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From: Nathan Bossart <[email protected]>
To: Amonson, Paul D <[email protected]>
Cc: Tom Lane <[email protected]>
Cc: David Rowley <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: Alvaro Herrera <[email protected]>
Cc: Shankaran, Akash <[email protected]>
Cc: Noah Misch <[email protected]>
Cc: Matthias van de Meent <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: Popcount optimization using AVX512
Date: Sat, 30 Mar 2024 15:03:29 -0500
Message-ID: <20240330200329.GA2047466@nathanxps13> (raw)
In-Reply-To: <20240330032209.GA2018686@nathanxps13>
References: <20240328213854.GA916732@nathanxps13>
	<BL1PR11MB5304E51336123CE6F041A920DC3B2@BL1PR11MB5304.namprd11.prod.outlook.com>
	<BL1PR11MB5304217FFEFB15124CD2EC9EDC3B2@BL1PR11MB5304.namprd11.prod.outlook.com>
	<20240329155940.GC1046039@nathanxps13>
	<20240329162211.GB1106154@nathanxps13>
	<BL1PR11MB5304D41D49B56986097713AFDC3A2@BL1PR11MB5304.namprd11.prod.outlook.com>
	<20240329191312.GA1411904@nathanxps13>
	<20240329200828.GB1411904@nathanxps13>
	<20240329205741.GA1593561@nathanxps13>
	<20240330032209.GA2018686@nathanxps13>

I used John Naylor's test_popcount module [0] to put together the attached
graphs (note that the "small arrays" one is semi-logarithmic).  For both
graphs, the X-axis is the number of 64-bit words in the array, and Y-axis
is the amount of time in milliseconds to run pg_popcount() on it 100,000
times (along with a bit of overhead).  This test didn't show any
regressions with a relatively small number of bytes, and it showed the
expected improvements with many bytes.

There isn't a ton of use of pg_popcount() in Postgres, but I do see a few
places that call it with enough bytes for the AVX512 optimization to take
effect.  There may be more callers in the future, though, and it seems
generally useful to have some of the foundational work for using AVX512
instructions in place.  My current plan is to add some new tests for
pg_popcount() with many bytes, and then I'll give it a few more days for
any additional feedback before committing.

[0] https://postgr.es/m/CAFBsxsE7otwnfA36Ly44zZO+b7AEWHRFANxR1h1kxveEV=ghLQ@mail.gmail.com

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com


Attachments:

  [image/jpeg] avx512_popcnt.jpg (17.0K, ../20240330200329.GA2047466@nathanxps13/2-avx512_popcnt.jpg)
  download | view image

  [image/jpeg] avx512_popcnt_small_arrays.jpg (13.8K, ../20240330200329.GA2047466@nathanxps13/3-avx512_popcnt_small_arrays.jpg)
  download | view image

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