FSB-to-DRAM Ratio Question

[jaydub868]

I'm going to buy a Core 2 Quad Q9550, which has a FSB of 1,333 (333x4). For the sake of this argument (and assuming cost is not a facotor), if I HAVE to choose one of the following memory configurations:
DDR3 at 1,333 CAS 6 (FSB-to-DRAM = 1:2)
DDR2 at 667 CAS 5 (FSB-to-DRAM = 1:1)

which would provide faster system performance (all other factors held constant), and why?

What I really want to know is:
- Does running a FSB-to-DRAM in excess of 1:1 provide a faster setup, or is the extra memory bandwidth wasted due to the FSB bottleneck?
- If it is wasted bandwidth, then wouldn't the slightly lower latency DDR2 provide slightly better performance than DDR3 in this case?

 

[monst0r]

Well obviously the DDR3 would provide much more memory bandwidth (almost double). I personally find no reason to go over a 1:1 ratio, because then it's synced with the CPU. That 1:2 ratio will be beneficial when OTHER components besides the CPU need to access the system memory (such as a graphics card). The price difference makes the DDR3 option null in my eyes.

 

[ausch30]

I have another option for you. DDR2 800 underclocked to 667 and timings tightend to 3-3-3 or 4-4-4 would give you outstanding performance for very little cost.

 

[jaydub868]

Well, for sure, the smart money would buy DDR 800 and underclock to 667 with a lower latency. But I never said I was smart and cost just isn't a issue. The question is, what do you gain by going over 1:1. Maybe not much, which brings up the issue I'm pondering. Intel has front side ratings of 1333 and 1600, but DDR3 memory already scales to FSB:Memory ratios of 1:2 and higher. What is there a market for such bandwidth, if 1:1 is pretty much ideal?

 

[Box Cutter]

if you run DDR3 1333 at 1:1, you have room to overclock the processor (assuming you get an FSB1600 mobo). And btw, you can't run the DDR3 at 1:2 because ( at least as far as I understand it) DDR3 runs at 3X the base FSB speed, not 2X like DDR2. Hence if your FSB is 333 (as it will be at stock settings), DDR2 will run at 666 on a 1:1 ratio, but DDR3 will run at 1000 in a 1:1 ratio. And either way, you get much more performance from simply overclocking the CPU to its max than worrying about whether the RAM is going to its max. The RAM just doesn't have anywhere near as big an effect on overall performance.

 

[jaydub868]

Thanks for the info. But are you sure about the 3X base FSB speed for DDR3? I thought it was just like DDR2 (2x).

 

[Zorg]

And btw, you can't run the DDR3 at 1:2 because ( at least as far as I understand it) DDR3 runs at 3X the base FSB speed, not 2X like DDR2. Hence if your FSB is 333 (as it will be at stock settings), DDR2 will run at 666 on a 1:1 ratio, but DDR3 will run at 1000 in a 1:1 ratio.

I don't know where you got that understanding from. DDR3 is still double data rate. as is DDR and DDR2. The difference is that DDR3 has a lower memory clock than the I/O clock, to enable higher I/O clocks. Here is a table from Wiki, note the fact that they all run at twice the I/O bus clock, aka base FSB clock.

DDR3 SDRAM - Wikipedia

 

[Box Cutter]

I got it from this forum thread. It says:

The 667, 800, or 1066 refer to the max speed (front side bus in MHz x the divider) for which it's rated. It's a little more complicated... these are DDR2 type so you divide those numbers by 2. So you'll get 667/2 = 333 MHz; 800/2 = 400 MHz; and 1066/2 = 533 MHz. The CPU : DRAM divider is discussed later in this document. If you want to run a FSB of 400 MHz then you’ll need AT LEAST DDR2-800. The same rule applies for the newer DDR3 memory. That is, divide that last number by 3 to get the max FSB. For example, DDR3-1333 and DDR3-1600. 1333/3 = 444 MHz and 1600/3 = 533 MHz.

It seems to me from this that DDR3-1333 can only run up to 444 before the ram starts to overclock.

EDIT: This thread is stickied at both the TH forums and the Anandtech forums, so if it was wrong you'd think someone would have fixed it by now.....

 

[apache_lives]

DDR3 to overclock the hell out of that cpu, and give you the upgrade path to Nahlem - Tri-Channel DDR3

 

[Zorg]

I got it from this forum thread. It says:

That is, divide that last number by 3 to get the max FSB. For example, DDR3-1333 and DDR3-1600. 1333/3 = 444 MHz and 1600/3 = 533 MHz.

It seems to me from this that DDR3-1333 can only run up to 444 before the ram starts to overclock.

No, that is not the case. DDR2 and DDR3 both run at double (as in double data rate) the base FSB frequency assuming a 1:1 divider. They still run at double the memory clock speed as determined by the divider (not the internal memory clock speed, but the adjusted clock speed based on the divider) . Here is a link to DDR3 specs from none other than Micron (do you trust them?) DDR3 SDRAM Part Catalog. Make special note of the "clock rate" and "data rate" columns. Notice that the data rate is double the clock rate just as in DDR2, not triple. Here is a link to DDR2 specs for comparison DDR2 SDRAM Part Catalog. The speed can be higher because the internal memory clock is lower.

EDIT: This thread is stickied at both the TH forums and the Anandtech forums, so if it was wrong you'd think someone would have fixed it by now.....

I agree that particular point should be looked at and modified.

I will PM graysky and link this thread so he can fix the stickies or post here and show me the error of my thinking.

 

[graysky]

I'm on it guys, thanks for bring it to my attention.