Slashdot Mirror
Rethinking Computer Design For an Optical World
holy_calamity writes "Technology Review looks at how some traditions of computer architecture are up for grabs with the arrival of optical interconnects like Intel's 50Gbps link unveiled last week. The extra speed makes it possible to consider moving a server's RAM a few feet from its CPUs to aid cooling and moving memory and computational power to peripherals like laptop docks and monitors."
So you can buy a new monitor again, and again and again. I bet this is what went through Steve Jobs' head when he they made macs hard to upgrade, that and a huge thunder of Ka-ching Ka-ching Ka-ching Ka-ching Ka-ching Ka-ching ...
because this appears to be another aspect of Wheel of Reincarnation.
I'm old enough to remember a time where a computer was a series of bitty boxes tied together with cables. Then someone decided to integrate a lot of the stuff onto a motherboard, with just loosely-related stuff connected by cables to the motherboard. Then the loosely-related stuff got put into cards that plugged into the motherboard. Then that stuff just got integrated into the motherboard.
And now it's being reborn as stuff in bitty boxes connected together with cables.
I wonder what enlightement will be like, because karma appears to have been a bitch.
Welcome to the Panopticon. Used to be a prison, now it's your home.
Yes and no. In a vacuum, electrons aren't terribly useful unless you're driving them with a particle accelerator. In wires, electrons aren't really doing the work anyway: electrical signals effectively travel as waves in the dielectric surrounding the wires and in particular between signal pairs. In that case, the signal travels at around half the speed of light in a vacuum (faster if you use expensive insulation like Teflon, slower for other plastics.)
Light in optical fiber is also slowed by the refraction coefficient of the material and by path-length extension in multimode fiber. However, on balance it's a bit faster.
The real gotcha is that electrical signals at outrageous bandwidths suffer from some really horrible losses due to both skin effects on the wires and dielectric losses in the insulation. At 50 Gb/s and 30 cm, you're doing well to detect the resulting signal, never mind decode it. Worse, the losses are highly frequency-dependent, so you have to do all sorts of ugly things to pre- and post-condition the signal to make it usable. Some of this can be overcome by cranking up the transmit power, but then you get into that property of wires known as "antenna." All of that processing at both ends takes time, too.
Just not worth doing, generally.
Likewise, putting a bunch of streams out in parallel requires all sorts of cleverness to put the separate lanes together again after transmission skew. A single optical stream is much easier to use, sort of like the communications equivalent of Amdahl's Law.
Lacking <sarcasm> tags,
Note that I am not a physicist, and not much of an electrical engineer. I may be entirely wrong.
I'm not qualified enough to say whether you're right or wrong, but you stated your case eloquently and if there's one thing that Hollywood, politics, and Star Trek have taught me, sounding right is more important than being right.
"Never let your sense of morals prevent you from doing what is right" - Salvor Hardin
Same goes for optical interconnect to memory: the flood may be Biblical when it arrives
But it won't be - the system is fundamentally limited by all of the rest of the components. A top end front-side bus can already push 80Gb; scaling that upto the 400Gbit that this optical link promises will probably be practical within a few years, but the latency of encoding and decoding a laser signal and pushing it over several meters is going to be a killer for computational applications. It will be great for USBX, and for high end networking it will challenge Infiniband (which currently tops out at around 300Gb). Infiniband is already used for networking high-performance computational clusters, but nobody is using it for the CPU to memory bus because of the high latency. Even with high bandwidth, computation still has to be carried out on the data, and so it still makes sense to put the data and processor as close together as possible.
In the last decade there were many research papers proposing that co-processors would be placed on DRAM cards, or Embedded DRAM would allow CPU and processors to be fabricated on a single die (e.g. 1, 2). But if you have a processor and DRAM connected to similar units via an optical interconnnect, guess what - the architecture begins to look awfully similar to a regular network with optical ethernet. So, it looks likely that this will be just another incremental improvement in architecture rather than the radical shift that TFA envisions.
Now admittedly electricity usually only travels at about 0.5c, IIRC, but I think she was giving the speed-of-light delay, not the speed-of-electrons delay.
Don't confuse propagation velocity of electromagnetic waves, which depends on dielectric constant and is around 0.8c in normal conductors, with drift velocity of electrons which is maybe a meter per hour.
http://en.wikipedia.org/wiki/Speed_of_electricity
http://en.wikipedia.org/wiki/Drift_velocity
http://en.wikipedia.org/wiki/Velocity_of_propagation
Electrons really move slowly in metal. In a vacuum tube like a CRT, pretty quick.
"Science flies us to the moon. Religion flies us into buildings." - Victor Stenger