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 during these times. 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.