Single channel vs. dual channel?

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Mariner
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Joined: Sat Oct 28, 2006 11:27

Single channel vs. dual channel?

Postby Mariner » Sun Feb 25, 2007 21:29

OK...

Nobody seems to approach this matter from this point of view, so here goes.
Let's assume we have a computer with a 800 MHz FSB.
We shall try tree different memory configurations, which are:

1. DDR2-800MHz 1GB memory in single channel.
2. DDR2-400MHz 2x512MB memory in dual channel.
3. DDR2-800MHz 2x512MB memory in dual channel.

Which one is faster? Please specify why.

Stick
Posts: 90
Joined: Mon Dec 19, 2005 16:12

Postby Stick » Tue Feb 27, 2007 11:10

Your question is to vague and I'll tell you why. Usually memory modules with lower speeds have better latencies than the ones with higher speeds, so applications which require a lot of memory I/O will run faster with those, even if the bandwidth is smaller. If you have applications that require bandwidth, the higher the memory speed, the better.

1. DDR2-800MHz 1GB memory in single channel.
- the available bandwidth in this case is ~6.4 GB/s - 64 bit
- it gives you the option of a later upgrade (laptops usually have only 2 memory slots, so you will not have to sell your old memory)
- probably higher latencies than DDR2-400

2. DDR2-400MHz 2x512MB memory in dual channel.
- the available bandwidth in this case is ~6.4 GB/s - 128 bit
- problems for future upgrades
- lower latencies

3. DDR2-800MHz 2x512MB memory in dual channel.
- the available bandwidth in this case is ~12.8 GB/s - 128 bit
- problems for future upgrades
- probably higher latencies than DDR2-400

If I would have to chose, it would be 1 x 1GB DDR2-800 for the easier upgrade in the future. Single channel has only about 5-10% speed loss compared to dual channel in real life application, even if on paper it should be half the speed of dual-channel config.

Hope it helps.
Last edited by Stick on Tue Feb 27, 2007 14:18, edited 1 time in total.
FJS Amilo M3438G, PM 760 (2,0 GHz), 2 x 1GB Kingston DDR2 533 MHz, 2x100 GB FJ S-ATA in RAID-0 config, GF Go6800 (3812 in 3dmark05)

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Mariner
Posts: 86
Joined: Sat Oct 28, 2006 11:27

Postby Mariner » Tue Feb 27, 2007 11:53

OK...

My point was, that the band width of the CPU front side bus was 6,4GB/s (800MHz).
So what is the great idea to exeed that with the memory and gain nothing because of the higher latencies?

Isn't the fastest alternative the nr. 2, where the bandwidth of the FSB and memory are the same (FSB: 800MHz and memory: 2x400MHz=800MHz).
The memory would be fastest in this case because of the lower latencies?

Or is there something I'm missing now???
Sorry, but I'm so lost with this...
Last edited by Mariner on Tue Feb 27, 2007 19:28, edited 1 time in total.

Stick
Posts: 90
Joined: Mon Dec 19, 2005 16:12

Postby Stick » Tue Feb 27, 2007 15:01

OK, made some corrections on the bandwidth for the memory (i forgot to multiply it by 2).
As for your other question, you got some of the things wrong. FSB max bandwidth depends on many things and it works totally different with Intel than with AMD.
For example the Core2Duo max FSB bandwidth is 8.5GB/s, which is equal with the max bandwith of double-channel DDR2-533. As you can see, the bandwidth is higher than the one of a single-channel DDR2-800. But remember one thing, max theoretical bandwidth on memory doesn't mean that the memory can hit that. If you want more info, read this nice article with comparisons on the C2D memory types:
http://www.xbitlabs.com/articles/memory ... guide.html
FJS Amilo M3438G, PM 760 (2,0 GHz), 2 x 1GB Kingston DDR2 533 MHz, 2x100 GB FJ S-ATA in RAID-0 config, GF Go6800 (3812 in 3dmark05)

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Mariner
Posts: 86
Joined: Sat Oct 28, 2006 11:27

Postby Mariner » Tue Feb 27, 2007 18:46

Thanks for the link!
I see now...


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