How important is CAS really?!: is PC3200 CAS3 better or worse then PC2700 CAS2?!

[Dance123]

Hi,

I am looking for dual-ddr RAM for the Asus P4P800 and I was wondering how much different CAS latency really makes in practice?! How relevant is CAS latency? I don't intend to overclock in case that matters. Is it true by the way that fast modules (like PC3200?) with fast timings (CAS2?) can give stability and other problems? Isn't it so that PC3200 modules are standard CAS3?

To simplify my question: What will give the best performance and why?!:

1/ dual-DDR PC3200 with CAS3 or

2/ dual-DDR PC2700 with CAS2

Thanks in advance!

Mike.

 

[nick1985]

you could probably get that cas2 2700 faster than that cas3 3200 IMO

 

[CanadianBacon]

Memory timings are more important for AMD systems than Intel, so if you're running a P4 I'd say don't bother paying extra, really. But Nick is right, you can probably pull of 3200 speeds at CAS 2 on that 2700Cas2, thats just the way it works.

 

[MercenaryForHire]

FWIW, my system performs identically at 266/CAS2 and 333/CAS2.5 - I think CAS3 might neuter it a bit though.

- M4H

 

[Lord Evermore]

Higher bandwidth makes a more significant difference then aggressive timings, as long as the bandwidth difference is more than just a few MHz. But it depends on whether the CPU bus is as fast. Extra bandwidth for memory without a CPU bus that can make use of it provides almost no benefit. If you're using a P4C with 800MHz bus, you'll get better performance from CL3 PC3200 than CL2 PC2700. But a 533MHz P4 will only run with 333MHz memory on an Intel chipset anyway, 400MHz won't even be an option. Overclocking is possible, but you won't be able to make the memory run faster than the CPU bus.

The difference between CL2 and CL3 is present, but I don't think it's really huge. Benchmarks may show better bandwidth for low latency memory and the like, however real world usage will have a less noticeable difference.

The cost difference between CL3 and CL2 or even CL2.5 is very minimal, there is almost no reason to get CL3. If 10 dollars difference is too much, you wouldn't be buying such a high end CPU and motherboard.

The other timings however do make the price go up quite a bit, without as much performance increase. 2-3-3-6 rated memory going to 2-2-2-5 isn't going to be as much of a jump as the difference between CL2 and CL3.

Also most CL2.5 memory is able to do 2-2-2-5 if it isn't overclocked, and most CL2 memory is easily able to do that. (It seems that 2-2-2-6 is actually a better setting though.) I'd think in order to only be rated at CL3, the chips used would pretty much be the bottom of the barrel.

 

[Lord Evermore]

Oh, and no PC3200 is not standard at CL3. There is no standard. Early modules did have very bad timings because they were brand new, the chips weren't making good yields. Only Crucial makes only CL3 PC3200 modules anymore (which I think may be just so they can provide a better guarantee of compatibility, but even so, many boards now won't even run modules at CL3). Corsair makes no CL3 modules of any speed. Kingston makes CL3 modules, but as I mentioned the price difference to CL2.5 is very small.

Stability with low latency isn't specific to simply using CAS2. The stability problems result from trying to use too low a latency setting with a combination of too high a bus speed. PC2700 memory is relatively fast, and performs perfectly well at 2-2-2-5 with good modules. PC3200 is the fastest official speed, and also has good modules that do 2-2-2-6. When you might have issues is if you tried to run a module rated for 333MHz at 400MHz or higher, and still use low latency settings. Although most modules can overclock to some degree, the chips aren't rated for such high speeds along with low latencies. Low latency in itself doesn't cause stability problems, you just have to have modules that are capable of low latency at the speeds you choose.

 

[ScrapSilicon]

all good except on that

I'd think in order to only be rated at CL3, the chips used would pretty much be the bottom of the barrel.

or what the dram maker had at the time to fulfill a time-sensitive contract..wherein to make a deadline ..have cl3 pkged PNY pc2100 that actually runs very well at cl2 ..at 166mhz but these are the exception ..

 

[Dance123]

The reason I was asking about PC3200 CAS3, is because Apacer's PC3200 is CAS3. Now you probably think why I would want Apacer when there are brands that have faster CAS? Well, this is the reason:

When checking the supported memory list for the P4P800 Deluxe over here http://www.asus.com/products/mb/socket478/p4p800/overview.htm it seems that lots of top brands like Corsair, etc.. doesn't allow to have 4 DIMMs of them installed as 2 pairs of dual-ddr. When checking the supported memory list for the P4C800, it seems that lots of brands even allow only 1 DIMM to be installed, so how can you even have dual-ddr operation that way?!!

With brands like Apacer you can have 4 DIMMs as 2 pairs of dual-ddr, so that's the reason I would prefer Apacer above Corsair. Does that make sense as I am perhaps seeing it all wrong?! I don't understand why not all brands can have 4 DIMMs installed in those mobos?!

 

[Lord Evermore]

I think it's related to the number of banks on the modules, and maybe the chip density and organization. Notice also that those are PC3500 modules of Corsair and one of the Kingstons, and that they're running an overclocked bus.

That list is only a list of modules they've tested. Just because they got it stable during their testing doesn't necessarily mean it will be for you, or vice versa, since you'll have a different processor, different board, different modules, et cetera (even if you got the same models, you might get different revisions and stuff like that), and your timings may be different, they probably used the SPD timings on the tests. The fact that 2 Kingston modules of the exact same type but one is 256MB and one is 512MB, and they have such different "support" indications, seems to point towards it being a density/organization thing.

I can say that I had several modules of Apacer PC133 that were cheap and worked perfectly.

In real usage, you'll probably be able to use 4 modules at a time just fine. I'm not even sure that the notes on the Asus page about "supporting" more than 2 or 1 slot actually means they don't work, but rather may mean that the documentation for those modules says they won't work and Asus didn't even test it.