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Optimal memory speed / fsb combinations

Last response: in Memory
April 5, 2003 12:04:13 AM

I'm trying to scope out components for a new PC, and I'm kinda confused by the array of options surrounding memory. Specifically, is there any speed advantage to be gained from matching front side bus speed with memory speed? Say, if I got PC3200 DDR RAM (PC3200 = 400MHz). Would I be better off getting a P4 with a 400 MHz front side bus to "match" the RAM? Would the extra bandwidth of a CPU with 533 MHz FSB be "wasted" waiting for memory? Or am I totally out of whack here? And if that's true, then what about the commonly used pairing of a 533 MHz FSB CPU with 266 Mhz DDR RAM? Does that just totally suck?

My question arises from this cryptic statement from

"The latest chip sets have demonstrated that performance is greatest when the memory bus runs at an integral multiple of the front side bus of the processor. So, the optimum memory performance is attained when the memory bus is synchronous with the processor, and latency settings are reduced to the lowest values possible."

An "integral multiplier"? So, if 533 * 2 = 1066, is it massively better to get 1066 RDRAMs instead?

Also, I notice that (at least on
P4 2.6 GHz 400 MHz FSB Retail = $236
P4 2.66 GHz 533 MHz FSB Retail = $225

Why would that be, if faster was necessarily better? Maybe there's some kind of formula taking processor FSB, memory, and CAS latency into account. Maybe faster is just better, and that's all there is to it. Maybe none of this matters 'cause the CPUs have fat 512K L2 cache and that removes most of the penalty of slow memory. Maybe I should seek treatment for obsessive compulsive disorder.

Please help me figure this out. Thanks.
April 5, 2003 12:52:25 AM

I'm not really a technical guru when it comes to RAM, but I will give some advice..The reason why RDRAM cost more is for a reason, and if you take a look at some articles on this website about i850 (search for i850) you'll learn that RDRAM is tops in the benchmarks. I've read several times where the FSB bus has to be synced up with the RAM. A Pentium 4 is bandwidth hungry and RDRAM is better suited for a Pentium 4 for optimal performance. RDRAM gives at least 2 Gigabits (per second) of bandwidth per stick. When paired up with two sticks, it will give the P4 some breathing room with 4 Gigabits of bandwith. Compare this with single channel DDR at 2 gigabits - that's all. That's only 1,000 more megs of bandwith than SDRAM. Take a look at this link for more..
April 14, 2003 9:16:56 PM

I too was wondering about this idea that they must be matched. I came on the forums and didn't have to look long to get enough info to form an awnser. Here is the basic idea I'v finnaly formed:

Its good to have Ram synched with the FSB for optimal performance. However, if the Memory Bus is higher(faster) then the Processor Bus, you'll see no impact(or improvement) in speed. You'll just have wasted extra money on the faster RAM. The Faster Ram is intended for faster FSB's, or if your a AMD user (like me) you need the faster ram to make room for your FSB overclocking. Conversely, if your FSB is higher then your memory Bus(speed), you've got a serious problem. Thats because now your processor is going idle waiting for your RAM to cough up information to the Processor. Thats baaaad for performance.

Thus its better to have a:
DDR266(266Mhz) Athlon and a Pc3200(400Mhz) Ram stick then a:
DDR333(333Mhz) Barton and a Pc2100(266mhz).

I plan on overclocking my AMD (when I get it). If I got the cheaper DDr266(133Fsb) Athlon 2200 and overclocked it to the speed of a 333(166Fsb) I'd have to at least have Pc2700 Ram. If I Got the barton and overclocked it to DDr400(200Fsb) I'd need at least Pc3200 ram.

Thats the gist of it from what I'v read and gathered.