4 ms·

Actually I do have servers in London and Sydney, so here goes. Speed of light is 299792458 m/s (according to google) Distance between London and Sydney is 16,

Actually I do have servers in London and Sydney, so here goes. Speed of light is 299792458 m/s (according to google) Distance between London and Sydney is 16,983Km (according to google) So, 16,983 / 299792.458 * 1000 = 56ms. Now: PING xyz.com (220.233.xxx.xxx): 56 data bytes 64 bytes from 220.233.xxx.xxx: icmp_seq=0 ttl=46 time=409.263 ms 64 bytes from 220.233.xxx.xxx: icmp_seq=1 ttl=46 time=432.955 ms 64 bytes from 220.233.xxx.xxx: icmp_seq=2 ttl=46 time=351.018 ms 64 bytes from 220.233.xxx.xxx: icmp_seq=3 ttl=46 time=373.894 ms 64 bytes from 220.233.xxx.xxx: icmp_seq=4 ttl=46 time=392.918 ms 64 bytes from 220.233.xxx.xxx: icmp_seq=5 ttl=46 time=412.357 ms Pretty much 10x the speed of light. So, not several orders of magnitude, but at least one. This gets way worse in datacetner networks.

gamegoblin 10y ago
SF -> NY latency is about 7x speed of light[1] https://ipnetwork.bgtmo.ip.att.net/pws/network_delay.html https://ipnetwork.bgtmo.ip.att.net/pws/network_delay.html
camhutch 10y ago
The speed of light in a glass fibre is 2x10^8 m/s (according to Google). However the path through a fibre is not straigt. According to physics.stackexchange.com (http://physics.stackexchange.com/questions/80043/how-fast-does-light-travel-through-a-fibre-optic-cable http://physics.stackexchange.com/questions/80043/how-fast-do...), the speed of light though a fibre would be equivalent to 1.42x10^8 m/s. Then you've got to account for the round-trip. You've only covered half the distance of the ping. You also have to account for a non-direct path from London to Sydney. All of this means you're looking at about 200ms or more as the lower bound, so your 10x becomes < 2x.
- deadgrey19 10y ago
  Fair point about only taking half the ping distance. On the other issues, I disagree. They are the precisely the point of my objection. Assuming that latency is governed by the speed of light fails to take into account all of those other effects. Many of which can be mitigated. And, this is an extreme case. London to Sydney is probably one of the longest paths you can take (except maybe London Auckland?) with many stretches of fibre. On shorter paths, the overheads stack up much more heavily. In datacenters, the time of flight is a few nano-seconds, but the overall latency is tens of micro-seconds to tens of milli-seconds. Isn't it odd that we accept 2x-5x (sometimes even 1000x) overhead over the best case and yet still argue that it's governed by the speed of light? How many other places in computing are we willing to accept that?

gamegoblin 10y ago

SF -> NY latency is about 7x speed of light[1] https://ipnetwork.bgtmo.ip.att.net/pws/network_delay.html https://ipnetwork.bgtmo.ip.att.net/pws/network_delay.html

camhutch 10y ago

The speed of light in a glass fibre is 2x10^8 m/s (according to Google). However the path through a fibre is not straigt. According to physics.stackexchange.com (http://physics.stackexchange.com/questions/80043/how-fast-does-light-travel-through-a-fibre-optic-cable http://physics.stackexchange.com/questions/80043/how-fast-do...), the speed of light though a fibre would be equivalent to 1.42x10^8 m/s. Then you've got to account for the round-trip. You've only covered half the distance of the ping. You also have to account for a non-direct path from London to Sydney. All of this means you're looking at about 200ms or more as the lower bound, so your 10x becomes < 2x.

deadgrey19 10y ago
Fair point about only taking half the ping distance. On the other issues, I disagree. They are the precisely the point of my objection. Assuming that latency is governed by the speed of light fails to take into account all of those other effects. Many of which can be mitigated. And, this is an extreme case. London to Sydney is probably one of the longest paths you can take (except maybe London Auckland?) with many stretches of fibre. On shorter paths, the overheads stack up much more heavily. In datacenters, the time of flight is a few nano-seconds, but the overall latency is tens of micro-seconds to tens of milli-seconds. Isn't it odd that we accept 2x-5x (sometimes even 1000x) overhead over the best case and yet still argue that it's governed by the speed of light? How many other places in computing are we willing to accept that?