8 ms·
The speed of light story is a fun conjecture, but given the discrepancy (30cm vs 5.5mm), I don't think that's it. Light travels slower in wires but not that muc
by marcan_42 5y ago
The speed of light story is a fun conjecture, but given the discrepancy (30cm vs 5.5mm), I don't think that's it. Light travels slower in wires but not that much slower, and increased design clocks don't explain the discrepancy either.
I think it's more likely that this is an artifact of the interconnect design. For example, the interconnect may be designed as a tree, where the path from core 0 to the L2 only visits a branch but the path to core1/2 has to traverse the root of the tree. If each stage adds a clock cycle, there's your 4 clock cycles: one to go to the root, one back down to the other side, and then the whole thing in the other direction for the reply from L2 with the data.
If the interconnect had been a ring bus instead, you'd find that the latency from cores 0 and 2 is the same, but higher for core 1 - assuming no other endpoints in the ring, but that's obviously not the case, since I/O needs to be on the bus too. It's possible he got the core numbers wrong in the die shot, and core 0 is the bottom left core. Then if the ring goes clockwise L2 - core 0 - core 1 - core 2 - I/O on the top right - L2, that's how you end up with core 0 being closer to the L2 in terms of clock cycles than the others, and the observation would be consistent with this design too.
- 123pie123 5y agothis is an interesting video (to me anyway!!) on the speed of electrons vs fields The Big Misconception About Electricity.. https://www.youtube.com/watch?v=bHIhgxav9LY https://www.youtube.com/watch?v=bHIhgxav9LY
- Dumble 5y agohttps://www.youtube.com/watch?v=2Vrhk5OjBP8 https://www.youtube.com/watch?v=2Vrhk5OjBP8
- jaywalk 5y agoThat video is hilariously wrong, which seems to be a common theme for Veritasium lately. I can't tell if he legitimately believes what he's saying, or if he produces these seemingly-correct-but-not videos to drive up engagement and views.
- scrollaway 5y agoCare to share the hilarity with the rest of the class?
- aemreunal 5y agoDisclaimer: I don't understand nearly enough about the physics of electricity, but this was a popular response video by ElectroBOOM that Veritasium also responded to in the comments: https://www.youtube.com/watch?v=iph500cPK28 https://www.youtube.com/watch?v=iph500cPK28
- mannykannot 5y agoTo be clear, you will not find that Veritasium is wrong here (whether hilariously or otherwise); it supports the essential features of the original claims.
- tsimionescu 5y agoHe is extremely misleading, if not technically wrong. The Poynting vector has nothing to do with why some small amount of current is temporarily induced into the light bulb. The actual electrical energy flows along the wires (inside and out). In an unrelated phenomenon, you transmit some tiny amount of electrical power directly through the air/vacuum between the battery and the bulb. In a different configuration, or if you inserted certain kinds of reflective materials, you could block (most of) this energy from ever reaching the bulb, without any change whatsoever in the current flowing along the wires. Also, if you submerged the whole circuit (or just the space around the battery and bulb) in a dielectric material with a low speed of light (say, in a piece of rock), you would get a vastly slower and weaker current, with no effect on the current flowing through the wires. Even the part where he explains the problem with the chain metaphor is wrong. If you actually had a mechanical chain and an engine moving it back and forth, you could extract energy from the movement of the chain either by exploiting friction (to heat up something, just like a resistor does) or by using gears that resist movement in the opposite direction (e.g. slipping the chain) to achieve movement in a single direction. There's nothing all that mysterious about how we extract energy from electricity, at least in practice. In fact, you could even get an equivalent of the small induced curent: if you recreate his extremely long circuit with a motor instead of the battery and an extremely sensitive motion detector instead of the light bulb 1m away, you would detect some motion [1m/speed of sound in material connecting them] seconds after the motor is started, assuming you are not floating in a perfect vacuum; and then vastly higher motion after 300,000km/speed of sound in chain seconds later. Nothing more mysterious going on with the EM field.
- krupan 5y agoElectroBOOM replied to this: https://youtu.be/iph500cPK28 https://youtu.be/iph500cPK28
- deleted 5y ago[deleted]
- AshamedCaptain 5y agoThe speed of light is a useless metric when measuring propagation delay of a signal in a wire since there is load of factors that limit it much more significantly, such as the width of the conductor / capacitance / resistance, the strength of the driver, etc. Starting point: https://en.wikipedia.org/wiki/Elmore_delay https://en.wikipedia.org/wiki/Elmore_delay And if you are going to do anything useful (e.g. toggling the signal), the max frequency at which you can do it is even lower than the limit set by the propagation delay. In any case, even TFA says the reason pretty explicitly: it's actually 4 pipelined stages, thus 4 clock cycles delay. Probably it was way too much power+area to drive the distance in one cycle.
- entangledqubit 5y agoRandom historical context (IIRC), in the early 2000s (around when the chip discussed was made) chip designers were dealing with the transition from gate/switch delays being a dominant factor to signal wire propagation delays becoming very significant. This was a side effect of the improving processes making smaller chip features.
- marcan_42 5y agoInteresting that that was around when this shift was happening. I know things have gotten a lot crazier with modern nodes...
- randomswede 5y agoAnecdotally, the DEC Alpha was the first chip to move from "clock is provided at the edge of the chip" to "clock is provided in the middle of the chip", due to a desire to minimise the skew from one edge of the CPU to the other. How true it is? Not the faintest, but maybe someone here was in a/the Alpha design team and can say yes/no.
- ece 5y agoMore asynchronous design?
- krupan 5y agoIn chips because of the layers of conductors and insulators you are actually charging a capacitor, not just sending a signal down a lone wire.
- AshamedCaptain 5y agoThat's how all conductors work, not just "in chips due to layers of conductors and insulators". The thing is that in most situations you just model the conductor away as a "perfect ideal wire" and everything usually just works since you don't care about these small delays and/or you have very high power. But when you enter sub-nanosecond and/or sub-mW territory...
- jhgb 5y agoBut IC interconnects have a very small cross section, so for them this problem is much more pressing.
- tom-thistime 5y agoYou're always driving a waveguide that has inductance and capacitance per unit length. But when resistance is high enough you are just charging a big distributed capacitor. It's a different regime from the high-conductivity situation (which looks like a transmission line). Either picture could apply at sub-ns speeds, it depends mainly on the resistance.
- kens 5y agoYes, a long bus is like a resistor-capacitor delay circuit. You can speed it up by using a bigger driver for more current, but that takes up more space and adds its own delay. I expect that the bus in the Xbox is split in two, with a second driver in the middle. This makes the first half faster, but the second half of the bus probably takes another clock cycle, which would explain the observed behavior.
- brucedawson 5y agoReading from L2 requires sending down a request and then waiting for the response. This makes me realize that the actual speed is 5.5 mm in 0.625 ns because it's a two cycle delay in each direction. I'll update the article. But, this request/response means that a ring bus seems unlikely because a unidirectional ring bus should make the sum of the request/response times identical for all CPUs, if I'm understanding correctly.
- kens 5y agoYou mentioned a document with a detailed description of the internals of the Xbox 360 CPU. I don't suppose that document is available to the public?
- brucedawson 5y agoSorry, I've never seen a non-NDA document with full details. Some information was released but it was a bit spotty.
- marcan_42 5y agoI was thinking bidirectional ring bus in my model; indeed if it were unidirectional you'd end up with the same result for all CPUs. FWIW, the Cell used a bidirectional ring bus network, and it's of the same generation and happens to share technology heritage with the XCPU... (very similar PowerPC core) :-)
- deleted 5y ago[deleted]
- BeeOnRope 5y agoSpeed of signal propagation though wires on modern chips is much slower than the speed of light. A bit less so on an old Xbox chips, but still a big difference. On a modern core the delay is enough that you can't come close to reaching from side one side of a core (not to mention CPU) to another in a single cycle.
- dlemire 5y agoDo you have a measure of this speed?
- xyzzy21 5y agoSo first, there are two types of velocity when it comes to signals on a transmission line which is what metal lines on chip are: phase velocity and group velocity. Phase velocity is close to the speed of light. It can be reduced by dielectric materials parameters so it's often lower than c. https://en.wikipedia.org/wiki/Phase_velocity https://en.wikipedia.org/wiki/Phase_velocity Group velocity is often much slower. Information transfer can never be faster the group velocity, rather than the phase velocity. So when it comes to "on chip communication" it is group velocity that matters. Group velocity depends on many factor of transmission line design such as line thickness, line width, dielectric thickness, etc. https://en.wikipedia.org/wiki/Group_velocity https://en.wikipedia.org/wiki/Group_velocity