5 ms·

The foot has been criticized for being an arbitrary measurement with no real relation to a repeatable physical distance. However, it turns out that a foot is w

The foot has been criticized for being an arbitrary measurement with no real relation to a repeatable physical distance. However, it turns out that a foot is within 2% of the distance light travels in a nanosecond! Because of this, the foot becomes really convenient when talking about latencies. For example, if something is 6 inches away from the cpu on a motherboard, the lowest possible latency to reach that is 0.5 nanoseconds. Time to push for the adoption of feet everywhere /s

doliveira 4y ago
Reminds me of the whole "Fahrenheit is awesome because 0°F is too cold and 100°F is too hot" (BTW, for me 0°C is too cold and 40°C is too hot)
- 8note 4y ago
  Celcius is nice because -30 is the limits of cold that I want, and +30 is the limits of hot
chrisseaton 4y ago
> For example, if something is 6 inches away from the cpu on a motherboard, the lowest possible latency to reach that is 0.5 nanoseconds. Isn't that going to be insignificant compared to everything inside the computer on both ends?
- mlonkibjuyhv 4y ago
  Not if you're making computers.
  - dredmorbius 4y ago
    To expand on this: if you're designing / building / assembling a computer or cluster, and components are 6" apart, then the minimum latency for communicating between those components is 1 nanosecond. Given clock speeds of multiple GHz, that means spending an entire clock cycle or more simply communicating between components. See also the case of the 500 mile email: https://www.ibiblio.org/harris/500milemail.html https://www.ibiblio.org/harris/500milemail.html (A Sendmail misconfiguration resulted in a maximum response time of 3 milliseconds, or roughly 500 miles of travel at the speed of light. The observed behaviour was that a uni campus computer could send email only within a 500 mile radius, as noted by the statistics department.)
    - chrisseaton 4y ago
      > Given clock speeds of multiple GHz, that means spending an entire clock cycle or more simply communicating between components. Yeah that's expected isn't it? That's why we have caches on die. Nobody is out there expecting main memory reads to retire in a clock cycle, let alone IO! I don't think even lower tier cache access retires in a single clock cycles. That's just not how processors work these days.
      - dredmorbius 4y ago
        It's not just single systems. It's clusters. It's datacentres. It's tools which span the globe. Or extend into space. The Web by default is now transacted over HTTPS. This means that every session requires a TLS handshake: - Client hello - Server hello + key - Client key exchange. - Server finished. - Client finished. - Data transfer begins. That's six exchanges, and three round trips. For an antipodal set of hosts, at 300ms per trip, that's nearly 2 seconds just to set up a session. If you're communicating with a Moon base, it's eight seconds. And if you're using a tool or protocol which presumes cheap or fast round-trips, and uses a lot of round trips, you may find it's unusable. Some years back a multi-campus site rolled out a remote-console tool that worked across platforms in datacentres --- we had both Linux and Windows hosts. Working locally with the DC one building over in the campus, or even with a facility elsewhere in the province, performance was laggier than local, but tolerable. The team operating out of Dubai was waiting five minutes to see login screens presented. Distance is time.
        wolf550e 4y ago
        TLS1.3 removes one roundtrip. HTTP3 (QUIK) remove more by combining the TCP handshake with the TLS handshake.
        dredmorbius 4y ago
        SPAs return those roundtrips with a vengence.
stn8188 4y ago
Love your comment, this quick rule is critical, but... Don't forget that the relative permittivity if PCB material is roughly 4, so the rule for circuit boards is 6" per nanosecond :)
Arcorann 4y ago
Of course, if one were to switch measurements for that reason it'd be better to use the Japanese shaku [1], which at 30.303 cm is closer to a light-nanosecond than a foot. [1] https://en.wikipedia.org/wiki/Japanese_units_of_measurement https://en.wikipedia.org/wiki/Japanese_units_of_measurement
InitialLastName 4y ago
A similar heuristic is very useful for acoustics: the speed of sound is close enough to 1 foot/ms to be a great rule of thumb for estimation.
- ChainOfFools 4y ago
  Or... rule of foot, for the compulsive unit-cancelers out there
_joel 4y ago
Welcome to the UK in 2022! You'll even get a crown logo emblazened on the nanosecond ;)
protomyth 4y ago
I thought if the US ever switched from Imperial we should just switch to light-nanoseconds since its so close and then make fun of the metric folks for being Earth-centric. Units of weight / volume would be a pain though since a cubic light-second of water is about 7.118 US gallons and weighs (@1g) about 59.227 lbs at the melting point of ice. Of course, we should still go with base 8 like the Yuki tribe (spaces between fingers instead of fingers because that's how many bottles you can carry).
- ArnoVW 4y ago
  Light nanosecond depends on 'second', which is litteraly earth centric :-)
  - protomyth 4y ago
    Less Earth centric than the distance from the North Pole to the Equator. We'll still use seconds on Mars.
    - dredmorbius 4y ago
      And you can use feet on Mars, despite there being no actual feet there. The second is 1/86,400 of a nominal Earth sidreal day. More or less. Slightly less with time as the Earth's rotation is in fact slowing.
deleted 4y ago
[deleted]

doliveira 4y ago

Reminds me of the whole "Fahrenheit is awesome because 0°F is too cold and 100°F is too hot" (BTW, for me 0°C is too cold and 40°C is too hot)

8note 4y ago
Celcius is nice because -30 is the limits of cold that I want, and +30 is the limits of hot

chrisseaton 4y ago

> For example, if something is 6 inches away from the cpu on a motherboard, the lowest possible latency to reach that is 0.5 nanoseconds. Isn't that going to be insignificant compared to everything inside the computer on both ends?

mlonkibjuyhv 4y ago
Not if you're making computers.
- dredmorbius 4y ago
  To expand on this: if you're designing / building / assembling a computer or cluster, and components are 6" apart, then the minimum latency for communicating between those components is 1 nanosecond. Given clock speeds of multiple GHz, that means spending an entire clock cycle or more simply communicating between components. See also the case of the 500 mile email: https://www.ibiblio.org/harris/500milemail.html https://www.ibiblio.org/harris/500milemail.html (A Sendmail misconfiguration resulted in a maximum response time of 3 milliseconds, or roughly 500 miles of travel at the speed of light. The observed behaviour was that a uni campus computer could send email only within a 500 mile radius, as noted by the statistics department.)
  - chrisseaton 4y ago
    > Given clock speeds of multiple GHz, that means spending an entire clock cycle or more simply communicating between components. Yeah that's expected isn't it? That's why we have caches on die. Nobody is out there expecting main memory reads to retire in a clock cycle, let alone IO! I don't think even lower tier cache access retires in a single clock cycles. That's just not how processors work these days.
    - dredmorbius 4y ago
      It's not just single systems. It's clusters. It's datacentres. It's tools which span the globe. Or extend into space. The Web by default is now transacted over HTTPS. This means that every session requires a TLS handshake: - Client hello - Server hello + key - Client key exchange. - Server finished. - Client finished. - Data transfer begins. That's six exchanges, and three round trips. For an antipodal set of hosts, at 300ms per trip, that's nearly 2 seconds just to set up a session. If you're communicating with a Moon base, it's eight seconds. And if you're using a tool or protocol which presumes cheap or fast round-trips, and uses a lot of round trips, you may find it's unusable. Some years back a multi-campus site rolled out a remote-console tool that worked across platforms in datacentres --- we had both Linux and Windows hosts. Working locally with the DC one building over in the campus, or even with a facility elsewhere in the province, performance was laggier than local, but tolerable. The team operating out of Dubai was waiting five minutes to see login screens presented. Distance is time.
      - wolf550e 4y ago
        TLS1.3 removes one roundtrip. HTTP3 (QUIK) remove more by combining the TCP handshake with the TLS handshake.
        dredmorbius 4y ago
        SPAs return those roundtrips with a vengence.

stn8188 4y ago

Love your comment, this quick rule is critical, but... Don't forget that the relative permittivity if PCB material is roughly 4, so the rule for circuit boards is 6" per nanosecond :)

Arcorann 4y ago

Of course, if one were to switch measurements for that reason it'd be better to use the Japanese shaku [1], which at 30.303 cm is closer to a light-nanosecond than a foot. [1] https://en.wikipedia.org/wiki/Japanese_units_of_measurement https://en.wikipedia.org/wiki/Japanese_units_of_measurement

InitialLastName 4y ago

A similar heuristic is very useful for acoustics: the speed of sound is close enough to 1 foot/ms to be a great rule of thumb for estimation.

ChainOfFools 4y ago
Or... rule of foot, for the compulsive unit-cancelers out there

_joel 4y ago

Welcome to the UK in 2022! You'll even get a crown logo emblazened on the nanosecond ;)

protomyth 4y ago

I thought if the US ever switched from Imperial we should just switch to light-nanoseconds since its so close and then make fun of the metric folks for being Earth-centric. Units of weight / volume would be a pain though since a cubic light-second of water is about 7.118 US gallons and weighs (@1g) about 59.227 lbs at the melting point of ice. Of course, we should still go with base 8 like the Yuki tribe (spaces between fingers instead of fingers because that's how many bottles you can carry).

ArnoVW 4y ago
Light nanosecond depends on 'second', which is litteraly earth centric :-)
- protomyth 4y ago
  Less Earth centric than the distance from the North Pole to the Equator. We'll still use seconds on Mars.
  - dredmorbius 4y ago
    And you can use feet on Mars, despite there being no actual feet there. The second is 1/86,400 of a nominal Earth sidreal day. More or less. Slightly less with time as the Earth's rotation is in fact slowing.

deleted 4y ago

[deleted]