Is Rambus Destined to Return?

An anonymous reader pointed us to an article running over at Tom's that talks about the world of ram and criticizes the performance of DDR. The article goes into DDR333, DDR400, and Rambus, and explains the issues at higher clockspeeds.

Comments

Re:Experience tells us

[Anonymous Coward]

RDRAM is currently only about 20% more expensive than DDR and has been closing that gap pretty quickly recently. It's not really a price issue anymore. You may have had a point when RDRAM was 200% more expensive but that was a while ago.

As an small OEM computer maker, I hope not

[bravehamster]

Seriously, ignoring pure performance considerations, RDRAM is garbage. It has to be put in pairs, and if those pairs aren't made by the same manufacturer, I've seen motherboards refuse to boot. Heat is a serious issue, and I've burned one finger too many on those heat spreaders. I've also seen an analog cable coming from the cdrom get stuck between the RIMM's and melt to the heat spreader. And price is still an issue, although it's improved quite a bit recently.

Expensive + Has to run in pairs + Runs very hot == Useless to me.

Re:As an small OEM computer maker, I hope not

[Zathrus]

No, his points are valid.

Interleaved memory designs (interleaving on a slot basis rather than interleaving on the RAM stick itself) causes many issues. First off, you have to have more slots for equivalent upgradeability. And more slots requires you to have more layers on the motherboard due to increased number of traces (although, admittedly, RDRAM has vastly fewer traces than SDRAM even so). It also requires more real estate on the board, which isn't debateable. Second, you start running into timing issues more often with interleaving than standard memory clocking. Sure, as you say, it depends how robust your controller is. But, funny thing, RDRAM either has amazingly shitty controllers, or they're just vastly more prone to lockups when you have slightly differing speed memory.

As for heat - it's not a tradeoff issue. DDR didn't double the heat of standard SDRAM, and RDRAM isn't merely twice as hot as DDR. It's absurdly hot. And heat is a major computer issue already between CPUs, chipsets, and graphics cards throwing off oodles of heat as is. I don't know of a manufacturer that has a fan blowing specifically over the RAM, but RDRAM could certainly benefit from this. Heat kills systems (more specifically, thermal changes kill systems, but you'll get faster thermal changes with hotter components), so why design a system with RDRAM that is so much hotter than the alternatives? For how little (if any) of a performance gain?

Oh, and you claim RDRAM is twice the speed. Ok. Want to compare apples to apples? Put RDRAM in a non-interleaved system (yes, they're out there. They're even predominant) and the memory bandwidth is only slightly higher than DDR. Or compare it to an interleaved DDR system (again, they're out there). Boom. You have a DDR system with nearly as much bandwidth as RDRAM.

And, frankly, bandwidth ain't all it's cracked up to be. Funny how DDR systems routinely spank RDRAM systems in real world benchmarks (not pure memory bench's). Why? Because latency is king. Particularly if you're multitasking. You'll hit different areas of memory so much that bandwidth will make little difference compared to latency. And RDRAM has really, really miserable latency. And it gets higher as you add more sticks. So while it's great for some things (video editing/streaming, etc), it sucks for most applications.

Re:Rambus as a company

[Dudio]

The article you reference talks about RDRAM performance with the P3. The P4 presents an entirely different picture, as it fully utilizes the bandwidth of the dual-channel RDRAM architecture. See this article for details. A brief quote from the conclusion:
The memory benchmarks from above show that Pentium 4 really requires the 3,200 MB/s of data bandwidth supplied by the two Rambus channels. I doubt that it will perform as well with DDR-SDRAM, unless two channels will be used.

The business practices, of course, are a different story.

Tom, as usual, not 100%

[Chris Burke]

It wasn't a bad article... I mean, the facts -do- show that the p4 runs better with RDRAM, and he addresses the consequences of that quite well, and quite neutrally. For that I commend him.

But he does misrepresent some issues. For example, signal integrity issues. I can say with complete assurance that Rambus is loaded with signal integrity issues. These issues get -very bad- as the clock frequency goes up. Also Rambus is -not-, strictly speaking, a serial bus. First, it is 16 bits wide, while pure serial would be 1. Second, the depiction of a DIMM as being a unterminated stub with significant SI issues is correct, but this doesn't go away with rambus, and this definition of "serial" fails as well. While the signals do pass through a RIMM continuously, eliminating the RIMM itself as the source of major SI problems, you still have each and every RDRAM device itself acting as an unterminated stub, each of which causes reflections of its own. Especially for devices with tolerances as low as RDRAM, this can be difficult to manage. While in the balance I'd have to concede that at a given clock frequency RDRAM has the SI advantage, remember that RDRAM needs 4x the clock frequency of DDR to match bandwidth.

Or you could have 2 channels of rambus, and only need 2x the frequency. Well, 2 channel DDR is becoming a reality. Not only does nForce support it, Sledgehammer will as well. Neither of these are Intel platforms, but I would guess that going dual-channel would be a natural step for VIA and others competing with Intel chipsets. It would especially make sense for p4, as it would more than make up the memory bandwidth disparity that currently exists.

Speaking of nForce, another thing I take issue with is the suggestion that the nforce's DIMM-slot population problems are indicative that DDR is crippled by SI issues. I think more likely is that this was the first chipset designed by a company whose experience lies solely with graphics cards, on which the ram is directly soddered to the PCB. Lack of experience in the harsher SI conditions of a computer motherboard are to blame.

Speaking of DIMM population, it's hard for me to see only having 2 DIMM's on some boards as a particularly black mark for DDR... That leaves you with 2GB per channel, the same as RAMBUS.

So, he was right about some things, insightful on others, but the picture is -not- so clear-cut in the image of rambus Inc.

Re:Tom, as usual, not 100%

[Perdo]

The Column was done by Frank Völkel. Based apon his lack of technical documentation, I'm going to guess it's just his opinion. I doubt very seriously that Tom Pabst himself agrees with the article. Tom tends to be much more objective in his articles than Frank is.

Tom contradicts himself?

[DrMrLordX]

Instead of focusing upon some benchmarks with overclocked CPUs squaring off against one another, why not pay attention to another article from Tom's that shows some benchmarks of the P4 2.0 ghz operating at factory spec on a number of different P4 boards with different memory configs(P4 + i850 + RDRAM vs P4 + Sis645/ViaP4X266/ViaP4X266a/i845 + DDR). As you can see from that article, at least on the pre-Northwood P4s, DDR did pretty well on the non-Intel chipsets, particularly on the Sis645 when PC2700 was used.

Re:Experience tells us

[Paladin128]

DDR is still faster than Rambus. PC1600 DDR SDRAM (100mhz DDR=200mhz effective), offers 1.6GB/sec of memory bandwidth, the same as PC800 RDRAM. Why does Rambus perform better on the P4?

The i850 chipset has a dual-channel RDRAM controller. It handles two channels of PC800 RDRAM, offering twice the memory bandwidth of PC1600, and about 25% greater than PC2100.

Conversely, the newer i845 DDR SDRAM memory controller offers single-channel support for PC2100.

If a dual-channel DDR motherboard was available for the P4, it would smoke the Rambus performance. Period. The article at Tom's stated that at the higher speeds DDR must have a relatively high CAS latency (2.5). This is still FAR lower than the latency in RDRAM. RDRAM is high-bandwidth, high latency. DDR is high-bandwidth, low latency.

I've been really disappointed with most articles at Tom's of late.