Add Latency numbers every programmer should know section

Author: donnemartin

README.md

@@ -1667,3 +1667,51 @@ Power           Exact Value         Approx Value        Bytes
 #### Source(s) and further reading
 
 * [Powers of two](https://en.wikipedia.org/wiki/Power_of_two)
+
+### Latency numbers every programmer should know
+
+```
+Latency Comparison Numbers
+--------------------------
+L1 cache reference                           0.5 ns
+Branch mispredict                            5   ns
+L2 cache reference                           7   ns                      14x L1 cache
+Mutex lock/unlock                          100   ns
+Main memory reference                      100   ns                      20x L2 cache, 200x L1 cache
+Compress 1K bytes with Zippy            10,000   ns       10 us
+Send 1 KB bytes over 1 Gbps network     10,000   ns       10 us
+Read 4 KB randomly from SSD*           150,000   ns      150 us          ~1GB/sec SSD
+Read 1 MB sequentially from memory     250,000   ns      250 us
+Round trip within same datacenter      500,000   ns      500 us
+Read 1 MB sequentially from SSD*     1,000,000   ns    1,000 us    1 ms  ~1GB/sec SSD, 4X memory
+Disk seek                           10,000,000   ns   10,000 us   10 ms  20x datacenter roundtrip
+Read 1 MB sequentially from 1 Gbps  10,000,000   ns   10,000 us   10 ms  40x memory, 10X SSD
+Read 1 MB sequentially from disk    30,000,000   ns   30,000 us   30 ms 120x memory, 30X SSD
+Send packet CA->Netherlands->CA    150,000,000   ns  150,000 us  150 ms
+
+Notes
+-----
+1 ns = 10^-9 seconds
+1 us = 10^-6 seconds = 1,000 ns
+1 ms = 10^-3 seconds = 1,000 us = 1,000,000 ns
+```
+
+Handy metrics based on numbers above:
+
+* Read sequentially from disk at 30 MB/s
+* Read sequentially from 1 Gbps Ethernet at 100 MB/s
+* Read sequentially from SSD at 1 GB/s
+* Read sequentially from main memory at 4 GB/s
+* 6-7 world-wide round trips per second
+* 2,000 round trips per second within a data center
+
+#### Latency numbers visualized
+
+![](https://camo.githubusercontent.com/77f72259e1eb58596b564d1ad823af1853bc60a3/687474703a2f2f692e696d6775722e636f6d2f6b307431652e706e67)
+
+#### Source(s) and further reading
+
+* [Latency numbers every programmer should know - 1](https://gist.github.com/jboner/2841832)
+* [Latency numbers every programmer should know - 2](https://gist.github.com/hellerbarde/2843375)
+* [Designs, lessons, and advice from building large distributed systems](http://www.cs.cornell.edu/projects/ladis2009/talks/dean-keynote-ladis2009.pdf)
+* [Software Engineering Advice from Building Large-Scale Distributed Systems](https://static.googleusercontent.com/media/research.google.com/en//people/jeff/stanford-295-talk.pdf)