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pcwalton's profile
Patrick Walton
Patrick Walton
Patrick Walton
@pcwalton

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Patrick Walton

@pcwalton

Research engineer at Mozilla

San Francisco, CA
pcwalton.github.io
Joined November 2009

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    Patrick Walton‏ @pcwalton 3 Jan 2019
    • Report Tweet
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    A plea to developers of non-cryptographic hash functions: Test performance with very small keys (like, 4-16 bytes). They’re extremely common. SipHash is disfavored in Rust these days because, despite having good perf on long keys, suffers a lot on very short ones.

    7:01 PM - 3 Jan 2019
    • 5 Retweets
    • 66 Likes
    • isiah meadows 🧢 gharel an end Addictive Colors β Pascal Hartig 😷 electrified filth Matt Denton shiva Teja Reddy
    6 replies 5 retweets 66 likes
      1. New conversation
      2. Matthew Fernandez‏ @smattrr 4 Jan 2019
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        Replying to @pcwalton

        Is the use case arbitrary length data? If you’re always hashing, e.g., 4 byte data then surely you can just use the data itself. For arbitrary length data seems you could wrap the hash function with something that branches elsewhere on small length. Too naive?

        1 reply 0 retweets 1 like
      3. Michael‏ @michael90187356 4 Jan 2019
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        Replying to @smattrr @pcwalton

        Actual value is fine in many cases. If you look at pointer and integer hashes in some STL implementations, they just return the value iirc. I have done this in directed mapped caches. One may need a shift depending on how the address is calculated.

        1 reply 0 retweets 1 like
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      2. Steve Canon‏ @stephentyrone 3 Jan 2019
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        Replying to @pcwalton

        Been saying this to everyone who will listen for years.

        1 reply 0 retweets 3 likes
      3. Patrick Walton‏ @pcwalton 3 Jan 2019
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        Replying to @stephentyrone

        I think it’s likely that DoS resistance is effectively impossible for small keys, so it’s not worth doing. The best hash function might be one that scales up number of rounds as the key size increases (and that the compiler can specialize if it knows an upper bound on key size).

        1 reply 0 retweets 1 like
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      1. Damian Gryski‏ @dgryski 3 Jan 2019
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        Replying to @pcwalton

        Perl uses two hash functions these days: one for short (<32 bytes) keys, and a different one for longer ones. Google's farmhash takes this to an extreme splitting the ranges to with different hash functions for 0-16, 17-32, 33-64, 65-96, 97-256, and 257+ bytes.

        0 replies 0 retweets 18 likes
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      1. New conversation
      2. Ben Jackson‏ @puremouron 5 Jan 2019
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        Replying to @pcwalton

        Didn’t @cmuratori recently claim to have created the fastest non crypto hash fn evar? https://mollyrocket.com/meowhash 

        1 reply 0 retweets 0 likes
      3. Casey Muratori‏ @cmuratori 5 Jan 2019
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        Replying to @puremouron @pcwalton

        Meow Hash is for large data. If you want fast non-crypto hashing of 16 bytes or less, you can just do a few AESDEC's (or ENC's) and it's effectively instant. Take a look at Go's internal hash for an example. Meow is still reasonable on small keys, but why pay for the setup?

        1 reply 0 retweets 1 like
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      1. Anarcho réformé patarchiste‏ @ObnoxiousJul 4 Jan 2019
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        Replying to @pcwalton

        like this SipHash?https://www.python.org/dev/peps/pep-0456/ …

        0 replies 0 retweets 0 likes
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      1. Adenilson Cavalcanti‏ @adenilsonc 4 Jan 2019
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        Replying to @pcwalton

        I tested some hashes sometime ago, I recall that xxhash had good performance. Alternatively, __crc32w loads 32 bits and uses a instruction on ARMv8-1.

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