79 lines
3.9 KiB
Markdown
79 lines
3.9 KiB
Markdown
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# Appendix
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## Powers of two table
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```
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Power Exact Value Approx Value Bytes
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---------------------------------------------------------------
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7 128
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8 256
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10 1024 1 thousand 1 KB
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16 65,536 64 KB
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20 1,048,576 1 million 1 MB
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30 1,073,741,824 1 billion 1 GB
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32 4,294,967,296 4 GB
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40 1,099,511,627,776 1 trillion 1 TB
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```
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## Source(s) and further reading
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- [Powers of two](https://en.wikipedia.org/wiki/Power_of_two)
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## Latency numbers every programmer should know
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---
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Latency Comparison Numbers
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--------------------------
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L1 cache reference 0.5 ns
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Branch mispredict 5 ns
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L2 cache reference 7 ns 14x L1 cache
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Mutex lock/unlock 100 ns
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Main memory reference 100 ns 20x L2 cache, 200x L1 cache
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Compress 1K bytes with Zippy 10,000 ns 10 us
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Send 1 KB bytes over 1 Gbps network 10,000 ns 10 us
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Read 4 KB randomly from SSD- 150,000 ns 150 us ~1GB/sec SSD
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Read 1 MB sequentially from memory 250,000 ns 250 us
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Round trip within same datacenter 500,000 ns 500 us
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Read 1 MB sequentially from SSD- 1,000,000 ns 1,000 us 1 ms ~1GB/sec SSD, 4X memory
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Disk seek 10,000,000 ns 10,000 us 10 ms 20x datacenter roundtrip
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Read 1 MB sequentially from 1 Gbps 10,000,000 ns 10,000 us 10 ms 40x memory, 10X SSD
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Read 1 MB sequentially from disk 30,000,000 ns 30,000 us 30 ms 120x memory, 30X SSD
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Send packet CA->Netherlands->CA 150,000,000 ns 150,000 us 150 ms
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Notes
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-----
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1 ns = 10^-9 seconds
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1 us = 10^-6 seconds = 1,000 ns
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1 ms = 10^-3 seconds = 1,000 us = 1,000,000 ns
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Handy metrics based on numbers above:
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- Read sequentially from disk at 30 MB/s
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- Read sequentially from 1 Gbps Ethernet at 100 MB/s
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- Read sequentially from SSD at 1 GB/s
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- Read sequentially from main memory at 4 GB/s
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- 6-7 world-wide round trips per second
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- 2,000 round trips per second within a data center
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#### [](https://github.com/donnemartin/system-design-primer#latency-numbers-visualized) Latency numbers visualized
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[ ](https://camo.githubusercontent.com/77f72259e1eb58596b564d1ad823af1853bc60a3/687474703a2f2f692e696d6775722e636f6d2f6b307431652e706e67)
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#### [](https://github.com/donnemartin/system-design-primer#sources-and-further-reading-14) Source(s) and further reading
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- [Latency numbers every programmer should know - 1](https://gist.github.com/jboner/2841832)
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- [Latency numbers every programmer should know - 2](https://gist.github.com/hellerbarde/2843375)
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- [Designs, lessons, and advice from building large distributed systems](http://www.cs.cornell.edu/projects/ladis2009/talks/dean-keynote-ladis2009.pdf)
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- [Software Engineering Advice from Building Large-Scale Distributed Systems](https://static.googleusercontent.com/media/research.google.com/en//people/jeff/stanford-295-talk.pdf)
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## Introduction of base 62
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- Encodes to `[a-zA-Z0-9]` which works well for urls, eliminating the need for escaping special characters
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- Only one hash result for the original input and and the operation is deterministic (no randomness involved)
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- Base 64 is another popular encoding but provides issues for urls because of the additional `+` and `/` characters
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## MD5
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- Widely used hashing function that produces a 128-bit hash value
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- Uniformly distributed
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