can i clock faster ram slower to get a lower cas?

[mikedev10]

i have a dell 7510, which uses ddr4 2133 ram. the fastest i can find is cas 13.

can i buy ddr 3000 ram, clock it down to 2133 and set a lower cas in the bios manually to 12 or 11?

 

[UsandThem]

Do Dell BIOS actually allow you to manually change RAM timings now? The last time I saw a Dell BIOS, it was pretty locked down on what you can change.

Even if you could change the CAS settings on DDR4 2133 speeds, the impact would be minimal at best. What do you do with your PC that would be impacted by RAM with a slightly different CAS latency? What RAM do you have in there now?

 

[Valantar]

CAS 13 for DDR4-2133 is excellent - the JEDEC standard is CAS 15. And anyway, what does this matter? RAM speed has a miniscule effect on performance. Don't sweat it unless you have an extremely specific workload that hinges on RAM latency only.

 

[mnewsham]

Latency is not a product of the CAS timings, latency is a function of your CAS timings AND your frequency.

They are tied together, if you lower the frequency to lower your CAS timings, you'll likely end up with higher latency unless you know what you're doing.

For example DDR4 2133 with CAS 13 has a first word latency of 12.19ns, fourth word 13.6ns, eighth word 15.48ns
DDR4-4133 with CAS 19 (some of the most expensive DDR4 kits you can buy) has a first word latency of 9.2ns, fourth word 9.92ns, and eighth word 10.89ns


So even though CAS 19 is much higher than CAS 13, the high frequency makes up the difference and the CAS 19 is far lower latency than the CAS 13.

 

[mikedev10]

so my thoughts on this even being possible were from reading a pdf of a micron stick i saw -
https://www.micron.com/~/media/documents/products/data-sheet/modules/sodimm/ddr4/atf16c2gx64hz.pdf

it looks like buying the 2G6 one could run it anywhere from 2666 cas 19 to 1333 cas 10. this question is probably more theoretical at this point as i have already bought the 2133 cas 13 stuff - but i did see the 3000 is only like $25 more. so my curiosity lies in whether it could be downclocked and run reliably at a sub 13 cas.

 

[dave_the_nerd]

Like mnewsham said - latency is not dependent only on CAS speed.

That said, in most cases what you're talking about will work. Used to be there were tools (Sandra-something?) that would read back all the "approved" speed/CAS (SPD) settings in your DIMM's firmware, not just the single combination that shows up on the box. That'd be where I'd start.

That said, once you start running stuff out of spec, there are no guarantees.

EDIT: It's interesting to me that RAM latency is still in the 10-20ns range. I mean, okay, yeah, that's really fast, but even 20 years ago with pre-SDRAM DIMMs it was 60-80ns. I guess CPUs have spoiled me; I would have expected better than a "mere" quadrupling.

Then again, a new CPU has more L3 cache than a 20-year-old computer had RAM, period. So maybe I'm just thinking about it from the wrong angle.

 

[Dasa2]

kingston 2666c15 has a jedec profile for 2133 12-14-14
so yes you may be able to buy a higher speed kit and run it at a lower speed with tight timings but it will require overclocking\stability testing
i have also used a 3200c14 kit at 3000c12 and with sub timings tweaked the 3000c12 was ~5% quicker in games

or you could just buy a kit that already has tight timings
HX421S13IBK2/16 CL13-13-13 @1.2V

buying a kit with tight timings is probably a bad value upgrade if you already have plenty of ram but if your looking to replace your ram with higher capacity kits i can see the value with looking into it since faster ram can improve cpu performance 10-30% in some games\programs that do overflow the cpu cache

 

[DrMrLordX]

Back in the DDR3 days of 2009, I had a DDR3-2200 kit that would run DDR3-1600 CAS6. Which was awesome, by the way. Loved my Pi Blacks!

So yes, high-clocked RAM can sometimes be made to run at low CAS/CL settings if you lower the clockspeed. My higher-density Silicon Power DDR3 will only do DDR3-1600 CAS7 despite being DDR3-2400 RAM. It simply is not as good as my old Pi Black RAM (though it's 2x8 GB instead of 2x2 GB, that has something to do with it).

With DDR4 you are basically looking at the same-ish latency if you move up 266 MHz in rating and 1 in CAS/CL

So:

DDR4-2133 CAS13
DDR4-2400 CAS14
DDR4-2666 CAS15
DDR4-2900 CAS16 (of course you are not going to find DDR4-2900, you will find DDR4-3000 instead)
DDR4-3200 CAS17

etc.

Though this will vary from one memory controller to the next, and if you care about iGPU performance at all (. . .) then higher clockspeed is almost always going to be better regardless of timings.