dcddr on a7n8x (afterthougt on Xtreeme's post from a month ago)

[epz]

A brief refresh.... Dual Channel (DC) support is pretty useless if the memory bus runs at the same speed as the FSB since the FSB is saturated anyhow. So doubling (theoretically) the memory bandwidth does nothing because the the FSB bandwidth is already maxed out.

Now comes a question. On a board like a7n8x where memory speed can be set independently from the FSB, if I'm running my FSB at 200 (400 DDR) and memory at 133 (266 DDR) using PC2100 I should presumably still max out the FSB but take advantage of the DC DDR technology. Cheaper memory same bang.

Have any of you guys tried anything like that?

[Urban_Squrill]

Welcome to TechIMO.
I tried that once but when I did my computer was unstable form my cpu. In theory that should work.

[epz]

Thanks for the reply and the welcome!

You are saying it was unstable from the CPU... does it mean you were overclocking the CPU? I was more thinking of a scenario when you are using a CPU with the 200 nominal FSB but PC2100 instead of PC3200 memory without loosing any of the memory bandwidth.

Theoretically this should work but practice does not always agree with theories So I was hoping to poll some coollective experience before venturing into benchmarking it myself.

Thanks again!

[Urban_Squrill]

Unstable form beacuse of the stupid heatsink. The theory looks solid to me.

[brainyron]

Sorry guys, but this doesnt work. I was running a configuration like that for quite a while and going to a single 512MB DIMM of DDR400 gave me a huge performance boost over TWO 512MB DIMMs of DDR266. Dual Channel is nice in theory, but in practice, it doesnt offer much performance boost except in certain circumstances.

[pinipig523]

dual channel ddr is not just a theory - it definitely works.

however, the results are limited when it comes to the athlons and their respective motherboards. this is due to the fact that the athlons cannot shuttle more than 3.2 GBps.... and this is almost achieved by a single ddr400 chip alone.

however, dual channel does push the bandwidth a bit more and it DOES get closer to the theoretical max of 3.2GBps. From experience, a single channel ddr400 hits about 2.8GBps, while the same ram in dual channel configuration can hit about 3.0-3.1GBps.... so in my case, there IS a difference, albeit a small one (5-8% increase in bandwidth).

the p4c on the other hand DEFINITELY owns with dual channel - because the theoretical shuttle limit is 6.4GBps.... dual channel is ALLOWED to flex it's muscle in this case. Sometimes hitting mid 5s GBps.

So you see, dual channel is NOT just a lovely theory - it really works! Just not so good with AMD.

[brainyron]

You're still vastly overestimating the value of Dual Channel. I ran 1Gig of DDR266, and going to 512 of DDR400 is a huge performance boost......400MHz bus in both cases. If dual channel worked exactly as it was supposed to, then the DDR266 would've been more than able to keep my bus supplied, but it didn't happen that way. I think dual channel MAY help push the ram to its theoretical max, but certainly does not double its throughput.

[pinipig523]

explain to me then - why is it when dual channel is used with p4c (with 800+mhz FSB) that the bandwidth according to pcmark and sysmark is almost double of when only single channel is used.

dual channel can "double" the bandwidth (theoretically speaking) but you need to have the hardware to fully utilize it.

AMD systems just do not have the capability to fully utilize dual channel other than pushing bandwidth close to its theoretical limit of 3.2GBps.

[pinoy87]

I thought dual channel needed to be run synchronous with the FSB.

[brainyron]

You thought wrong. Dual channel doesnt even need two identical DIMMs, although that ensures optimal compatibility.