Dual Channel Ram guide.
Why I wrote: To correct some misconceptions that are out there about what is dual channel. And to try and help with some of the questions. “Will this be Dual Channel”. This only applies to Notebooks not Desktops, they are very different, as Notebooks have two slots A & B channels and Desktops have four slots 2A & 2B channels.
What is dual channel? The RAM bus is 64 bit, dual means using 2 (Intel) which gives you a 128 bit bus (effective), capable of twice the bandwidth. AMD is done different but the outcome is the same 64 vs 128
Do I have it? Most current Notebook Motherboards support it, remember it is the Motherboard that determines if you can use it, not the RAM. RAM is sold in “Dual Channel kits” as both a convenience and marketing ploy.
Do I need it? Yes/No, RAM is very fast but it does help, depending on your system, some more than others. Current Intel's take less of a performance hit not using it, AMD on the other hand makes much more use of it. Intel being 8% to 10% (est) improvement, AMD 20% to 30% (est) improvement.
Do I need the same size RAM sticks to run in Dual Channel? Yes (
AMD at this point says yes)/No (
Intel) but you do usually want the same speed and latency as every thing clocks down to the slowest of both. Example 533Mhz CL4 mixed with 667Mhz CL5 will go with the slowest of both 533Mhz CL5 (not always with the latency) and slower than if either were matched. Same size sticks will run slightly faster symmetric RAM array (Interleaved). If the RAM is of different sizes it runs in what is called “asymmetric RAM array” (
Intel Flex Memory/ AMD does not support). The greater the disparity between your 2 sticks the greater the performance hit vs sticks of the same size. The difference seems to be approximately in proportion to the ratio of the the smaller stick x2 to the entire amount of RAM. An example with Intel, 1GB stick and 2GB stick. 2/3, 1GB(smallest stick)x2 to 3GB(the entire amount). Well 2/3 of the total improvement, of the Dual Channel running with the same size sticks (symmetric RAM array) 10%, the asymmetric RAM array example is about 6.7% so a 3.3% less than symmetric RAM array. With 512MB stick and 2GB stick, 512MB(smallest stick)x2 to 2.5GB(the entire amount). 1/2.5 4% improvement. AMD's will show greater improvements and losses, Dual/Single keep that in mind.
What do I do? One absolute is the fastest, lowest latency is always the best unless it exceeds the the
FSB speed or the supported speed for your memory, RAM will usually just clock down. You want to have an amount of RAM that minimizes how often your CPU goes to Page File whether Dual Channel or not. RAM bandwidths are likely 3000MB to 4500MB/s 60ns to 100ns latency , HDD's are 25MB to 60MB/s plus 6ms to 20ms random access times. RAM is exponentially faster than HDD. The truth is there are no absolute answers to what to do. Clearly more RAM running Single Channel or asymmetric RAM array is better than symmetric RAM array with out enough to avoid the Page File.
Are some brands better than other? Yes/NO, all RAM of a rated speed and latency performs the same regardless of brand. Some of the expensive brand name RAM offers things like heat spreaders. Reducing heat is always a good thing but not always necessary and does not affect speed. Some people who overclock like these features. Buy from a reputable retailer and get a lifetime warranty and you will be fine with any RAM.
Conclusions: With Intel's I don't think people should worry as much about their RAM configurations. Only up to 10% difference. Make sure you have enough, AMD people you should care a little more up to 30% difference but not at the expense of too little RAM. Those with IGP's might want to consider symmetric RAM array more than others.
Let me try and give an example to try and illustrate the difference between the Single Channel and the two Dual Channels listed above. It is simplified but I think valid. We have two bottles exactly the same.
For Single Channel we fill both with 1 liter We turn one upside down to drain as soon as it is empty with no delay we turn the second one upside down until empty. We have timed it and we know our quantity. So we can get a ounce/s number
Symmetric RAM array some call “true” Dual Channel. Same two bottles fill 1 liter each, turn them upside down at the same time until empty. We have timed, to get the ounces/s, I am going to guess the ounce/s is going to be double the previous (don't laugh I know Einstein is rolling in his grave).
Finally the big controversy, Asymmetric RAM array. We fill one with 1 liter and the other with 2 liters turn them upside down until empty. We have timed, to get ounces/s.
What does that tell us? Well if you have 128 bit bus with 3GB's it is not Single Channel. It is not the same as symmetric bottle array, it is less ounces/s but it is more than Single Channel because it uses the 128 bit.
Here are some links that support some of what I say, nothing spells it out, which is why I wrote and got results on my own. These links actually disagree on some terminology. They are not notebooks, I could not find anything on notebooks but some info is applicable.
Link #1: Intel desktops. RAM Channel explanations.
Link #2: Describes RAM Channels on a server.
Link #3: Discusses Intel's "Flex Memory"
I would like to thank John Ratsey as he provided me with the Intel benchmarks I use, I have a very small sample but it is the best I could do.
If anyone has a Turion and can run matching sticks and different size sticks please PM me if you would like to help me.
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