5 ms·

The problem with putting RAM at the end of a cable is that RAM latency is highly dependent on distance from the CPU. Assuming RAM is 2 inches away from the cpu

The problem with putting RAM at the end of a cable is that RAM latency is highly dependent on distance from the CPU. Assuming RAM is 2 inches away from the cpu, and assuming the CPU is operating at 3 ghz, you are looking at 3 billion round trips per second over the distance of 2 inches, or a grand total of 12 billion inches of wire being crossed per second. Electric signals are limited by the speed of light, though. In a vacuum, light travels at 299,792,458 meters per second, or 11,802,852,677 (11 billion) inches per second. Do you see the bottleneck?

mechagodzilla 6y ago
That's not really how CPUs / RAM have worked since the 1980s. The CPU doesn't operate synchronously out of RAM, fetching data on each clock cycle. It operates from a hierarchical series of on-die caches, with only the L1 cache (typically a few 10s of kilobytes at most) operating close to how you describe. Logic on the CPU will send a series of commands to the RAM (along with data if it's a write), and there will be many commands 'in-flight' around the system at any given time, to hide the relatively high latencies involved (many 10s of nanoseconds). The actual propagation delay (around ~1 nanosecond/foot for cables) barely figures into it. Even the cables can have many 'bits' in flight at a time on them, and typically each 'direction' of the link forwards a clock along with the data (or it's embedded in the data stream) to avoid messy synchronization issues.
peter_d_sherman 6y ago
I do see the bottleneck! But, short of having a "how to subvert the speed of light" discussion, which would be exceedingly long (and probably unproductive!) -- you've given me an idea! Yes, you are correct that the speed of electrical propagation is limited, and does limit things! We're completely agreed there! OK, so here's my idea! F--- the electrical cable entirely! So, we have Quantum computers, Quantum bits, and all sorts of research on how to compute things with them, all kinds of professors and papers and companies and venture capital all researching how to do Quantum Computation... Well, I say F--- all of that! (In a polite, friendly, "yes your work is really genius and I acknowledge all of your brilliant contributions kind of way!"). Here's the grail! The grail of Quantum is NOT Quantum Computation -- but rather Quantum Communication! In other words, get rid of the damn electrical cable, and figure out a way to do that communication with a pair of connected-at-a-distance Quantum bits; that is -- Quantum Communication! In other words, Quantum Communication -- should replace wires and busses of all sorts, serial, parallel, electrical, light, what-have-you. That's the damn grail!... (pardon my language!) Not Quantum Computing!!! (brilliant though it is, I am not trying to knock it in any way, far from, quite the opposite!) Quantum Communication -- replacing wires and busses of all sorts, serial, parallel, electrical, light -- would be the Holy Grail -- of being able to build bigger computers (like imagine close-to-infinite RAM -- on one motherboard! Close-to-infinite RAM! Well, it would be off that motherboard... but Quantum Communication, if it were available, would, for all intensive purposes, make that RAM so connected, would have the net effect -- of physically existing on the motherboard, even though in actuality it would not be so!) So, to answer your question... YES, I DO SEE THE BOTTLENECK! But... equal-and-oppositely... Do you see my point?
- mrguyorama 6y ago
  You cannot communicate over a distance faster than the speed of light even using quantum phenomenon.
  - peter_d_sherman 6y ago
    About 120+ years ago or so, if you would have said to the common man that it was possible for a heavier-than-air aircraft to fly, they would have said that that feat of engineering was physically impossible... https://en.wikipedia.org/wiki/Wright_brothers https://en.wikipedia.org/wiki/Wright_brothers Today, after 120 years or so (roughly) of heavier-than-air flight, if you were to say that heavier-than-air aircraft flight was possible, no one would even do so much as "bat an eye" at that statement... I had an old Physics Teacher who once (very wisely!) said: "Given time, the impossible becomes possible, the possible becomes probable, and the probable becomes damn-near-likely to occur!" It was like that for heavier-than-air flight. I think that future history will show your statement to be incorrect at some point -- even though, in fairness to you, your statement might very well be true today, tomorrow, and in the immediate, near-term future... "Given time, the impossible becomes possible, the possible becomes probable, and the probable becomes damn-near-likely to occur..." Also, as a general rule, if something is apparently "not possible" (which yours truly highly doubts) -- then anyone who's worth their salt as a scientist needs to ask WHY multiple times -- and to keep "tunneling downward" across multiple levels, multiple substrata of WHY... See, reach a certain level of WHY (which is really a cause, which sets of the whole cause-effect-cause-effect-etc. chain), understand it -- and you can control all of the effects which proceed out of that initial cause... If we consider waves, for example, those which could occur in a rope or steel cable tied between two posts (compare to radio transmitter and receiver, even though there is no physical medium between them), but if we consider a rope or steel cable, and the waves which can travel along that medium -- they will travel at a certain speed, relative to wavelength (incidentally, all of the "can't go faster than light speed" stuff -- is tied to wavelength, and wavelength equations of hertzian waves -- that's the WHY). But, there's something else going on in that rope or steel cable. And that is, the instantaneous tension of a part of the rope or steel cable -- when a certain waveform is applied, for example, when someone starts vibrating or oscillating that rope or steel cable... See, the waveform propagates at one speed relative to the rate of oscillation. That is one form of propagation. Waves of that nature -- will have a certain speed relative to their wavelength (size). But if the rope or steel cable is in tension between two points, two poles or what have you -- it will propagate changes in that tension near instantly -- much faster than the waveform formed by the aforementioned oscillation or vibration! The same principle is at play with railroad tracks. That is, a train moves at one speed, one velocity, but if you're far ahead of the train, if you put your ear to the train tracks (and I highly highly recommend that no one do this -- this is said for explanatory purposes only!), but if someone were to put their ear to the track of a train which was some distance away -- they would hear the vibrations propagated in the train track at a much higher speed, a much higher velocity, than the speed of the train! Well, same thing with light speed in this case. Light is the train, the medium in which light travels through is the train track. The train track is in tension, and by increasing vibrations as the train gets closer, it "knows" that a train is approaching! The vibrations in the train tracks (train tracks have extremely high tension!) -- move at a faster rate of speed than the oncoming train! So, while you might be right in your assessment in the short term, that Quantum Communication is impossible, I respectfully disagree as to what the long-term prospects and possibilities in this area are... You might be right today... but for how much longer will that truth hold?
corebuffer 6y ago
honest question: what about throughput? I see the issue on latency, but suppose this system is being consumed by a large GPU or a traditional out of order CPU or DMA mechanism. If you can start reads and writes over a high throughput pipe on many chips, can't you also get higher throughput to keep multiple parallel operations busy while requests are in flight?
rasz 6y ago
No, I dont. Its ~30cm/ns, Fastest RAM available _ever_ never went below 7ns.

mechagodzilla 6y ago

That's not really how CPUs / RAM have worked since the 1980s. The CPU doesn't operate synchronously out of RAM, fetching data on each clock cycle. It operates from a hierarchical series of on-die caches, with only the L1 cache (typically a few 10s of kilobytes at most) operating close to how you describe. Logic on the CPU will send a series of commands to the RAM (along with data if it's a write), and there will be many commands 'in-flight' around the system at any given time, to hide the relatively high latencies involved (many 10s of nanoseconds). The actual propagation delay (around ~1 nanosecond/foot for cables) barely figures into it. Even the cables can have many 'bits' in flight at a time on them, and typically each 'direction' of the link forwards a clock along with the data (or it's embedded in the data stream) to avoid messy synchronization issues.

peter_d_sherman 6y ago

I do see the bottleneck! But, short of having a "how to subvert the speed of light" discussion, which would be exceedingly long (and probably unproductive!) -- you've given me an idea! Yes, you are correct that the speed of electrical propagation is limited, and does limit things! We're completely agreed there! OK, so here's my idea! F--- the electrical cable entirely! So, we have Quantum computers, Quantum bits, and all sorts of research on how to compute things with them, all kinds of professors and papers and companies and venture capital all researching how to do Quantum Computation... Well, I say F--- all of that! (In a polite, friendly, "yes your work is really genius and I acknowledge all of your brilliant contributions kind of way!"). Here's the grail! The grail of Quantum is NOT Quantum Computation -- but rather Quantum Communication! In other words, get rid of the damn electrical cable, and figure out a way to do that communication with a pair of connected-at-a-distance Quantum bits; that is -- Quantum Communication! In other words, Quantum Communication -- should replace wires and busses of all sorts, serial, parallel, electrical, light, what-have-you. That's the damn grail!... (pardon my language!) Not Quantum Computing!!! (brilliant though it is, I am not trying to knock it in any way, far from, quite the opposite!) Quantum Communication -- replacing wires and busses of all sorts, serial, parallel, electrical, light -- would be the Holy Grail -- of being able to build bigger computers (like imagine close-to-infinite RAM -- on one motherboard! Close-to-infinite RAM! Well, it would be off that motherboard... but Quantum Communication, if it were available, would, for all intensive purposes, make that RAM so connected, would have the net effect -- of physically existing on the motherboard, even though in actuality it would not be so!) So, to answer your question... YES, I DO SEE THE BOTTLENECK! But... equal-and-oppositely... Do you see my point?

mrguyorama 6y ago
You cannot communicate over a distance faster than the speed of light even using quantum phenomenon.
- peter_d_sherman 6y ago
  About 120+ years ago or so, if you would have said to the common man that it was possible for a heavier-than-air aircraft to fly, they would have said that that feat of engineering was physically impossible... https://en.wikipedia.org/wiki/Wright_brothers https://en.wikipedia.org/wiki/Wright_brothers Today, after 120 years or so (roughly) of heavier-than-air flight, if you were to say that heavier-than-air aircraft flight was possible, no one would even do so much as "bat an eye" at that statement... I had an old Physics Teacher who once (very wisely!) said: "Given time, the impossible becomes possible, the possible becomes probable, and the probable becomes damn-near-likely to occur!" It was like that for heavier-than-air flight. I think that future history will show your statement to be incorrect at some point -- even though, in fairness to you, your statement might very well be true today, tomorrow, and in the immediate, near-term future... "Given time, the impossible becomes possible, the possible becomes probable, and the probable becomes damn-near-likely to occur..." Also, as a general rule, if something is apparently "not possible" (which yours truly highly doubts) -- then anyone who's worth their salt as a scientist needs to ask WHY multiple times -- and to keep "tunneling downward" across multiple levels, multiple substrata of WHY... See, reach a certain level of WHY (which is really a cause, which sets of the whole cause-effect-cause-effect-etc. chain), understand it -- and you can control all of the effects which proceed out of that initial cause... If we consider waves, for example, those which could occur in a rope or steel cable tied between two posts (compare to radio transmitter and receiver, even though there is no physical medium between them), but if we consider a rope or steel cable, and the waves which can travel along that medium -- they will travel at a certain speed, relative to wavelength (incidentally, all of the "can't go faster than light speed" stuff -- is tied to wavelength, and wavelength equations of hertzian waves -- that's the WHY). But, there's something else going on in that rope or steel cable. And that is, the instantaneous tension of a part of the rope or steel cable -- when a certain waveform is applied, for example, when someone starts vibrating or oscillating that rope or steel cable... See, the waveform propagates at one speed relative to the rate of oscillation. That is one form of propagation. Waves of that nature -- will have a certain speed relative to their wavelength (size). But if the rope or steel cable is in tension between two points, two poles or what have you -- it will propagate changes in that tension near instantly -- much faster than the waveform formed by the aforementioned oscillation or vibration! The same principle is at play with railroad tracks. That is, a train moves at one speed, one velocity, but if you're far ahead of the train, if you put your ear to the train tracks (and I highly highly recommend that no one do this -- this is said for explanatory purposes only!), but if someone were to put their ear to the track of a train which was some distance away -- they would hear the vibrations propagated in the train track at a much higher speed, a much higher velocity, than the speed of the train! Well, same thing with light speed in this case. Light is the train, the medium in which light travels through is the train track. The train track is in tension, and by increasing vibrations as the train gets closer, it "knows" that a train is approaching! The vibrations in the train tracks (train tracks have extremely high tension!) -- move at a faster rate of speed than the oncoming train! So, while you might be right in your assessment in the short term, that Quantum Communication is impossible, I respectfully disagree as to what the long-term prospects and possibilities in this area are... You might be right today... but for how much longer will that truth hold?

corebuffer 6y ago

honest question: what about throughput? I see the issue on latency, but suppose this system is being consumed by a large GPU or a traditional out of order CPU or DMA mechanism. If you can start reads and writes over a high throughput pipe on many chips, can't you also get higher throughput to keep multiple parallel operations busy while requests are in flight?

rasz 6y ago

No, I dont. Its ~30cm/ns, Fastest RAM available _ever_ never went below 7ns.