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."

Comments

Re:dumb monitor

[bsDaemon]

you mean like an imac? /ducks (disclaimer: typed from a 24" imac while at work)

Here we go again

[overshoot]

This is eerily reminiscent of Intel's flirtation with Rambus: they were so focused on bandwidth that they sacrificed latency to get it. Yeah, the Pentium4 series racked up impressive GHz numbers but the actual performance lagged because the insanely deep Rambus-optimized pipeline stalled all the time waiting for the first byte of a cache miss to arrive.

Same goes for optical interconnect to memory: the flood may be Biblical when it arrives, but while waiting for it to arrive the processor isn't doing anything useful.

Now, peripherals are another matter. But if bandwidth were all it took, we'd be using 10 Gb/s PCI Express for memory right now.

Re:Here we go again

[demonbug]

This is eerily reminiscent of Intel's flirtation with Rambus: they were so focused on bandwidth that they sacrificed latency to get it. Yeah, the Pentium4 series racked up impressive GHz numbers but the actual performance lagged because the insanely deep Rambus-optimized pipeline stalled all the time waiting for the first byte of a cache miss to arrive.

Same goes for optical interconnect to memory: the flood may be Biblical when it arrives, but while waiting for it to arrive the processor isn't doing anything useful.

Now, peripherals are another matter. But if bandwidth were all it took, we'd be using 10 Gb/s PCI Express for memory right now.

I was thinking the same thing regarding latency and remote memory. If you've got your memory 1 physical meter away, you're already looking at something like 6.6 ns round-trip latency (in a vacuum) just for light traveling that physical distance; seems like once you include switching plus getting to/from the optical interconnect you're looking at some pretty serious latency issues compared to onboard RAM (I think DDR3 SDRAM is on the order of 7-9 ns).

light speed lag leads to higher latency

[Chirs]

Without factoring in speed of light drops due to index of refraction changes, at a distance of 1 meter you're looking at latencies of 7 nanoseconds just for travel time. The bandwidth may be decent but the latency is going to be an issue for any significant distance.

The 1990s called.

[PPH]

They want their rats nest of cables back.

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.

Computer architecture must have the Bhudda-nature

[idontgno]

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.

Speed of whatever

[overshoot]

I don't think that light travels that much more quickly than electrons do.

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.

Re:a few extra feet

[Mordok-DestroyerOfWo]

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.

Re:a few extra feet

[smooth wombat]

Or, as our esteemed Professor Farnsworth remarked:

Yes, yes, anything with that many big words could easily be the solution.

Re:LightPeak

[The Master Control P]

I recommend reading the programmer's guide to a modern graphics architecture; Caching is essential to them.

Modern GPU architectures face the same clock speed/bus speed disparity and memory latency problems as CPUs and have taken their response much farther. They have several thousand registers per core and an L1 size & speed cache per processor group. Cache misses carry a typical penalty of several hundred cycles.