Re: General purpose hashing func in pgbench

From: Ildar Musin <i(dot)musin(at)postgrespro(dot)ru>
To: Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>
Cc: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: General purpose hashing func in pgbench
Date: 2017-12-21 11:38:14
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Hello Fabien,

20/12/2017 10:36, Fabien COELHO пишет:

> As there may be several hash functions included in the long run. I'd
> suggest that the hash function should be named more precisely, eg
> "hash_fnv1a".
Done. Added "hash_murmur2" too, see below.
> The image looks like the distribution is more regularly scattered than
> actually randomized... Maybe this is because the first highest 256
> values are really scattered by the process multiply/modulo process. Or
> maybe this is an optical effect?
After your comment I searched the internet for different hashing
algorithms comparison wrt randomness and found an interesting post at
stackexchange [1]. According to author's research the murmur2 algorithm
has the best randomness rate (among those he tested). So I implemented
it (using original code by Austin Appleby as a reference [2]) and
conducted few experiments. Results are in attachement. Indeed, comparing
to murmur2 the FNV distribution seems pretty regular.
> ISTM that there are undesired utf8 chars in a comment. Should be kept
Oops, I copy-pasted the algorithm name from wikipedia, didn't notice
there were some fancy unicode hyphens.
> I would put the actual hash computation in a separate function rather
> than inlined in the evaluator.
> Add the submission to the next CF?
I think it is not commitfest ready yet -- I need to add some
documentation and tests first.


Ildar Musin
Postgres Professional:
Russian Postgres Company

Attachment Content-Type Size
pgbench_hash_v2.patch text/plain 2.5 KB
fnv_vs_murmur2_2K.png image/png 473.1 KB
fnv_vs_murmur2_500.png image/png 627.9 KB

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