Frequency or Latency - which is more important ?

Christer · 2007/02/09

Hello all!

I came across a motherboard with two DDR slots and two DDR2 slots and this triggered a question. Which is more important, frequency or CL? Which of the below would be the "best" choice (all are Kingston 1GB kits of matched modules at the same prize):

DDR PC3200/400MHz CL3
DDR2 PC4200/533MHz CL4
DDR2 PC5400/667MHz CL5

Thanks for your time,
Christer

Whiskeyman · 2007/02/09

The best depends on the processor used, plans of future updating of the processor and/or thoughts of overclocking.

Rockster2U · 2007/02/09

Christer:

There you go again - another question that not only is a great question, but is going to lead to a lot of investigation and cerebral excercise. Correct answer is going to be partially processor dependent, but your DDR2 PC5400/667MHz CL5 is probably the best choice in most circumstances. I don't pretend to fully understand the Latency issues with DDR2 as I haven't worked with AM2 boards much. Guess I'll have to bone up while watching other responses to a most thought provoking post.

Chiles4 · 2007/02/09

Just a couple of months ago, MaximumPC magazine wrote an article which was basically a "showdown" between memory frequency and latency. They used Corsair's fastest RAM vs. their lowest latency RAM. Unfortunately, I don't remember the winner. Whichever side won, it wasn't a clearcut enough win for MaxPC to make any blanket statements about the two attributes. They did declare a winner in their specific scenario - whether that can be applied to all "scenarios ", that's hard to say.

Don't know about Socket AM2 but I've always heard that in Socket 939 at least, cpu speed is king and that memory is a bit of a stepchild in affecting overall performance. It's possible that the biggest factor affecting RAM performance (at least in a Socket 939) aside from the obvious clock speed is setting the RAM's command rate to 1T instead of 2T. I was told about this and it gave me a major boost in memory throughput. Didn't seem to translate to any improved gaming performance but you know how that goes.

Disclaimer: Using 1T on a Socket A platform is not worth the instability it brings but can often be used on Socket 939 with no problems.

Christer · 2007/02/09

The backgound story is a friend whos computer broke down. It begun with inconsistent behaviour and finally, when I removed and reseated connectors and RAMBUS modules, it became consistently dead. My conclusion was that it is either the motherboard or RAMBUS, probably the former. Since I didn't know for sure and 2 x 256 MB RAMBUS modules are twice as costly as a new motherboard plus 512 MB PC3200, my friend decided for one of the options below. Either of these will provide the quickest solution and he needs his computer.

Option #1 is a motherboard for the existing 2.0 GHz P4 plus 512 MB PC3200. (A motherboard for the existing RAMBUS modules would be very difficult to find, I guess and possibly at a higher price.)

Option #2 is a Socket 775 motherboard, an Intel Core 2 Duo E6400 and ... ... which brings us to my question.

The motherboard is chosen to be able to reuse the existing hardware which means that it needs an AGP 8X slot and two IDE connectors (four devices). These requirements limit the choices and I decided on ASRock 4CoreDualVSTA

Since I posted the question, he has been told that he would have to wait a week or so for those components and went with Option #1 which was immediately available. Option #1 was actually my unspoken recommendation since Option #2 would result in a computer with "mismatched" hardware. The question is still interesting, though so, keep your thoughts coming.

(I know what I will be doing this weekend ... ... reinstall his software and ... ... drink his beer!)

Christer

Christer · 2007/02/09

I forgot to mention that I went google on the subject but found no conclusive answer. When DDR2 was new, it was outperformed by DDR due to the higher CL of DDR2. DDR2 is developed towards higher frequencies but the higher the frequency, the higher the CL.

Christer

Christer · 2007/02/13

I read Can AMD'S 65 nm Core Fight Back? and found an interesting fact regarding the memory management in Memory Speeds Can Vary and I quote:

The processor can only use integers to derive the memory clock speed, which means that it will divide the processor clock speed by the memory divider. As you can see in the table, none of the new processors runs the memory at the full 400 MHz speed (DDR2-800). Although a few megahertz in memory clock speed will not impact your overall performance very much, DDR2-700 instead of DDR2-800 certainly does make a difference in synthetic benchmarks.
Click to expand...

This means (I think) that it will run at a frequency derived from processor speed divided by the lowest multiplier that will render a frequency within the memory specifications.

A 2.4 GHz processor will run the memory at 2400/6 = 400 MHz (which is "full steam ahead ").
A 2.5 GHz processor will run the memory at 2500/7 = 357 MHz.
A 2.6 GHz processor will run the memory at 2600/7 = 371 MHz.
A 2.7 GHz processor will run the memory at 2700/7 = 386 MHz.
A 2.8 GHz processor will run the memory at 2800/7 = 400 MHz (which is "full steam ahead ").

The above applies to Socket AM2 and DDR2 memory. I have not yet found out if the "same" applies to Socket 939 and DDR memory.

Christer

Christer · 2007/02/13

The above applies to Socket AM2 and DDR2 memory. I have not yet found out if the "same" applies to Socket 939 and DDR memory.
Click to expand...

I have exercised my mouse clicking on the internet and have come to the conclusion that it is a DDR2 "problem" with the higher frequencies (400 MHz DDR2-800 in the examples in the post above).

All processors for Socket 939 run at even decimal frequencies (2.0, 2.2, 2.4, 2.6, 2.8) and that renders the memory (DDR-400) to run at 200 MHz in all cases.

Christer

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