Memory: CAS Timings VS Speed

[photoshop]

Ok guys

I've seen a few discussions on here but havent found the definitive answer.
I know a lot of people like to use fast DDR3 such as PC3-14400+ but in notice the timings are not as tight as slower DDR3 such as PC3-12800.

How does this actually work in real life situations?
Is faster RAM really quicker buy a big margin than slower more tighter ram?

 

[Rroff]

Depends on what your doing but on current Intel platforms realworld difference between any half decent RAM is a very few percent at best.

From my experience overall you get the best results from a blend of frequency and reasonable timings for example 2133 cas 9. Though 1866 tends to be better value for money, I've bought 2400 stuff due to the extra headroom to play with and running 2133 with tight timings.

 

[lord filbuster]

It depends on the difference between the timings and the clock speed.

The timings are referenced in terms of the clock speed, so it is relatively easy to calculate the theoretical difference between PC14400 and PC12800

The clock speeds of these two are 900MHz and 800MHz (1800MHz and 1600MHz transfer rate), so a CAS latency of 1 would mean that the delay between an access request by the memory controller and the data being available is 1s/900000000 = 1.111ns and 1.25ns respectively.

So if the CAS latency of the faster stick is say 7, and the slower stick is 6, then the delay is 7.77ns and 7.5ns, so the slower clock speed actually has lower latency. However, if the faster ram had a clock speed of 1000MHz and CL 7, then the delay would only be 7ns.

In general, higher clock speeds are still better, because the maximum transfer rate only depends on clock speed, not CAS latency. DDR-1800 has a bandwith of 14400MB/s compared to 12800MB/s for DDR1600.

To be honest, there probably isn't a difference outside of benchmarks anyway. Unless the memory access is completely random, the memory controller can do some clever tricks to minimise the effect of the latency, and I doubt the peak bandwidth of the RAM is going to be a bottleneck in many systems.

 

[HangTime]

In my experience since the advent of DDR3 (perhaps even DDR2) it has all been much of a muchness apart from perhaps a few niche situations. Personally when purchasing I'd start by choosing the speed you want, then weigh up the relative cost of different latencies. In terms of tweaking existing RAM, I wouldn't bust a gut over it as the difference between say CAS9 and CAS11 isn't going to have a dramatic impact compared to the old days of say DDR at CAS2 vs CAS3, especially if you have to give up much speed.

One thing I have noticed is that bizarrely timings seems to have got higher on the slower memory being sold these days. When I moved to DDR3 I bought PC12800 CAS7, but CAS7 seems pretty hard to come by these days - OCUK don't sell anything under CAS9 for example! (of course, chances are some higher speed rated memory could manage it when downclocked)

 

[Rroff]

One thing I have noticed is that bizarrely timings seems to have got higher on the slower memory being sold these days. When I moved to DDR3 I bought PC12800 CAS7, but CAS7 seems pretty hard to come by these days - OCUK don't sell anything under CAS9 for example! (of course, chances are some higher speed rated memory could manage it when downclocked)

Seems a lot lately your getting just quality binned ICs of the same model in a lot of RAM modules so the 1600 CAS9/10 stuff uses the same ICs as the 2400 CAS10/11 just speed binned. In the past there was a couple more people making ICs and for instance hynix had several models from low to high end now they seem to mostly be concentrating on one middle of the road model (possibly due to recent events fire, storms, economy, etc.).