Horton

Re: Popcount optimization using AVX512

Nathan Bossart <nathandbossart@gmail.com>

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From: Nathan Bossart <nathandbossart@gmail.com>
To: "Amonson, Paul D" <paul.d.amonson@intel.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, David Rowley <dgrowleyml@gmail.com>, Andres Freund <andres@anarazel.de>, Alvaro Herrera <alvherre@alvh.no-ip.org>, "Shankaran, Akash" <akash.shankaran@intel.com>, Noah Misch <noah@leadboat.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Date: 2024-03-29T15:35:15Z
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. Fix __attribute__((target(...))) usage.

  2. Use __attribute__((target(...))) for AVX-512 support.

  3. Fix code for probing availability of AVX-512.

  4. Optimize visibilitymap_count() with AVX-512 instructions.

  5. Optimize pg_popcount() with AVX-512 instructions.

  6. Inline pg_popcount() for small buffers.

  7. Avoid function call overhead of pg_popcount() in syslogger.c.

  8. Refactor code for setting pg_popcount* function pointers.

  9. Inline pg_popcount{32,64} into pg_popcount().

  10. Remove MSVC scripts

  11. Use ARMv8 CRC instructions where available.

  12. Use Intel SSE 4.2 CRC instructions where available.

On Thu, Mar 28, 2024 at 10:03:04PM +0000, Amonson, Paul D wrote:
>> * I think we need to verify there isn't a huge performance regression for
>>   smaller arrays.  IIUC those will still require an AVX512 instruction or
>>   two as well as a function call, which might add some noticeable overhead.
> 
> Not considering your changes, I had already tested small buffers. At less
> than 512 bytes there was no measurable regression (there was one extra
> condition check) and for 512+ bytes it moved from no regression to some
> gains between 512 and 4096 bytes. Assuming you introduced no extra
> function calls, it should be the same.

Cool.  I think we should run the benchmarks again to be safe, though.

>> I forgot to mention that I also want to understand whether we can
>> actually assume availability of XGETBV when CPUID says we support
>> AVX512:
> 
> You cannot assume as there are edge cases where AVX-512 was found on
> system one during compile but it's not actually available in a kernel on
> a second system at runtime despite the CPU actually having the hardware
> feature.

Yeah, I understand that much, but I want to know how portable the XGETBV
instruction is.  Unless I can assume that all x86_64 systems and compilers
support that instruction, we might need an additional configure check
and/or CPUID check.  It looks like MSVC has had support for the _xgetbv
intrinsic for quite a while, but I'm still researching the other cases.

-- 
Nathan Bossart
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