Horton

Re: pgbench - add pseudo-random permutation function

Fabien COELHO <coelho@cri.ensmp.fr>

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From: Fabien COELHO <coelho@cri.ensmp.fr>
To: Hironobu SUZUKI <hironobu@interdb.jp>
Cc: PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Date: 2018-09-12T09:26:53Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. pgbench: Function to generate random permutations.

  2. Add basic support for using the POPCNT and SSE4.2s LZCNT opcodes

  3. Further improve code for probing the availability of ARM CRC instructions.

Attachments

Hello Hironobu-san,

> However, the implementation of the scatter operation in this patch overflows 
> in many cases if the variable:size is 38 bit integer or greater. Because the 
> variable:size and the item of the array:primes[] which stores 27-29 bit 
> integers are multiplicated. If overflow occurs, the scatter operation does 
> not satisfy bijective.

Indeed. Again, thanks for the debug! As you contributed some code, I added 
you as a co-author in the CF entry.

Attached a v3, based on your fix, plus some additional changes:
  - explicitly declare unsigned variables where appropriate, to avoid casts
  - use smaller 24 bits primes instead of 27-29 bits
  - add a shortcut for multiplier below 24 bits and y value below 40 bits,
    which should avoid the manually implemented multiplication in most
    practical cases (tables with over 2^40 rows are pretty rare...).
  - change the existing shortcut to look a the number of bits instead of
    using 32 limits.
  - add a test for minimal code coverage with over 40 bits sizes
  - attempt to improve the documentation
  - some comments were updates, hopefully for the better

The main idea behind the smaller primes is to avoid the expensive modmul 
implementation on most realistic cases.

-- 
Fabien.