每个程序员都应该知道的一些时间数据

原文：Every
Programmer Should Know These Latency Numbers

L1 cache ：………………………………………………..0.5ns

branch mispredict(转移、分支预测) : ……………..5ns

L2 cache :………………………………………………… 7ns

互斥锁\解锁 :…………………………………………….. 25ns

1k字节压缩(Zippy) : …………………………………….3000ns = 3us

在1Gbps的网络上发送2k字节 : ……………………….20,000ns = 20us

SSD随机读 ：……………………………………………..150,000ns = 150us

从内存顺序读取1MB ：…………………………………250,000ns = 250 us

同一个数据中心往返 ：………………………………….500,000ns = 0.5ms

从SSD顺序读取1MB ： …………………………………1,000,000ns = 1ms

磁盘搜索:…………………………………………………..10,000,000ns = 10ms

从磁盘里面读出1MB
: …………………………………..20，000，000ns = 20ms

### Latency numbers every programmer should know

L1 cache reference ......................... 0.5 ns

Branch mispredict ............................ 5 ns

L2 cache reference ........................... 7 ns

Mutex lock/unlock ........................... 25 ns

Main memory reference ...................... 100 ns

Compress 1K bytes with Zippy ............. 3,000 ns = 3 μs

Send 2K bytes over 1 Gbps network ....... 20,000 ns = 20 μs

SSD random read ........................ 150,000 ns = 150 μs

Read 1 MB sequentially from memory ..... 250,000 ns = 250 μs

Round trip within same datacenter ...... 500,000 ns = 0.5 ms

Read 1 MB sequentially from SSD* ..... 1,000,000 ns = 1 ms

Disk seek ........................... 10,000,000 ns = 10 ms

Read 1 MB sequentially from disk .... 20,000,000 ns = 20 ms

Send packet CA->Netherlands->CA .... 150,000,000 ns = 150 ms

Assuming ~1GB/sec SSD


Visual chart provided by [ayshen](https://gist.github.com/ayshen)

Data by [Jeff Dean](http://research.google.com/people/jeff/)

Originally by [Peter Norvig](http://norvig.com/21-days.html#answers)

Lets multiply all these durations by a billion:

Magnitudes:

### Minute:

L1 cache reference 0.5 s One heart beat (0.5 s)

Branch mispredict 5 s Yawn

L2 cache reference 7 s Long yawn

Mutex lock/unlock 25 s Making a coffee

### Hour:

Main memory reference 100 s Brushing your teeth

Compress 1K bytes with Zippy 50 min One episode of a TV show (including ad breaks)

### Day:

Send 2K bytes over 1 Gbps network 5.5 hr From lunch to end of work day

### Week

SSD random read 1.7 days A normal weekend

Read 1 MB sequentially from memory 2.9 days A long weekend

Round trip within same datacenter 5.8 days A medium vacation

Read 1 MB sequentially from SSD 11.6 days Waiting for almost 2 weeks for a delivery

### Year

Disk seek 16.5 weeks A semester in university

Read 1 MB sequentially from disk 7.8 months Almost producing a new human being

The above 2 together 1 year

### Decade

Send packet CA->Netherlands->CA 4.8 years Average time it takes to complete a bachelor's degree