[置顶] Every Programmer Should Know These Latency Numbers

转自：   https://dzone.com/articles/every-programmer-should-know

Every Programmer Should Know These Latency Numbers

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This is interesting stuff; Jonas Bonér organized some general some latency data by Peter Norvig as a Gist, and others expanded on it. What's interesting is how, scaling time up by a billion, converts a CPU instruction cycle into approximately one heartbeat, and yields a disk seek time of "a semester in university".

### 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 representation of latencies](http://i.imgur.com/k0t1e.png)

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