Rambus Takes Another Shot At High-End Memory

An anonymous reader writes "Tom's Hardware is running an article about Extreme Data Rate memory (XDR DRAM for short), which was developed by Rambus and now entered mass production in Samsung's fabs. Right now, Rambus says the memory is only for high-bandwidth multimedia applications such as Sony's Cell processor, but the company ultimately hopes to push XDR into PCs and graphics cards by 2006. Time will tell if Rambus has learned from the mistakes it made with RDRAM a few years ago."

Comments

It will be awhile

[I_am_Rambi]

before AMD might even thinking about accepting it. Since AMD now puts the memory controller on chip, AMD will have to see proff that it is faster. AMD will not go for DDR until it gets faster. Their reasoning, DDR2 adds cost and decreases performance. Without help from AMD, Rambus might be heading down the same track.

Re:It will be awhile

[MBCook]

All the more reason to move to FBDIMMs. AMD would put one memory controller on their chips, and it would work with SD, DDR, DDR2, Rambus, XDR, or anything else someone wants to put on. Makes things easy. Becuase the physical interface is constant and buffered, you don't get the problems of needing a different socket for every kind of RAM out there.

Unfortunatly, no one seems to be pushing for this despite the headaches it would remove. All you'd have to do is make your memory controller able to recieve faster (like going from DDR333 to DDR400). Plus, with the memory not directly connected, memory makers would not only compete evenly (since the user wouldn't need to know the difference between DDR2 and XDR except speed and price), but they could add other things like an extra cache level in front of the memory just by replacing RAM. And it would mean that the computer you bought today would take the memory that was available 3 years from now. Right now SDRAM costs a FORTUNE. But if you had a computer that takes FBDIMMs, instead of paying $50 a stick for 256mb sticks, you could buy at the price of DDR today (say 512mb for $25 or whatever it is today).

Just think, you wouldn't need to buy new types of RAM for your PC every 2 years.

Re:It will be awhile

[MBCook]

One other thing I forgot. With FBDIMMs it would be easy to replace your DRAM with SRAM (if prices dropped enough) because the refresh circuitry is on DIMM. That means one less thing that the memory controller has to do, which means less complexity and less silicon (not that the refresh logic takes up a huge ammount, but every little bit). When magnetic RAM comes along, you wouldn't need yet another memory controller.

And (since I think it's serial, instead of parallel like current RAM) it would SERIOUSLY decrease the pincounts of the Opteron and northbridges. Think if you could have quad channel memory in your desktop as an option. Right now the CPU would need THOUSANDS of pins to do that. But you might be able to do it with the current 939 pins on an Opteron if you used FBDIMMs.

Ah, dreams.

Re:It will be awhile

[joib]

IMHO, FBDIMM is just Intel's hedge against RAMBUS going bust. The point of RAMBUS was to reduce pincount per chip by reducing the width of the channel between the memory controller and the chip, and to decouple the notion of a memory controller controlling specific banks of memory. FBDIMMs are to solve the same problem, except RAMBUS is shipping already.


Reducing pincount is one important reason for FB-DIMM, but the real reason for it is to get out of the capacity/speed tradeoff game. See, many systems need lots of memory. However, with current DDR-400 or DDR2-667 you can only put two devices per channel. If you want more RAM than what fits in two devices, you have to reduce the speed. FB-DIMM gets around this problem by using point-to-point links between the devices.

Yes, this increases latency a little bit, but there really isn't any other practical way to increase speed without reducing capacity. However, FB-DIMM compensates for the increased latency by allowing many outstanding transactions on each channel; because of this, latency under high load is actually supposed to be lower than for traditional RAM tech with the same specs.

Re:It will be awhile

[Moraelin]

One reason the AMD 64 works so well is precisely because they _reduced_ latency. That's basically the great advantage that the IMC (Integrated Memory Controller) offers.

Funny abstraction layers and everything being agnostic of everything else is a nice CS theoretician fantasy. In a CS theory utopia everything should be abstracted, or better yet virtualized. Any actual hardware or other implementation details should be buried 6 ft deep, under layers after layers of abstraction or better yet emulation.

The problem is that reality doesn't work that way. Every such abstraction layer, such as buffering and translating some generic RAM interface costs time. Every single detail you play agnostic about, runs you the risk of doing something extremely stupid and slow. (E.g., from another domain: I've seen entirely too many program implementations that, in the quest to abstract and ignore the database, end up with a flurry of connections just to save one stupid record.) Performance problems here we come.

The AMD 64 runs fast precisely because it has one _less_ level of abstraction and virtualization. Precisely because their CPU does _not_ play agnostic and let the north-bridge handle the actual RAM details. No, they know all about RAM, and they use it better that way.

So adding an abstraction layer right back (even if one that moves the north-bridge on the RAM stick) would solve... what? Shave some 10% out of the performance? No, thanks.

Or you mention SRAM. Well, the only advantage to SRAM is that it's faster than DRAM. Adding an extra couple of cycles of latency to it would be just a bloody stupid way to get DRAM performance out of expensive SRAM. Over-priced under-performing solutions, here we come.

Wouldn't it be easier to just stick to DRAM _without_ extra abstraction layers to start with? You know, instead of then having to pay a mint for SRAM just to get back to where you started?

Not meant as a flame. Just a quick reflection on how the real world is that-a-way, and utopias with a dozen abstraction layers are in the exact opposite direction.

Never mind

[LittleLebowskiUrbanA]

if they plan on charging exorbitant prices for their memory again. I inherited a network full of fairly fast (2ghz) Dell boxes using RAMBUS. Sure is fun spending about $300 for a 512 upgrade. Of course you can only install this crap in pairs so there goes your slots.... Junk.. Rather buy a cheap new box than a memory upgrade using this overpriced crap.

Good marketing sense

[gbulmash]

Smart plan not to try to make it main RAM. By going after multimedia applications like HDTV, video games, etc. they're targeting a market historically willing to pay a premium to get the best performance. I'll be really interested to see the graphic cards based on it and how they compare with the alternatives.

Pathetic attempt at FPing

[tibike77]

Well, looks like they haven't learned much from their old mistakes, but are trying to avoid the consequences... smart move targetting heavy bandwidth apps for now.

In the long run, if they can't significantly drop manufacture prices to (let's say) 150% or even 200% of "regular" (by that date) RAM, the boost in speed a computer with "XDR DRAM" will get compared to (again, let's say) "PC800 RDRAM" will be not significant... and I'll bet (regular) people would rather choose 8 GB of "PC800 RDRAM" over 2 GB of "XDR DRAM" any time of the day.

Bottom line: they're either stuck with "speciality hardware" (like graphic cards or high-end servers) or they have to drop (manufacture) prices rapidly if they want to keep selling.

Re:Why do we use DRAM in this day and age?

[tomstdenis]

...SRAM is much more expensive to produce? It also takes more power and generates more heat.

That and the benefits of cache go DOWN as the size of the cache goes up. Past a MB or two the benefits would be lowered. Also as the # of address lines goes up the access gets slower. And finally a bigger bottle neck is that "external memory" is external.

So unless you want to pay for a cpu with a GB of onboard "memory" in the form of SRAM.... the benefits won't be that high.

Tom

Re:Why do we use DRAM in this day and age?

[be-fan]

Because SRAM takes up 6 transistors per bit, while DRAM takes up 1 transistor per bit. The biggest mainstream CPUs run about ~150m transistors, and that's only enough (if everything were cache), about 3MB.

latency?

[tomstdenis]

8GB/sec is good but not if the latency is higher than DDR.

People seem to forget that the "Random" part of RAM is kinda crucial.

Tom

Re:latency?

[be-fan]

Not necessarily. It depends on the application. In "streaming" applications (hint: 3D rendering like on a graphics card!) the latency doesn't matter nearly as much as bandwidth.

Rambus seems to forget

[onyxruby]

Rambus seems to forget their attempt to shanghai the entire memory business through fraud a few years ago. Perhaps they should be reminded that the IT community has not. They should sell their IP and disolve themselves to avoid losing their stockholders any more money.

I have adamantly refused to purchase any system that would use their memory for years, and more to the point have made that decision for others that depend on me making that decision. That's a lot of computers over the years were talking about. I am also far from alone.

Re:Why do we use DRAM in this day and age?

[Jeff DeMaagd]

That post still reflects a lack of understanding in the technology.

For a given area of silicon, you could have 1 gigabit of DRAM or 128 Megabit of SRAM. Is it worth that trade-off? One can make more chips, but making chips uses a lot of expensive and toxic chemicals, and fab time isn't free either.

Time Will Tell?

[cacepi]

Time will tell if Rambus has learned from the mistakes it made with RDRAM a few years ago.

Well, Rambus has expanded their latest lawsuit blitz to include DDR2 patent claims, so do you think they've learned?

Why does RAM suck so much?

[digitalgimpus]

1. Fast RAM is still expensive.

2. RAN changes to quick. I buy RAM for one computer, it's only for that computer. No portability.

I get a hard drive, I can put that in my new system. I get a new mouse, can use that on my new system. Display? Yep. Graphics card? Most likely.

RAM? Not likely.

IMHO they need to standardize RAM like AGP or PCI-X. That way users feel more comfortable investing in it... you can upgrade and keep your RAM.

Re:The numbers don't lie!

[forkazoo]

That would be an unusual special case. First off, most (non realtime) 3D rendering isn't terribly bandwidth or latency sensitive. Assuming the CPU is fast enough that it isn't the main bottleneck, such apps will tend to be more sensitive to latency than to bandwidth. When tracing a ray, for example, one may need to access data from all over memory to do hit-testing, but not need very much information in total. So, the relatively poor latency characteristics of RDRAM don't really suggest a keen funtansticness for 3D rendering. And, considering that current single channel DDR400 has as much bandwidth as dual channel RDRAM did... Well, I'm just surprised that your app would have such a benefit. I'd suspect that there were other differences that caused such a difference in your benchmarks. Do you have any more specifc information, such as what app you use, what sort of scene it was, and what the test systems were?

If you were dealing with slightly different steppings of the same CPU (I assume a P4?) it would be possible that you had two CPU's of the same clock speed, but the newer stepping was less efficient per clock. The P4's, over time, have been tweaked to be less and less efficient over time, in order to facilitate higher clock speeds. RDRAM was popular with the very first generation of P4's, so it'd be logical that the benchmark you saw may have been a newer core. That shouldn't explain a 20% speed difference, but it's an example of a small thing that may have contributed to making the memory system appear to be the determinant item in performance.

Re:The numbers don't lie!

[captaineo]

The test case was intensive ray tracing with Pixar's RenderMan on two systems:

3.06 GHz Pentium 4, 512KB cache, 533MHz FSB, RDRAM
3.00 GHz Pentium 4, 1MB cache, 800MHz FSB, DDR400 RAM

The DDR system is only 86% as fast as the RDRAM system (the RDRAM system is 16% faster). This is despite the DDR system having been purchased almost two years later, and having more cache!

The DDR system does pull ahead for compositing tasks (by quite a bit - in some cases it's twice as fast). I assume this is due to the larger cache.

But ray tracing takes about 90% of my total render times, so it's far more important to optimize. I am disappointed that I can't buy hardware today with the same RAM performance as I got two years ago.