate and tRC, etc.

There are many guides on the web.
For example, GN's is a popular one, and very thorough:
https://www.gamersnexus.net/guides/3333-memory-timings-defined-cas-latency-trcd-trp-tras
Maybe this other one can serve better, although it's very old (dates back to DDR3), also a long read but it's divided in comprehensive sections:
 https://hardwaresecrets.com/understanding-ram-timings/

Anyways...  let's imagine you have DDR5 6400 32-39-39-102

So, you already understand that the 6400 (Mhz) listed there is about the speed of the memory.
But what about the other four main timings (not three as you mention), the 32-39-39-102 and what do each of those represent?
So in that "32-39-39-102" order....

• CL: CAS Latency. The time it takes between a command having been sent to the memory and when it begins to reply to it. It is the time it takes between the processor asking for some data from the memory and then returning it.
   
• tRCD: RAS to CAS Delay. The time it takes between the activation of the line (RAS) and the column (CAS) where the data are stored in the matrix.
   
• tRP: RAS Precharge. The time it takes between disabling the access to a line of data and the beginning of the access to another line of data.
   
• tRAS: Active to Precharge Delay. How long the memory has to wait until the next access to the memory can be initiated.

As you can realize, the shorter these timings are, the better.
Ideally, you want the highest speed and shortest timings, but that does not come cheap (or easy, if you're into RAM overclocking).
In reality, it's not like very small differences will be perceived in "real world usage" by the user (most times they're not). 

OK, now let's imagine a quick and basic p!ss-match between two DDR5 modules that you can find on the market:  

6400 CL32 (32-39-39-102)  versus  6800 CL34 (34-45-45-108)

• 6400 CL32
  At 6400, a cycle takes (1000/3200 = ) 0,3125 ns
  32 of those cycles then totals to 10.0 ns
   
• 6800 CL34
  At 6800, a cycle takes (1000/3400 = ) 0,2941... ns
  34 of those cycles then totals to 10.0 ns

Hmmm... curious, eh? ...the latency is the same.
People looking into one and the other will think that the latency in the 6800 CL34 kit is higher (versus the 6400 CL32 kit), but it isn't.
You see, there are more cycles (34) on the 6800 CL34 kit - but then those cycles happen at a faster rate.

In the end...... "but between the two kits, if it was my money??"   
Honestly, I'd get the cheapest of the two. Period.
I really think people overthink the subject and overspend too often. For example, in this particular "versus" that I just mentioned, yes the 6800 CL34 is the better one, but I doubt anyone will be able to tell a single difference between those two kits (none, nada, zilch) in real life usage or gaming, perhaps not even if registering and noting down average FPS to units and decimals like an utter nerd.  ...I don't now, in this case, and instead of splitting hairs, use the rest of the money on beer (I like beer!).

Edited November 8, 2022 by LucShep

[BitMaster]

To say it short: HIGH MHz & low latency is the goal

[Gun Jam]

Thanks for the feedback guys this is helpful

I plan on 13th gen somday

Thanks