how important is cas 2.0 vs 2.5?

[Paladin7]

i am building a p4/2.53ghz rig w/512mb pc2700. is it worth it to buy the lower latency memory?

 

[magnux]

*shrug* Personally, I don't believe so. My "Cas 2.5" (as stated by Corsair) runs at Cas2 with absolutely no voltage increases or anything, however, I have noticed (@ googlegear.com) that the price difference is all of maybe $3-$5 (now, unforunately that wasn't the case when I bought my RAM). I'm also assuming your looking at Corsair RAM, I can't really comment on any other brands.

 

[BowlingNut]

my samsung pc2700 is rated to run at cas 2.5@333ddr, but i've seen it runn 2-2-5-3 at 400ddr. however, it is a gamble just like procs.
if its a small price difference, go for it. if its more than 10-15bucks, dont bother - memory is normally rated conservatively IF you use a quality provider like samsung or corsair.

 

[naddicott]

Hard to say about the memory itself, but if you're interested in the performance differences between the settings, check out Thugsrook's Corsair XMS 5 stick shootout thread.

 

[Paladin7]

hmmm....good input. i guess i should add that i am buying corsair ram and i am not planning on overclocking anything. how does one run cas2.5 @ 2.0? is it an overclock or what?

 

[caboob]

you adjust those settings in your board's bios if it will let you play with those settings, of course.

 

[Davegod]

my mobo doesnt even give an option for cas2.5, only 2 or 3, so apparently my crucial 2.5 stuff is running at 2, no probs at all. Generally though I'd say its worth getting cas2 over 2.5 if your mainly after gaming or something intensive, but not important for general use.

 

[rIpTOr]

The is almost no performance gain from CAS2.5 to CAS2 in DDR RAM. Why? Because DDR runs at the rising and falling edges of the clock so that equates to a 4th of a difference instead of a whole half clock cycle (2.5) as in SDRAM.

 

[ozihcs]

Uhh, come again? This is latency we're talking about, not bus speed. With a lowered latency of 25%, the processor will, on average, wait 25% less per memory read than with a CAS of 2.5. This does not affect the speed of burst reads (afaik, correct me if I am wrong), but I don't see how the internal doubling of DDR is related to the percentile gain in performance .. 25% is 25% regardless of how many intervals you divide over.

 

[Need4Speed]

you'll never actually see that 25% gain though. here is a snippet from crucial on this ...

"CAS latency 2 parts process data a little quicker than CAS latency 2.5 parts in that you have to wait a half clock cycle less for the initial data. However, after the first piece of data is processed, the rest of the data is processed at equal speeds. Latency only affects the initial burst of data. Once data starts flowing, there is no effect.

Bear in mind, a clock cycle for a system properly using a PC2100 DDR module is 8 nanoseconds, so the difference between CAS latency 2 and CAS latency 2.5 for PC2100 parts is 4 billionths of a second. You probably won't be able to notice the difference.

All of Crucial's PC1600 DDR parts are CAS latency 2."

If the price is the same, buy the lower latency....if the 2.5 is cheaper, get that instead. Your memory bandwidth will see much more of a performance increase from an increase in FSB.

 

[Need4Speed]

However, after the first piece of data is processed, the rest of the data is processed at equal speeds. Latency only affects the initial burst of data. Once data starts flowing, there is no effect. ... that's what's important here...unless you are processing a lot of little pieces of data, the lower latency will be negligable.

 

[MowSow]

Cool info guys !