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

[Anonymous Coward]

Uh, you realize that with FBDIMMs, there is an unavoidable latency for serializing/deserializing between the memory controller and the DIMM? And that as cache lines get longer (Pentium M has 64-byte lines, PPC970 has 128-byte lines) the serialization will increase the pressure per FBDIMM lane. Then, in the effort to reduce the lane blocking for transferring large cache line reloads, they'll want to increase the lane frequency even more, which means you're going to have to end up getting rid of your FBDIMM for FBDIMM-2.

Besides, it's in the interests of the memory makers to make you throw away your old memory so that they can sell you new memory.

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.

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.

Why do we use DRAM in this day and age?

[Dancin_Santa]

SRAM is much faster, closer to the core of the CPU, and plentiful (if the chip manufacturers wanted it to be).

Who needs a gig of RAM when you can have a gig of cache?

If they need swap space, they can always write back out directly to a disk-based swap file.

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.

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

[Anonymous Coward]

SRAM is not a limited resource. The analogy is not correct.

As demand for gold increases, the cost of gold rises because of the scarcity of the element. However, for SRAM, the production of it can be ramped up to whatever level is necessary to meet demand.

Higher efficiency in the manufacturing of SRAM would lead to lower prices (though not necessarily lower than the current chip prices).

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.

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.

Re:Good marketing sense

[hackerjoe]

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.

Is this really true? I think you're smoking crack... HDTV and game consoles are consumer electronics, which is a market almost entirely driven by price. I don't know about HDTV, but I can't think of any way the popular consoles could be construed as including "premium" parts...

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.

I'm in the same category

[Anonymous Coward]

I went out of my way to NOT spec machines with Rambus memory, even when Intel was trying to (ahem) ram it down our throats. The bang for the buck just wasn't there at the time and then Rambus started extorting money from the industry. So that's probably several thousand machines that I either approved or influenced the buying decision for...hopefully, others in our shoes did the same.

Cheers,

Re:Rambus seems to forget

[GoMissedAtTheMAP]

If memory serves me , Rambus, while not an innocent bystander, was closer to being a victim of the large manufacturers conspiring to put them out of business. I did some quick research and apparently a judge just issued a summary judgement against Hyinx, stating that Hyinx infringed on 4 of Rambus' patents (total of 26 times). I agree that Rambus took advantage of some poor guidelines of JEDEC, but they do not deserve all the bad press. They don't manufacturer the memory, they patented the interface between memory and processor. It seems like most people on here view anything that is 'for profit' as evil, and that song is getting old.

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:Why does RAM suck so much?

[rrowv]

> 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.

You may be interested in FB-DIMMs, if they ever come out. Basically a standard (buffered) interface to all RAM you might want to put on there. Just make a new buffer chip and you're set.

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.

Latency

[dpilot]

There's another aspect of latency here that's being ignored. Here and elsewhere in this thread tree folks are talking about circuitry issues, like the memory controller, DRAM itself, DDR, etc. Those are all valid, but there's one more that's being neglected - wires, drivers, and receivers. By simply putting the DRAM somewhere away from the CPU/Northbridge, up on a DIMM socket, you take a big hit in latency. Even getting Zero-access DRAM wouldn't speed things up that much, because of the physical-related delays.

Oh, I agree with your abstraction comment.

Putting faster things into an FBDIMM just won't do that much, because the speed is physically in the same spot. I did an extensive study of this back prior to 1990 and found these results, and the consolidation of L2 and even Northbridge onto the CPU shows that it's still valid, today. Main memory is going to be slow. Main memory is always going to be slow, because that's a side effect of being "big". Main memory is always going to be "big" as long as the appetite for bits exceeds what can fit onto one chip. Learn to live with it.

Incidentally DRAM latency grows beyond minimum the moment you multiplex row and column addresses. There is a Trcd(max) spec where access is purely row-limited, but in practice that's just about impossible - access is almost always limited by Column access. Trade speed for pins.

Beyond that, even SDR traded off latench for bandwidth, compared to EDO. (I've designed both.) I don't think DDR is that bad a deal, compared with SDR, though I haven't actually done a DDR design, myself. At the very least, DDR offers the half-cycle latency options, and the DDR designs have been architected to scale far higher in frequency than SDR ever was.

*We* Should Not Let Them

[Bob9113]

We are a big section of the opinion makers in computer hardware. We have the ability to affect the public opinion on XDR. To a large extent we were the ones most adversely affected by the last round, and we are the ones who can shift public opinion now.

This should be like a usenet death penalty. The free market is there to reward those companies that serve their customers and punish those that do not. It is a good system, but it tends to have a short attention span. Tell your friends. Tell your purchasing deparment. Keep Rambus from coming back from the dead and send a message to other companies who think about abusing submarine patents. It's the same thing as harsh criminal sentencing, except that the free market has a far better track record of responding to example punishment (that is to say; if you support harsh criminal sentincing, you should support this on the same ideological grounds, and if you don't support harsh criminal sentencing because it doesn't work, you should still support this because it does).