A few days ago, I remembered about a famous collection of numbers that every programmer should know about timing of typical computer operations. I realized it would be interesting to see how much light travels in that amount of time. Without further ado, here is the table:

Operation Time Distance
3.3 GHz CPU Cycle 0.3 ns 9 cm
L1 cache reference 0.5 ns 15 cm
Branch mispredict 5 ns 1.5 m
L2 cache reference 7 ns 2.1 m
Mutex lock/unlock 100 ns 30 m
Main memory reference 100 ns 30 m
Compress 1K bytes with Zippy 10,000 ns 3 km
Send 2K bytes over 1 Gbps network 20,000 ns 6 km
Read 1 MB sequentially from memory 250,000 ns 75 km
Round trip within same datacenter 500,000 ns 150 km
Disk seek 10,000,000 ns 3000 km
Read 1 MB sequentially from disk 30,000,000 ns 9000 km
Send packet CA-Netherlands-CA 150,000,000 ns 45000 km

This table shows interesting things for latency. It gives an idea of how incredibly fast are processors today in executing one instruction: a Laser Range Finder using "time of flight" is able to shoot a bunch of photons, and physically measure the time it takes for these photons to bounce and come back. In fact, even a simple arduino can count fast enough to measure distances with a time of flight approach.


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