参考文献

Latency numbers every programmer should know

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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

Handy metrics based on numbers above:

  • Read sequentially from HDD at 30 MB/s
    • 以 30 MB/s 的速度从 HDD 顺序读取
  • Read sequentially from 1 Gbps Ethernet at 100 MB/s
    • 以 100 MB/s 的速度从 1 Gbps 以太网顺序读取
  • Read sequentially from SSD at 1 GB/s
    • 以 1 GB/s 的速度从 SSD 顺序读取
  • Read sequentially from main memory at 4 GB/s
    • 以 4 GB/s 的速度从主内存顺序读取
  • 6-7 world-wide round trips per second
    • 每秒 6-7 次全球往返
  • 2,000 round trips per second within a data center
    • 数据中心内每秒 2,000 次往返

Latency numbers visualized

img

Minute

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4
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

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2
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

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

Week

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4
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

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2
3
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

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