Whats the difference between CAS latency 2, 2.5, and 3?

[Asnrefugee]

For people who had experience with different CAS latency, how much of a difference is there?

 

[JBT]

The faster mhz wise the less the difference seems to be. For people who like to waste money on a negligable performance gains 2 is good. 2.5 if for the cost effective people who go for price/performance, and 3 is for cheapskates.

No im kidding though. ALot of the PC4000 and up stuff runs CAS 3 but is expensive and it doesn't seem that the latency really affects performance much.

2 = Fast
2.5 = pretty fast
3= decent speeds


I would go for atleast 2.5 but if it will break the bank 3 isn't really all that bad.

 

[RussianSensation]

There was a review i've seen long time ago that showed the performance delta was much greater between 2-3-2-5 and 2-2-2-5 than between 2.5-2-2-5 and 2-2-2-5. I cant remember where it was though. It was tested on a P4 system I believe.

The performance delta should be 2-3% I imagine, not more. Of course 3% here, 2% there, 4% another place and it all adds up (that is the logic here). But spending a lot of money extra on 2 vs 2.5 is not the best choice. In the time of rising ram prices, it is better to buy MORE ram than faster speed/latency ram. In the time of low prices, you should buy faster ram/lower latency.

If you have A64, since the memory controller is onboard, the difference should matter even less adn it has been shown that running memory at 1T vs 2T is a lot more important for A64 systems.

Finally, PC2100-PC3500 ram with cas 3 latency => usually poor overclockers, that is why many shy away from them. Also sometimes ram can run 2.5-3-3-6 on an AMD system and 2-3-3-6 on an Intel system, and other times you can buy the ram rated at 2.5-4-4-7 and it will run at 2-3-3-6 or vice versa so it all depends. Of course in theory 2-2-2-5 should be the fastest for Intel systems, 2-2-2-11 for Nforce2 systems and 2-2-2-10 1T for A64 systems.

Also Duvie and articles on the web proved that running lower latency (say 250FSB:200mhz RAM 2-2-2-5 (PC3200)) produces just as fast if not a faster system for P4 as opposed to running higher memory speed and higher latency (ie. 250FSB:250mhz RAM 3-4-4-8 (PC4000). So CAS timing alone wont affect performance as much as "overall" slow latency ram.

 

[danielf]

I´ve also read that for systems with a FSB of 333mhz, DDR333 might run faster than a DDR400.
I also read the following article on Tomshardware (http://www6.tomshardware.com/motherboard/20040119/index.html).
In the conclusions they say:
"The moral of the story is clear: while we still recommend buying brand-name products to ensure compatibility (especially for dual-channel systems), but you don't necessarily need the fastest timings. In today's market, you only need fast modules if your computer will be computing a lot or encoding video. For any other application, slower RAM will definitely cut the mustard."

 

[VirtualLarry]

Originally posted by: danielf
I´ve also read that for systems with a FSB of 333mhz, DDR333 might run faster than a DDR400.

This was true for the KT400 chipset, not sure if it were true for any others. Generally, AMD systems run better with the FSB of the CPU running sync with the FSB of the DRAM. Anything else, introduces a slight speed hit. So there is no advantage on a KT400 to running your CPU at 166Mhz FSB (x2), and the DRAM at 200Mhz (x2). Best to run them at the same.