Why run dual channel on 400Mhz FSB?

[TooBored]

This is actually 2 questions.

1) Why would you want dual channel RAM on any AMD board? The new boards are running 200Mhz front side bus pumped to 400. PC 3200 ram matches the bandwith to the proc exactly (8 byte lane X 400 = 3.2 GB/s). So what's the point of doubling your memory bandwidth, it just means the memory will have more bandwith than the FSB, making your FSB the bottleneck instead of the ram. I could see why it's important on an Intel board b/c they quad pump the FSB, which means if you run PC 3200 dual channel on an intel board w/800Mhz FSB, THEN your memory bandwidth catches up to the bandwidth of the FSB, but it just doesn't make sense for and AMD board, I wonder if it just comes standard with the new NVIDIA chipsets.

2)The dual channel AMD boards I have looked at all had 3 DIMMs, but the manuels warned against using all 3, and had clandestine requirements like "if all 3 dimm's are used, 1 must be single-sided RAM". First of all, I've never seen RAM identified as single or double sided in its descriptions (though I suppose a picture could tell you). Second, if you do manage to run all 3, does it kick the RAM out of Dual Channel (b/c you are now no longer running just 2 identical modules) This is important b/c I was wanting to put 1.5 GB of RAM into my new system, and this looks like it would be problematic with Dual-channel boards.

 

[Crashman]

why not get a 1GB and a 512MB chip?

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[TooBored]

If I did that, it wouldn't run dual channel. If there is no performance difference between dual or single channel, I'll just run a a non-dual channel board.

 

[etp777]

Memory bandiwdth tends to be more theoretical than FSB/CPU bandwidth, by how much, depends on the memory controller(or cpu, where this is integral). Because of said fact, memory that looks like it has same banwidth often has a lesser real world bandwidth than the cpu/fsb, which would be where dual channel could come in handy. Or in case of a quad pumped FSB, as you already said. This is only rough understnading here, not from experience, as I don't build AMD systems.

 

[Crashman]

No, nVidia chipsets allow you to run "dual channel" with different memory sizes. Really. That's because nVidia uses the two channels to stagger memory timings for decreased memory controller latency, they don't pair them up as 128-bit exactly like Intel does. Intel treats dual channel as 1 128-bit channel, while nVidia addresses the channels separately.

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[TooBored]

In other words, we have 2 technologies, both named dual-channel? I understand the Intel concept b/c thats what they did when RDRAM first came out, Intel treats 2 smaller sticks as 1 big stick. But if what you say about AMD's dual channel is true, then all the hubub about "dual channel must used matched pairs of EXACT memory" doesn't matter for AMD. Thanks Crashman, I'll have to go do some more research about dual-channel.

 

[SoDNighthawk]

Because it frees the AGP buss up. The new dual channel option prevents the graphics card from hoggin the AGP buss and slowing down the CPU cycles and the extra mamory in it's self improves system wide performance.

Call it a sudo hyperthreading or a Very much larger bandwidth pipe that prevents system bottle necks.

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[Crashman]

Correct, all the hubbub about matched modules is unimportant to nVidia, so long as both modules can support the speed/timing of the first. Saying "I don't need a dual channel chipset, so I won't buy one" also eliminates some very good boards like the Abit NF7-S version 2.0, even in single channel it's a top board.

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[TooBored]

Ok, so I tried to go find out more about dual channel, but entries like this: <http://www.vibecomputers.com/forum/viewtopic.cfm?tid=6513
worry me. If I go for 1GB in one of the channels and 512 i the other, then 1GB of the memory (512 from each module) will run dual-channel, and the other 512 from the gig stick will run single channel. Now admittedly, this would still be fast, but the cost from running 1GB + 1 512 vs the cost of 512 X 3 is prohibitive enough that I wouldn't go for the 1GB + 512MB unless I was looking at a discernible performance increase. Also, if matched pairs aren't important, how come there are still so many manufacturers selling them? Was there ever a reason to buy matched pairs, or is it just marketing?

 

[etp777]

Intel uses realdual channel, and this is where matched pairs are more important. Also, matched pairs tend to be higher quality ram, with bettertimings, so you don't end up with one fast stick and one slow one(Which of course results in both of them running at the slwoer speed)

 

[Crashman]

I thought you understood Intel uses matched pairs, so unless you don't think people buy P4's, I can't understand why you'd ask.

But there's another reason for matched pairs, in that nVidia's solution also stresses memory timings a bit more and can make the module puke, buying a matched pair gives you more assurance that you won't suffer a timing related system crash.

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[TooBored]

Well, I thought Intel dual-channel was only for their RDRAM boards, which HAD to be dual-channel b/c RDRAM had a narrow bus. If I understand what you're saying, then Intel is still using the same model for it's dual-channel DDR, i.e the controller treats 2 sticks as 1 big stick. The question I have now is that the Athalon 64 has an integrated memory controller that supports 1 channel, correct? I know the Athalon 64FX procs have integrated controller that supports 2 channels, but I'm speaking of the normal 64 right now. If the processor handles the memory controller, and it only handles 1 channel, how can they make dual-channel socket 754 boards? If it's as simple as just adding the extra controller onto the motherboard, than forgive my ignorance, but I thought it was more complicated. As for the memory timing issue, I plan to use brand name and not try to push the latencies, so I shouldn't be in trouble in that respect. Anyway, all these questions are because I think a good system can be made or broken by the Mobo, so want to know what I'm getting with my chipset.

 

[Crashman]

Intel's doing dual channel by making 2 64-bit DIMMs act as 1 128-bit DIMM. nVidia is doing dual channel on the nForce2 as 2 separate channels. AMD does their own thing, Opteron does dual channel like Intel, simply splitting a 128-bit bus into 2 64-bit paths. I don't know of any dual-channel Socket 754 boards, I'm fairly certain the basic A64 has a single channel memory controller (as opposed to the Opteron).

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[Crashman]

Intel offers dual channel on the 865 and 875 series chipsets, as well as some 7xxx series worstation/server chipsets like the old 7205.

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[TooBored]

Ok, I guess I was confused by the fact the the NForce2 chipset enabled dual channel for earlier Athlon procs. Now that I look, it seems like there may be no dual-channel mobos for Athlon 64. I was partially confused b/c when reading memory requirements, a lot of the boards have 3 dimm slots, 1 slot supports up to 1 GB, and the other 2 together can support up to 1GB. When I read this, I thought "Ahh, one channel for the 1st dimm slot, and one channel for the other 2 dimm slots" Now it just appears as if each slot can actually support up to 1 GB, but the Athlon can only address up to 2GB. Oh well, thanks for explaining, Crash. Now I can finally just evaluate Athlon64 mobos by chipset and features, and not worry about #$%# single or dual channel, since they're all single-channel.

 

[Crashman]

There is only 1 high end chipset for the A64, it's the nForce3 250. And there's only one top quality low end chipset, the SiS 755. Good luck finding boards for them right now, the nForce3 250 is new, and better boards for the 755 have been delayed indefinately.

But I know boards with the nForce3 250 are now in production, so if you look around you might find some...or some sites with an arival date in the near future.

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[ytoledano]

but the manuels warned against using all 3, and had clandestine requirements like "if all 3 dimm's are used, 1 must be single-sided RAM"

I think this has to do with the number of memory banks your board can handle. Usually, single sided is one bank and double sided is two (I've never heard otherwise). Check to see how many banks you mobo can handle.

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[TooBored]

Well, I was looking at the Chaintech ZNF3-150 Zenith here:
http://www.newegg.com/app/ViewProductDesc.asp?description=13-152-031&depa=0
or the Asus K8V SE here:
http://www.newegg.com/app/ViewProductDesc.asp?description=13-131-490&depa=0

This is basically for my New system that needs to be viable for the next 3-4 years, and I wanted it to be able to run Half-life 2 and Doom 3 and the like. But, with new chipsets in the works for both ATI and NVIDIA, it looks like it might be worthwhile to hold off a couple months. Of course, when building a comp, you can always get a better deal by holding off a few months, but I'd like to see how the new graphics cards hold up. These boards are still viable choices however, because unless I see about a 66% price drop, I'm not going to bend over for an Athlon 64FX.

 

[TooBored]

Oh, in response to vtoledano: yeah, I think the board supported 5 channels, but only 2 GB of ram. B/C with 5 channels, you COULD theoritically run 2.5GB (I've never seen a 1GB one-sided). So the board requires (the chaintech one I mentioned above) requires you to a)Only use 2 GB and b) use a 1-sided if you use all 3. Oh yeah, and in the fine print in the index it says "AMD recommends against using DIMM1 + DIMM2 + DIMM3". That was the bulls**t that would probably make me not buy this board.

 

[VernonNemitz]

Hello. I may have just encountered something that may be a little bit relevant to this discussion. I have been seeking an Athlon64 motherboard that supports 3GB of 400DDR. I wasn't interested in fancy stuff like dual-channel or even overclocking, but I did want the sheer capacity at the rated speed. Anyway, there are Gigabyte and Asus models that claim to support 3GB, but -- due to a THG article a few months back indicating problems when only 2 DIMMs were plugged in -- I decided to email the companies and find out if what they claimed was true. Between the replies, I gather that the VIA chipset can only handle 4 banks of DDR400, and 2GB of double-sided DIMMS were mentioned. There was also an implication that more than 4 banks of slower RAM could be handled. However, the size of a bank was not specified, and so I sent off more emails and am waiting for more information, because I'm almost certain that single-sided 1GB DIMMs have just recently started entering the market. Maybe those 3 DIMM slots can indeed take 3GB of DDR400 and the system will work correctly. But if anyone here has more information about this, I'd sure like to know! Thanks!