True latency

[Guest2]

I decided to put this together for poeple to quickly compare frequency and CAS letency to speed.

I don't know much about DDR5 but I guess there is more to it than "DDR5 6400C28 is about as fast as DDR4 3200C14"


DDR4
4000C14 14/4000 = 0.00350
3800C14 14/3800 = 0.00368
4266C16 16/4266 = 0.00375
4400C17 17/4400 = 0.00386
3600C14 14/3600 = 0.00388
4000C16 16/4000 = 0.00400
4600C19 19/4600 = 0.00413
4800C20 20/4800 = 0.00416
4500C19 19/4500 = 0.00422
3200C14 14/3200 = 0.00437
3600C16 16/3600 = 0.00444
4000C18 18/4000 = 0.00450
3600C17 17/3600 = 0.00472
4000C19 19/4000 = 0.00475
3200C16 16/3200 = 0.00500
3200C17 17/3200 = 0.00531

DDR5
6400C26 26/6400 = 0.00406
6400C28 28/6400 = 0.00437
6000C32 32/6000 = 0.00533
6000C36 36/6000 = 0.00600
6200C38 38/6200 = 0.00612
6400C40 40/6400 = 0.00625
5600C36 36/5600 = 0.00642
5200C38 38/5200 = 0.00730
4800C38 38/4800 = 0.00791

 

[philo-sofa]

Interesting!

Was under the impression that true latencies were generally static or improved? e.g. the normal DIMMs were roughly DDR2-800 CL5, DDR3-1600 CL10 and DDR4-3200 CL 15. This seems like a bit of a jump.

 

[Blackbadger]

To be Pedantic it's not calculated as Cas/Mhz, however (CAS latency/RAM clock speed) x 2000 = latency in nanoseconds.

True latency is another thing all together, which can be influenced by many factors. Whilst for a rough ballpark the cas-frequency can be a good start to gauge where you may be. Many other factors can make a significant impact on latency ><+- 25%.

In no order factors such as

-Motherboard/Vendors
-Platform AMD/Intel
-Single/Dual/Quad channel
-Single/Dual Rank
-Bios revisions
-Secondary & tertiary timings can cumulatively have a significant impact DDR5 6400 CAS 40-40-40-84 vs DDR5 6400 CAS 40-36-36-64 etc
-Geardown on/off etc.
-Drivers

Getting a good balance of clock speed/latency is generally the way to go, no magic answer as different use cases require one more than the other. However measuring true latency isn't so simple.

All that said, currently ddr5 is in it's infancy, caveat emptor with the first generation.