====== 杂项 ======
===== Latency Numbers Every Programmer Should Know =====
* [[https://colin-scott.github.io/personal_website/research/interactive_latency.html|最新数据与历史数据 ]]
Latency Comparison Numbers (~2020)
----------------------------------
L1 cache reference 0.5 ns
Branch mispredict 3 ns
L2 cache reference 4 ns
Mutex lock/unlock 17 ns
Main memory reference 100 ns
Compress 1K bytes with Zippy 2,000 ns 2 us
Send 2,000 bytes over commodity network 44 ns 0 us
SSD random read 16,000 ns 16 us
Read 1 MB sequentially from memory 3,000 ns 3 us
Round trip within same datacenter 500,000 ns 500 us
Read 1 MB sequentially from SSD* 49,000 ns 49 us
Disk seek 2,000,000 ns 2,000 us 2 ms
Read 1 MB sequentially from disk 825,000 ns 825 us 1 ms
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
* [[https://gist.github.com/jboner/2841832|2012年的数据]]:
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