Dual channel RAM how it works please read

Everyone in here is thowing in ideas but as far as I can tell no-one is giving 100% correct information.

Facts:
1. Dual channel ram does double the memory bandwidth
2. To get dual channel in most cases you need 2 sticks the same size and same speed.


Now to fact 1 everone seems to think it isn't true but thats becasue your using a laptop and the FSB is your limiting factor not the ram. On a 800mhz FSB you can get 6.4 GB/s of data. A single 667mhz stick can give 5.3Gb/s of data so when you put them in dual channel you get 10.6 GB/s but the FSB can't support that so you only see a small gain instead the diffrence from 5.3 GB/s to 6.4GB/s. Also these values are theoretical so you don't get them in real life and teh numbers will normally be lower when you benchmark. Another thing that stops you getting these values are the RAM timings which is effectivly the time it takes for the ram to respond to a command so if you have slower timings teh ram will respond slower and in effect you get Data slower.


With fact 2 that is in general there are some motherbaods which will give you dual channel even if the sticks are diffrent sizes but this is very rare and I wouldn't count on it. Also with say a 2Gb stick and 1 Gb stick on one of these boards you get 2Gb in dual channel and 1Gb not in effect.


So whats all this mean? well if you have an 800mhz FSB and 667Mhz ram in dual channel you are wasting alot of that rams potential. If you are running it in single channel the FSB isn't being 100% utilized. So to get the best perfomace you can from all your hardware their are 2 options.

1. make your FSB faster so it can handle the bandwidth of teh ram which on a laptop is very hard.

2. make the ram slower so its bandwidth mathces the FSB and lower the timings on the ram (I know it sounds crazy but it works). Although you still aren't really getting the full potential out of your ram because yes it can go faster but atleast this way you are utilizing it better.


Go read some reviews on the effects ram faster than 800mhz has on PC's. Becasue in everty case it comes has no perfomance gain becasue the FSB is the bottleneck not the ram. And it all works the same way for laptops.

Also in alot of cases the perfomance changes you get from changing ram stuff is minimal. And can casue stability issues so unless you really want all teh perfomance you can get I wouldn't be to worried about it.

 

It theoretically doubles that bandwidth, real world performance is a completely different story, and most modern machines (any core system and up) supports dual channel with different capacities, speed has always defaulted to the lowest module, IE 533 mixed with 667, both would run at 533.

 

First of all, thanks for making this thread and for your response in the other thread. Since you seem to know what you're talking about, I was was wondering if you could give me your opinion on whether or not 3Gbs of single channel ram is worth it. Since I will not be using more than 2Gb 95% of the time, it seems like the single channel memory would just be a hinderance to the systems performance. But everybody keeps telling me that more memory will outweigh the performance loss of dual channel, I just dont see how that is possible if the extra memory does not get used on a regular basis.

Also, why does dell sell laptops with 667Mhz ram if its not being taken advantage of? Why not just sell them with 400Mhz ram? It sounds like if you dont downclock it, the 667Mhz ram will actually hurt your performance a tiny bit. Are they just using it as an excuse to charge extra or something?

 

I know you didn't ask me but I can't resist. If I was going to use less than 2GB RAM 95% I would use 2X1GB configuration. But running w/3GB is not running in single channel it will still be dual channel.

The OP talking about the limitations of FSB (theoretically not real world) is true but not just w/notebooks, 1066Mhz is still not able to take advantage of his 667Mhz Dual Channel example as it can only handle 8.4GB not the theoretical 10.6GB in his example.

The strict syncing of FSB speed and RAM speed is flawed. In the real world the Memory Controller and CPU need to go out and look for things that it has to find so it will never function at it's theoretical max. So why do company's put in the faster RAM if theoretically it has greater bandwidth? If you were looking for something would you want to find it faster or slower, then when you found it move it faster (the later is how faster speed makes up for higher latency). The fastest I can find 667Mhz in Dual Channel moving in benchmarks is about 5GB so under even the 667Mhz FSB's capabilities let alone the 800Mhz FSB it is not limited by the FSB. So that is why they do it, it does run faster. Oh and just to confuse things Turions can handle about 19GB of which about 12GB is memory.

 

Thanks for the response.

I want to run 3Gb because I'd like to have it there just incase I ever need it and for the future, but running single channel seems like a bad idea. Basically I want 3Gbs for any situation where I might need it, but I dont want having it to slow the system down at all when I dont need. Since you say it will still run dual channel with 3Gb, Im guessing you think it will run asyncronous dual channel. Is there any way that I can be sure that my Inspiron 1520 has this capability? Will asyncronous dual channel produce the same bandwith as regular dual channel when it has the 1Gb card and the first 1Gb of the 2Gb card running dual channel?

I chatted with a dell rep, but I dont think that they really knew what they were talking about. First they told me that it has two 1.5Gb chips. Then I corrected them and explained what asyncronous dual channel does and they said that it has that, but I have a feeling that they didnt really know.

 

"So why do company's put in the faster RAM if theoretically it has greater bandwidth? If you were looking for something would you want to find it faster or slower, then when you found it move it faster (the later is how faster speed makes up for higher latency). The fastest I can find 667Mhz in Dual Channel moving in benchmarks is about 5GB so under even the 667Mhz FSB's capabilities let alone the 800Mhz FSB it is not limited by the FSB."
So I guess having a higher frequency and being able to find things faster is better than having tighter timings and a lower frequency?

I guess if running at 667Mhz, the actual bandwith of the memory is only 5Gb, mabey the actual bandwith of the FSB is significantly lower than the theroetical value as well, so I guess either could be holding it back? Or perhaps the computer manufactures have found that when run at 667mhz the actual memory bandwith is closest to the actual FSB bandwith?

 

As stated about 10% reduction, and yes your Dell will run like that.

Asyncronous dual channel, nice word, see it all the time. Even syncronous dual channel effectively runs asyncronous at times. But I don't think anyone wants to hear. So get 3GB and enjoy.

 

Sorry, just to clarify, you're saying that asyncronous dual channel, when in dual channel (using the 1st 1Gb of each chip) will still run 10% slower than regular dual channel? Or mabey the bandwith tests test the entire memory capacity, so the 10% decrease is the result of factoring in the addidional 1Gb?

 

1st off Rich115 you are correct I misread it was not your mistake it was mine. Yes in general the faster speed is going to give you better performance than slower with tighter timings (standard not some freaked out 1.5 vs 9). Now no it is not FSB holding back, it is the RAM that does not reach it's theoretical potential. On your last post I am not so much saying the 1st 1GB of the 2GB stick is dual channel, it is just dual channel, I can't split hairs. Let me give a bad example. If I have two 1GB sticks and the data is randomly placed then I go to retrieve and it is half on one and half on the other so I can use the full 128 bit bandwidth. But it never works like that so sometimes even w/that configuration I am only puling off 1 stick which means my effective bandwidth is 64 bit not 128 bit. But if I double the size of one of the sticks I can not even get a 50% 50% split so the odds increase that I will only actually be pulling from 1 stick and thus only 64 bit bus. Best I can do explaining right now. Hope it makes sense.

 

What apps are you running or planning to run? Most likely you will see no real world difference( not counting capacity) between 2x1GB or 1x1 and 1x2 GB in terms of real world performance, benchmarks will be another story, but in the end, they dont really matter.

 

Thanks for the input powerpack. and yes i know even the 1066FSB desktop procs don;t see full potential off 667mhz.

However i can most definitely tell you that when you over clock the FSB up to 1400 like i have then you don;t reach full ram bandwidth until 700mhz at which point my comp gets 7800MB/s in sandra and other ram benchmarking apps and although the theoretical of that setup should be 11200MB/s I can up my rams speed all the way to 1000mhz and I actually lose bandwidth. Because once i get past 700mhz i have to loosen the timings and that loses me more bandwidth than the extra mhz.

From my testing it seems that the theoretical bandwidth would nearly be reached if the RAM had 1-1-1 T1 timings however no ram has that. It also seems that the higher the latency the less bandwidth you get per mhz. Its all a bit of a balancing act if you really want every bit of speed you can get you should be prepared to spend quite a few hours on it. Took me about 12 hours to get the sweet spot on my PC. Its just increase/decreasing frequencies and timings and seeing the results and once you find the combination that gives you the highest numbers while being stable keep it .

 

Another thing people miss when they talk about dual channel is latency.

One thing that dual channel helps in is reduce latency. Not in the sense of timings but it allows one stick to be read or written and the second one to receive the request and prepare for it. If we're not talking about the new 800FSB chips then think NForce2. This is where the boost came from, it wasn't from the doubling of bandwidth which is limited by the FSB. It was the ability to accept more requests and reduce the response time of the chipset.

The new 800FSB chips need the doubling of bandwidth to keep up.

 

BenArcher:
I missed the overclocking of the RAM in earlier posts. I do understand latency (somewhat), I suspect the 1:1:1 would help but sometimes if the data was pulled from only 1 stick and restricted you to 64 bit bus slow down. I guess I am saying theoretical is impossible but no problem trying! What is your opinion on “dual channel” it gets kind of a bad rap around here which I do not understand. It is not clear to all if thier MoBo supports, if they have two sticks you are running "dual channel" at some level even if size does not match. I say pull out 1 stick and tell me it does not help! Most tests do not test large amounts (>1GB) of RAM so do not tell me it is because of less RAM.

 

My opinion is that dual channel is very useful. Although currently laptops see the least gain from it. As a single stick is now so fast that you don;t need the boost it gives you. But if you test any older system like my old inspiron with its 333mhz RAM and you pull out a stick you definitely notice the diffrence. And the same with any older PC and with current over clocked PC's. What has happened in the last 3 years is that FSB has doubled at most (P4's can have an 800mhz FSB) while the speed of ram has gone from 333mhz upto over 1000mhz which is triple. So the need to double the ram bandwidth has become less. But I still think its a good idea and I still get advantages from it.

Also something for people to note with timings.
your latency @ 667mhz with timing of 5 is the same as @ 533 with 4 or 400 with 3. So there is not really an advantage from the lower timings unless you find a middle ground like @ 450mhz with 3 would be superior to any of the above.

But to everyone just test it all out change settings and benchmark and watch the changes and pic what works best for you.

 

I understand much of what you say but I have an AMD and for disscusions of FSB/RAM I have 1600Mhz so one reason why I disagree or am confused on your FSB comments. What is your response?

 

yeah im not sure with AMD at the moment I don't know how their stuff works. So I'll do some research and find out.

Like on intel stuff 800mhz means 200mhz quad pumped.

but I have no idea where the 1600mhz on a AMD chip comes from or if they are even comunicating the same way :s

 

In AMD land the connection between the RAM and the CPU is the speed of the CPU(Imagine having a 2.4ghz FSB). The reason why it's not faster than intel is because the RAM speed is limiting. They get the RAM speed from dividing the CPU speed to get the HT speed which is divided again.

Ok so we all know that using dividers causes slow downs because of the latency caused by different clock speeds. This performance problem isn't a huge problem for AMD because of the on-board mem controller. It takes nearly no hit.

The 1600mhz HT speed is just the rate the CPU communicates with the rest of the system.

 

I have posted some Sandra memory bandwidth results for 2 x 1GB, 1 x 2GB and 1GB + 2GB in this thread. These results are for the Intel 965GM chipset.

John