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Replace random(), pg_erand48(), etc with a better PRNG API and algorithm.

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Standardize on xoroshiro128** as our basic PRNG algorithm, eliminating
a bunch of platform dependencies as well as fundamentally-obsolete PRNG
code.  In addition, this API replacement will ease replacing the
algorithm again in future, should that become necessary.

xoroshiro128** is a few percent slower than the drand48 family,
but it can produce full-width 64-bit random values not only 48-bit,
and it should be much more trustworthy.  It's likely to be noticeably
faster than the platform's random(), depending on which platform you
are thinking about; and we can have non-global state vectors easily,
unlike with random().  It is not cryptographically strong, but neither
are the functions it replaces.

Fabien Coelho, reviewed by Dean Rasheed, Aleksander Alekseev, and myself

Discussion: https://postgr.es/m/alpine.DEB.2.22.394.2105241211230.165418@pseudo
This commit is contained in:
Tom Lane
2021-11-28 21:32:36 -05:00
parent f44ceb46ec
commit 3804539e48
50 changed files with 543 additions and 480 deletions
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7
src/backend/tcop/postgres.c
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@ -42,6 +42,7 @@
#include "catalog/pg_type.h"
#include "commands/async.h"
#include "commands/prepare.h"
#include "common/pg_prng.h"
#include "executor/spi.h"
#include "jit/jit.h"
#include "libpq/libpq.h"
@ -2355,13 +2356,13 @@ check_log_duration(char *msec_str, bool was_logged)
/*
* Do not log if log_statement_sample_rate = 0. Log a sample if
* log_statement_sample_rate <= 1 and avoid unnecessary random() call
* if log_statement_sample_rate = 1.
* log_statement_sample_rate <= 1 and avoid unnecessary PRNG call if
* log_statement_sample_rate = 1.
*/
if (exceeded_sample_duration)
in_sample = log_statement_sample_rate != 0 &&
(log_statement_sample_rate == 1 ||
random() <= log_statement_sample_rate * MAX_RANDOM_VALUE);
pg_prng_double(&pg_global_prng_state) <= log_statement_sample_rate);
if (exceeded_duration || in_sample || log_duration || xact_is_sampled)
{