Quad Channel DDR

[JPH1121]

What is preventing either AMD or Intel from doing this? I know that dual channel offers significant performance gains over single channel, so why have htey not stepped up to quad channel to get that much more performance? Doing so, particularly with an AMD processor should net some huge performance gains right?

Could quad channel DDR2 potentially make the performance difference between the upcoming Intel Core processors and AM2 (considering that AM2 is ONLY an advancement to DDR2 memory and excludes any performance benefit)

Thanks!

 

[Bassyhead]

It depends on how much the CPU's front side bus can handle. Theoretically, the bandwidth of the FSB is a function of the clock rate, multiplication (single, double, quadruple data rate, etc) and the bus width. Too much RAM bandwidth isn't very productive if the CPU can't accept it. I remember when I got my nForce 420 board (the original, that is). The chipset featured a dual channel memory controller, however the Athlon XP's bus could barely theoretically match the bandwidth of a single channel of PC2100, so the dual channel controller was mostly a moot point only increasing bandwidth 5-10%. (side note: the dual channel controller was mainly to increase performance of the onboard GeForce 2 graphics adapter).

 

[RaynorWolfcastle]

I'm pretty sure it has to do with the complexity of routing a PCB with 4 channels of DDR. I'm not sure how many of the pins on a memory stick are data lines, but I'm thinking that maintaining reasonable signal skew would be nearly impossible, it would also be necessary to add layers to the board (adds cost).

 

[BassBomb]

definatley would add number of pins to a cpu ALOT, pcb would be harder to manufacture with so many lines as stated

 

[BrownTown]

As has been stated above, you need way mroe traces, and ther aint anywhere on the motherboard to put them. Also, no current CPUs can use 4 channels of bandwidth to any real effect. However, when we get 4 and 8 core CPUs they will need that bandwith, so either we get faster RAM, or more channels. I'm not sure how fast DDR3 is supposed to be, but that will probably up the bandwidth alot.

 

[imported_Tick]

Originally posted by: BrownTown
As has been stated above, you need way mroe traces, and ther aint anywhere on the motherboard to put them. Also, no current CPUs can use 4 channels of bandwidth to any real effect. However, when we get 4 and 8 core CPUs they will need that bandwith, so either we get faster RAM, or more channels. I'm not sure how fast DDR3 is supposed to be, but that will probably up the bandwidth alot.

Dual chanel DDR3 would provide way more than enough bandwidth for a Quad-Core. DDR3 is fast enough to allow graphics cards to process thousands of pixels in a sixtieth of a second.

 

[BrownTown]

DDR3 has nothing to do with graphics cards. GDDR3 and DDR3 are not related in any way except that they have the number 3 in them. To my understanding GDDR3 is closest to DDR2. The high speeds are due to the fact that hte RAM is phyiscally closer to the GPU.

EDIT, also GPUs have to calculate millions of pixels in 1/60 seconds to have a good framerate, not just thousands

 

[BassBomb]

what happend to serial memory?

 

[RaynorWolfcastle]

Originally posted by: BassBomb
what happend to serial memory?

It still exists, Rambus' XDR DRAM that will be used in the PS3 is serial. Rambus has all kinds of patents surrounding XDR DRAM and serial RAM technology and they've also proven that they're litigation-happy, so between royalties and the threat of litigation, companies aren't all that keen on working with them.

As an aside, FB-DIMM technology basically puts an IC on the stick of RAM, this IC serializes/deserializes signals among other things, so the net effect is very close to using true serial RAM. In addition, it reduces the number of data lines necessary between the northbridge and the RAM.

 

[imported_Tick]

Originally posted by: BrownTown
DDR3 has nothing to do with graphics cards. GDDR3 and DDR3 are not related in any way except that they have the number 3 in them. To my understanding GDDR3 is closest to DDR2. The high speeds are due to the fact that hte RAM is phyiscally closer to the GPU.

EDIT, also GPUs have to calculate millions of pixels in 1/60 seconds to have a good framerate, not just thousands

DDR3 and GDDR3 are very close technically. I'll agree GDDR3 is a hybrid between DDR2 and DDR3 thought.

 

[BrownTown]

it was my understanding that GDDR3 was clsoer to DDR2, but really it is its own thing seperate from both DDR2 and DDR3, designed for graphics applciations.

Also, I was looking at the Blackford chipset and the slides for it seem to show 4 FB-DIMM channels. This is the one with Dual Indenependant FSB (DIB), so it makes sense that you could increase the number of channels. Also, FB-DIMMs use less traces than current techs, so it would be easier to fit 4 channels onto one motherboard.

Not sure when that comes out though, think it was supposed to already be out, but was delayed because Intels partners couldn't get the FB-DIMMS working quick enough. (Blackford will initially use Preslers, then Woodcrest)

 

[fsardis]

whatever happened to QDR (quadruple data rate) RAM? about 2 years ago i remember they were talking about it.

 

[BrownTown]

its called DDR2

 

[fsardis]

i suppose you are right since DDR2 is in essence QDR. But the name they gave it back then never made it to the public...
i think QDR sounds better than the stereotypical DDR2.

 

[IntelUser2000]

What is preventing either AMD or Intel from doing this? I know that dual channel offers significant performance gains over single channel, so why have htey not stepped up to quad channel to get that much more performance?

Easy answer. Imagine how much bigger the motherboard have to be when 4 DIMMs have to be used at once. With Dual Channel nowadays you can upgrade later since it has 2 additional slots. With quad channel you have to expand to 8 slots. That makes the board big. And Quad Channel is A LOT of traces.

If you say got 4x256MB to get 1GB, to upgrade to 2GB you have to buy 4x512MB. Which is very inconvenient FOR YOU(and us).

 

[Peter]

Originally posted by: fsardis
i suppose you are right since DDR2 is in essence QDR.

No it's not.

QDR would have an _external_ data rate of four bits per clock - as seen on the Pentium-4 front side bus for example.

DDR2 has an _external_ data rate of two bits per clock just like DDR. The difference is, DDR2 achieves that from a four-wide clock-halved (!) core, while DDR has a two-wide nominal-clock core.

DDR2 at the same clock speed is no faster than DDR, it just has more clock headroom thanks to the internal clock-halving.

QDR would have achieved twice the data rate of DDR.


Besides, we already have quad DDR channels. Just buy a dual opteron board, and there you go. Twin DDR channels per CPU makes four.

Four channels on a single CPU or chipset? Trace routing hell. Pin count nightmare.

 

[BrownTown]

well, the ICs are quad pumped, so in a way that is quad data rate, although of course the external signaling rate is twice the IC rate so it is only double pumped. But it DID allow us to get twice the bandwidth as DDR, DDR400 -> DDR2-800. If you really want to increase bandwidth though its probably best to have the RAM physically on the PCB next to the core like in graphics cards. That was the trace lengths are alot shorter and you go ramp up the clock speed.

 

[Peter]

Like I said, it's about frequency headroom, and no, DDR2 is exactly NOT quad pumped. Four cells IN PARALLEL at 1/2 clock it is.

RAM down on the PCB is what we have on graphics cards, and it allows massive speeds there - at the expense of flexibility and upgradability.