dual channel

quantumisation

quantumisation

quantumisation

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24 Feb 2007
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im ashamed to ask this but sick of wondering. For optimal performance you should match your fsb and memory speed but when dual channel comes in to play, does this mean you can run both modules at 1/2 the speed and still have the maximum bandwidth?

e.g 1066mhz fsb and 2x533 modules in dual channel - is this optimal?

or is it only optimal if you have 2x1066mhz modules in dual channel for that fsb?

This is my weak spot when it comes to computers please help, some figures for bandwidth would be good for both of those scenarios i suggested if its easy for anyone to work out
 
it doesnt really

the only relevant bit might have been how memory speed is calculated but doesnt mention dual channel there. Im going to assume the following is correct

1x drr2 800mhz = (800 x 64) / 8 = 6400 hence PC6400 - (i know this bit is correct)

however does this mean in dual channel for e.g

2x ddr2 800mhz (dual channel) = (800 x 128) / 16 = 6400

or

2x ddr2 800mhz (dual channel) = (1600 x 128) /16 = 12800

therefore 800mhz in dual channel would be the same as a 1600mhz module in single?
 
I don't think that Dual Channel has anything to do with the speed of your memory. Dual Channel just means that there are more pipelines between the memory controller and the memory - instead of having a single pipeline for send and receive, there are multiple pipelines so that send and receive operations can be carried out simultaneously. Something like that anyway, I am sure someone else can explain it better. It certainly doesn't result in a doubling of performance as your calculations would suggest.

Whether running dual channel or not, matching the memory speed with FSB is optimal. Unless overclocking when a higher memory speed may be beneficial.
 
i wasnt suggesting it doubles speed, just the bandwidth which i would imagine is just as good as doubling speed for performance. Everyone knows the textbook 'dual channel doubles your bus width for memory' but not so many seem to know the real life implications otherwise id have had more replies right?
 
quantumisation said:
i wasnt suggesting it doubles speed, just the bandwidth which i would imagine is just as good as doubling speed for performance. Everyone knows the textbook 'dual channel doubles your bus width for memory' but not so many seem to know the real life implications otherwise id have had more replies right?
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OK, but I was confused by your calculations. Basically, if you are trying to put a general figure on the performance increase then you wont be able to. That is because the performance is application-dependant. Different applications will use the memory bandwidth in different ways and so while you might see a good performance increase in games for instance, you might not see any benefit at all in other applications.
Also, the performance difference will be different depending on the memory controller, the RAM, and other parts of the chipset too probably.

Forgive me if I'm still not helping but I'm still not sure what you are asking.
 
just how dual channel makes any difference with as much technical information behind it as possible. 128bit instead of 64bit bus makes things better seems to be the limit of what most people know on the matter
 
put simply, dual channel doubles the available ram bandwidth at any given speed. the reason you dont see double the memory performance is that it's the cpu thats limited as to how fast it can process that information.

for example, the old socket A AMD cpu's had a stock fsb of 200mhz. that just so happens to the the speed of a single stick of ddr400. Just as with ddr that internal fsb is doubled to get 400mhz they both have a limit of 3.2gb/sec @ 200mhz (ddr400 is also known as pc3200 ;)) Thus, when nvidia introduced socket A to dual channel ram, doubling theoretica bandwidth to 6.4gb/sec, there was very little difference in performance. the reason was because the cpu's are limited to to the same bandwidth as a single stick of ddr, get it? you cant increase performance by stuffing 6.4gb/sec down a cpu's throat if it can only process 3.2gb/sec.

P4's had their cpu's internal fsb quad pumped (800mhz with an fsb of 200mhz), so they didnt have that problem.


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As for your questions regarding what is optimal, you're over-complicating it. all cpu's are dual channel these days. they run optimally on ram rated at their working fsb in dual channel. if its 1066 then that doesnt mean 2x533, it means 2x1066 moduals.
 
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Just consider the RAM rating you're referring to (PC6400) as a performance index. PC6400 RAM is guaranteed to run at speeds of up to 400MHz (800MHz DDR effective).

Intel chips have a quad-pumped FSB so in order to run 1:1 with an actual FSB of 266MHz (1066MHz effective (4 x 266MHz)) you only need PC-4200 (266MHz (533MHz DDR Effective)) RAM. People generally buy kits rated above PC-4200 speeds to allow for overclocking, so a PC-6400 kit would allow for an FSB of 400MHz which allows for a CPU overclocking margin of 50%.

Dual Channel doesn't come into the overclocking aspect it just doubles the potentially usable bandwidth, the RAM speed is the frequency at which it operates. All you really need to do is have the RAM in the correct slots and make sure it's enabled in the BIOS, I think it yields a ~10% performance over single channel.

OT Edit - @James: Ah, so you're johnny bravo at the unspeakable ***** forums :p
 
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