Cpu memory limit

[Phenz]

So I'm looking into PC compatibility issues, and one point of concern that I found out is that CPUs have a memory speed limit. Specifically, I'm looking at 12700k, which has a memory speed limit of up to 4800 DDR5. So will running a higher speed than this be 1. Still beneficial, over the same capacity 4800? And if so, by how much as compared to a CPU that supports it? 2. Simply run the memory as 4800? or 3. Cause a worse performance than 4800, or even simply be incompatible (crashes, not booting up)?

 

P.S.: How important are ram timings for things like casual gaming / everyday use? Would a difference between 16.5 ns first word latency and 12.5 ns first word latency even be noticeable, or what change in fps in games?

[RONOTHAN##]

1 minute ago, Phenz said:

which has a memory speed limit of up to 4800 DDR5.

Not exactly. The 4800MT/s rating is the officially supported speed of the 12700K, but that doesn't mean that you can't run any higher speeds. It just means that if a higher speed doesn't work, you can't use that as a reason to RMA the CPU. 

 

In practice, every 12700K should be able to do 6000MT/s, with good ones on a good board being able to do 7000+. 

 

4 minutes ago, Phenz said:

So will running a higher speed than this be 1. Still beneficial, over the same capacity 4800? And if so, by how much as compared to a CPU that supports it?

Yes. The performance improvements will be more dependent on the workload that anything. For reference, there is no desktop CPU that has an official speed spec above 5600MT/s, and yet kits rated at DDR5 8000+ exist. 

 

7 minutes ago, Phenz said:

2. Simply run the memory as 4800? or 3. Cause a worse performance than 4800, or even simply be incompatible (crashes, not booting up)?

Incompatible is a possibility, but that's only if you go for a very high speed. 6000 CL30 should just work.

 

8 minutes ago, Phenz said:

How important are ram timings for things like casual gaming / everyday use? Would a difference between 16.5 ns first word latency and 12.5 ns first word latency even be noticeable, or what change in fps in games?

It's complicated. They don't really affect performance unless they're rather bad (I.E. 6000 CL30-38-38 and CL36-36-36 kits perform pretty similarly, though CL40-40-40 and CL36-44-44 kits perform a bit worse). Kits with good timings though are rather cheap nowadays, so there really isn't much reason to get something other than the ~10ns kits. 

[Phenz]

On 4/27/2024 at 11:58 PM, RONOTHAN## said:

Not exactly. The 4800MT/s rating is the officially supported speed of the 12700K, but that doesn't mean that you can't run any higher speeds. It just means that if a higher speed doesn't work, you can't use that as a reason to RMA the CPU. 

 

In practice, every 12700K should be able to do 6000MT/s, with good ones on a good board being able to do 7000+. 

 

Yes. The performance improvements will be more dependent on the workload that anything. For reference, there is no desktop CPU that has an official speed spec above 5600MT/s, and yet kits rated at DDR5 8000+ exist. 

 

Incompatible is a possibility, but that's only if you go for a very high speed. 6000 CL30 should just work.

 

It's complicated. They don't really affect performance unless they're rather bad (I.E. 6000 CL30-38-38 and CL36-36-36 kits perform pretty similarly, though CL40-40-40 and CL36-44-44 kits perform a bit worse). Kits with good timings though are rather cheap nowadays, so there really isn't much reason to get something other than the ~10ns kits. 

What about motherboards, can ram be run above the stated speed limit on ram for the specific motherboard without any impact on speed?

[RONOTHAN##]

9 hours ago, Phenz said:

What about motherboards, can ram be run above the stated speed limit on ram for the specific motherboard without any impact on speed?

That depends on how you define the maximum rated speeds of a motherboard. Boards list both a JEDEC frequency supported and an XMP setting they support. JEDEC speeds should always work, but the XMP settings they list is usually a bit optimistic (I.E. Gigabyte B760M Aorus Elite X AX lists 8266MT/s support, yet getting a CPU that actually is capable of that is almost impossible). 

 

Either way, the only thing that actually can be affected by maximum frequency is stability. It will always scale like you'd expect it to, it's just that those rather high speeds are a bit more iffy whether they'd work or not.