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After 50 years of effort, researchers made silicon emit light

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Re: After 50 years of effort, researchers made silicon emit light

#61

Post by Sheppy96 »

adwn wrote: post #57
Sheppy96 wrote: post #37

Earlier quoted context omitted.

Transmission time isn’t really the main issue, it’s more about the work required to get a memory request through the levels of the hierarchy to DRAM and back. Probing each level of cache, propagating through the miss queues, translation (maybe with TLB miss), waiting for the DRAM controller, etc.

What? That doesn't make sense. If cache probing would be the cause for DRAM accesses being slow, we wouldn't need caches. We would just access DRAM directly! It's the other way around: DRAM accesses are slow, that's why we need caches. > translation (maybe with TLB miss) In most architectures, the caches are physically addressed, so TLB lookups occur before even L1 cache access. Successful TLB lookups are extremely f…

I wasn’t suggesting probing caches is the main cost, I only wanted to describe that there is a long journey to DRAM in current architectures of which signal propagation is such a small part.

You are totally right that if you can make the resultant communication speed faster you could theoretically do away with caches. However this approach wouldn’t solve that problem on its own. Also forget not that cache is expensive and DRAM is cheap!

Yes I’m aware that caches can be physically addressed and you could reorder the sequence I described. No you can’t skip the TLB, but a hit will be faster since you don’t have to perform translation.

Re: After 50 years of effort, researchers made silicon emit light

#62

Post by gibybo »

dannypgh wrote: post #55
datenwolf wrote: post #50

Gaaah, please stop advertising optical computers as the technology that will overcome Moore's law. It makes no effing sense. Wavelength of the light emitted by these devices: ~4000nm Latest generation commodity CPU transistor structure size: 7nm Add to that that photons really don't like being trapped; you essentially need a delay line and optical amplifier to hold them indefinitely (that's essentially the core techn…

It's definitely not a continuation of Moore's law as it has nothing to do with transistor density, but it may mean that the performance people expect from computers - which is why people are usually talking about Moore's law - may continue increasing. I don't see how the wavelength is comparable to transistor size because as you switch to the optical realm, the benefit of information propagation at speeds near c (or…

I thought electric charge in conductors already moved very close to C? https://en.wikipedia.org/wiki/Speed_of_electricity

Re: After 50 years of effort, researchers made silicon emit light

#63

Post by [deleted] »

dannypgh wrote: post #55
datenwolf wrote: post #50

Gaaah, please stop advertising optical computers as the technology that will overcome Moore's law. It makes no effing sense. Wavelength of the light emitted by these devices: ~4000nm Latest generation commodity CPU transistor structure size: 7nm Add to that that photons really don't like being trapped; you essentially need a delay line and optical amplifier to hold them indefinitely (that's essentially the core techn…

It's definitely not a continuation of Moore's law as it has nothing to do with transistor density, but it may mean that the performance people expect from computers - which is why people are usually talking about Moore's law - may continue increasing. I don't see how the wavelength is comparable to transistor size because as you switch to the optical realm, the benefit of information propagation at speeds near c (or…

[deleted]

Re: After 50 years of effort, researchers made silicon emit light

#64

Post by mcnamaratw »

gibybo wrote: post #62
dannypgh wrote: post #55

Earlier quoted context omitted.

It's definitely not a continuation of Moore's law as it has nothing to do with transistor density, but it may mean that the performance people expect from computers - which is why people are usually talking about Moore's law - may continue increasing. I don't see how the wavelength is comparable to transistor size because as you switch to the optical realm, the benefit of information propagation at speeds near c (or…

I thought electric charge in conductors already moved very close to C? https://en.wikipedia.org/wiki/Speed_of_electricity

Exactly. If we're pedantic about it, a charge signal moves at the speed of light. But the signals are what computers use.

Re: After 50 years of effort, researchers made silicon emit light

#65

Post by whatshisface »

gibybo wrote: post #62
dannypgh wrote: post #55

Earlier quoted context omitted.

It's definitely not a continuation of Moore's law as it has nothing to do with transistor density, but it may mean that the performance people expect from computers - which is why people are usually talking about Moore's law - may continue increasing. I don't see how the wavelength is comparable to transistor size because as you switch to the optical realm, the benefit of information propagation at speeds near c (or…

I thought electric charge in conductors already moved very close to C? https://en.wikipedia.org/wiki/Speed_of_electricity

Only if you consider 70% or so to be close. There's some room for improvement over copper wires. Now, if there are any physicists here who want to jump in, I have a question about that. I heard waveguides are dispersive, would sending pulses of light through tiny channels slow it down as well?

Re: After 50 years of effort, researchers made silicon emit light

#66

Post by shsbsuss »

whatshisface wrote: post #65
gibybo wrote: post #62

Earlier quoted context omitted.

I thought electric charge in conductors already moved very close to C? https://en.wikipedia.org/wiki/Speed_of_electricity

Only if you consider 70% or so to be close. There's some room for improvement over copper wires. Now, if there are any physicists here who want to jump in, I have a question about that. I heard waveguides are dispersive, would sending pulses of light through tiny channels slow it down as well?

But light in a dielectric medium also travels slower.

If n=1.3 -> v = c/1.3 = 77% of c

Re: After 50 years of effort, researchers made silicon emit light

#67

Post by karkisuni »

dr_dshiv wrote: post #53
datenwolf wrote: post #50

Gaaah, please stop advertising optical computers as the technology that will overcome Moore's law. It makes no effing sense. Wavelength of the light emitted by these devices: ~4000nm Latest generation commodity CPU transistor structure size: 7nm Add to that that photons really don't like being trapped; you essentially need a delay line and optical amplifier to hold them indefinitely (that's essentially the core techn…

I like to think that comments like this make me smarter with 10x efficiency. In this case, the pattern is: 1. Identify and refute common conception 2. Compare easily understood numbers 3. Assert basis of knowledge 4. Introduce a clear, tangible model for alternative thinking 5. Appropriate use of emotional resonance to capture and maintain attention

Five Easy Steps to Becoming a 10x Commenter!

Re: After 50 years of effort, researchers made silicon emit light

#68

Post by pwildani »

MrBuddyCasino wrote: post #59
adwn wrote: post #57

Earlier quoted context omitted.

What? That doesn't make sense. If cache probing would be the cause for DRAM accesses being slow, we wouldn't need caches. We would just access DRAM directly! It's the other way around: DRAM accesses are slow, that's why we need caches. > translation (maybe with TLB miss) In most architectures, the caches are physically addressed, so TLB lookups occur before even L1 cache access. Successful TLB lookups are extremely f…

> In most architectures, the caches are physically addressed, so TLB lookups occur before even L1 cache access. So to see if a memory location is contained in a cache line, a TLB lookup is needed to first get the physical address? I wouldn't have expected this, can you expand on why this is the case?

If you use virtual addresses to index your caches, you have to clear them all on every process context switch.

With physical addresses, you just have to clear your TLB cache.

Re: After 50 years of effort, researchers made silicon emit light

#69

Post by dooglius »

MrBuddyCasino wrote: post #59
adwn wrote: post #57

Earlier quoted context omitted.

What? That doesn't make sense. If cache probing would be the cause for DRAM accesses being slow, we wouldn't need caches. We would just access DRAM directly! It's the other way around: DRAM accesses are slow, that's why we need caches. > translation (maybe with TLB miss) In most architectures, the caches are physically addressed, so TLB lookups occur before even L1 cache access. Successful TLB lookups are extremely f…

> In most architectures, the caches are physically addressed, so TLB lookups occur before even L1 cache access. So to see if a memory location is contained in a cache line, a TLB lookup is needed to first get the physical address? I wouldn't have expected this, can you expand on why this is the case?

See Linus Torvalds' thesis, section 4.3.3 "The case against virtual data caches": https://www.cs.helsinki.fi/u/kutvonen/index_files/linus.pdf

Re: After 50 years of effort, researchers made silicon emit light

#70

Post by kevin_thibedeau »

karkisuni wrote: post #67
dr_dshiv wrote: post #53

Earlier quoted context omitted.

I like to think that comments like this make me smarter with 10x efficiency. In this case, the pattern is: 1. Identify and refute common conception 2. Compare easily understood numbers 3. Assert basis of knowledge 4. Introduce a clear, tangible model for alternative thinking 5. Appropriate use of emotional resonance to capture and maintain attention

Five Easy Steps to Becoming a 10x Commenter!

There's no profit in taking five steps.
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