Horton

Re: pgbench - add pseudo-random permutation function

Thomas Munro <thomas.munro@enterprisedb.com>

View thread
From: Thomas Munro <thomas.munro@enterprisedb.com>
To: Fabien COELHO <coelho@cri.ensmp.fr>
Cc: hironobu@interdb.jp, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2018-09-26T04:49:09Z
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.

On Wed, Sep 19, 2018 at 7:07 AM Fabien COELHO <coelho@cri.ensmp.fr> wrote:
> > I reviewed 'pgbench-prp-func/pgbench-prp-func-4.patch'. [...] I thinks
> > this patch is fine.

modular_multiply() is an interesting device.  I will leave this to
committers with a stronger mathematical background than me, but I have
a small comment in passing:

+#ifdef HAVE__BUILTIN_POPCOUNTLL
+ return __builtin_popcountll(n);
+#else /* use clever no branching bitwise operator version */

I think it is not enough for the compiler to have
__builtin_popcountll().  The CPU that this is eventually executed on
must actually have the POPCNT instruction[1] (or equivalent on ARM,
POWER etc), or the program will die with SIGILL.  There are CPUs in
circulation produced in this decade that don't have it.  I have
previously considered something like this[2], but realised you would
therefore need a runtime check.  There are a couple of ways to do
that: see commit a7a73875 for one example, also
__builtin_cpu_supports(), and direct CPU ID bit checks (some
platforms).  There is also the GCC "multiversion" system that takes
care of that for you but that worked only for C++ last time I checked.

[1] https://en.wikipedia.org/wiki/SSE4#POPCNT_and_LZCNT
[2] https://www.postgresql.org/message-id/CAEepm%3D3g1_fjJGp38QGv%2B38BC2HHVkzUq6s69nk3mWLgPHqC3A%40mail.gmail.com

-- 
Thomas Munro
http://www.enterprisedb.com