Ivan's wiki

This page is read only. You can view the source, but not change it. Ask your administrator if you think this is wrong.
# Numbers every programmer should know
**Numbers every programmer should know** is a short document listing latencies of various computer operations. The list is given by a computer scientist [Jeff Dean](https://en.wikipedia.org/wiki/Jeff_Dean).

It illustrates how various high-level operations exponentially take longer time to perform compared to various low-level operations.

## Content
```
Latency Comparison Numbers (~2012)
----------------------------------
L1 cache reference                           0.5 ns
Branch mispredict                            5   ns
L2 cache reference                           7   ns                      14x L1 cache
Mutex lock/unlock                           25   ns
Main memory reference                      100   ns                      20x L2 cache, 200x L1 cache
Compress 1K bytes with Zippy             3,000   ns        3 us
Send 1K bytes over 1 Gbps network       10,000   ns       10 us
Read 4K 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 disk    20,000,000   ns   20,000 us   20 ms  80x memory, 20X 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

Credit
------
By Jeff Dean:               http://research.google.com/people/jeff/
Originally by Peter Norvig: http://norvig.com/21-days.html#answers

Contributions
-------------
'Humanized' comparison:    https://gist.github.com/hellerbarde/2843375
Visual comparison chart:   http://i.imgur.com/k0t1e.png
Interactive Prezi version: https://prezi.com/pdkvgys-r0y6/latency-numbers-for-programmers-web-development/latency.txt
```

## Criticism (Opinion)
While the list is illustrative, the claim that every programmer needs to "know" these numbers is not justified. Certainly it's not necessary to know these numbers by heart for one to be an effective programmer. The name "numbers every programmer should know" is thus a misnomer.

The list mixes different categories of programming which sit at completely different hierarchies. A high-level web programmer almost never needs to know about mutex lock/unlock or branch misprediction; A low-level embedded programmer almost never needs to know the packet roundtrip between California and Netherlands.

In addition, these numbers evolve as the technology continues to evolve. Because we're talking about latencies, they are subjected to eternal factors (electromagnetic interference, network congestion, ...). Therefore, even if you do know these numbers by heart, there is no context in which you will find their value useful.

As mentioned, the list should only be understood as illustrative.
```
L1 cache < Branch mispredict < ... < Send packet CA -> Netherlands -> CA
```

## Source

 - https://gist.github.com/jboner/2841832