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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
Download Patch File Download Diff File Expand all files Collapse all files

3
src/backend/access/gin/ginget.c
View File

@@ -16,6 +16,7 @@
#include "access/gin_private.h"
#include "access/relscan.h"
#include "common/pg_prng.h"
#include "miscadmin.h"
#include "storage/predicate.h"
#include "utils/datum.h"
@@ -787,7 +788,7 @@ entryLoadMoreItems(GinState *ginstate, GinScanEntry entry,
}
}
#define gin_rand() (((double) random()) / ((double) MAX_RANDOM_VALUE))
#define gin_rand() pg_prng_double(&pg_global_prng_state)
#define dropItem(e) ( gin_rand() > ((double)GinFuzzySearchLimit)/((double)((e)->predictNumberResult)) )
/*

5
src/backend/access/gist/gistutil.c
View File

@@ -19,6 +19,7 @@
#include "access/htup_details.h"
#include "access/reloptions.h"
#include "catalog/pg_opclass.h"
#include "common/pg_prng.h"
#include "storage/indexfsm.h"
#include "storage/lmgr.h"
#include "utils/float.h"
@@ -507,7 +508,7 @@ gistchoose(Relation r, Page p, IndexTuple it, /* it has compressed entry */
if (keep_current_best == -1)
{
/* we didn't make the random choice yet for this old best */
keep_current_best = (random() <= (MAX_RANDOM_VALUE / 2)) ? 1 : 0;
keep_current_best = pg_prng_bool(&pg_global_prng_state) ? 1 : 0;
}
if (keep_current_best == 0)
{
@@ -529,7 +530,7 @@ gistchoose(Relation r, Page p, IndexTuple it, /* it has compressed entry */
if (keep_current_best == -1)
{
/* we didn't make the random choice yet for this old best */
keep_current_best = (random() <= (MAX_RANDOM_VALUE / 2)) ? 1 : 0;
keep_current_best = pg_prng_bool(&pg_global_prng_state) ? 1 : 0;
}
if (keep_current_best == 1)
break;

3
src/backend/access/nbtree/nbtinsert.c
View File

@@ -19,6 +19,7 @@
#include "access/nbtxlog.h"
#include "access/transam.h"
#include "access/xloginsert.h"
#include "common/pg_prng.h"
#include "lib/qunique.h"
#include "miscadmin.h"
#include "storage/lmgr.h"
@@ -965,7 +966,7 @@ _bt_findinsertloc(Relation rel,
if (P_RIGHTMOST(opaque) ||
_bt_compare(rel, itup_key, page, P_HIKEY) != 0 ||
random() <= (MAX_RANDOM_VALUE / 100))
pg_prng_uint32(&pg_global_prng_state) <= (PG_UINT32_MAX / 100))
break;
_bt_stepright(rel, insertstate, stack);

5
src/backend/access/spgist/spgdoinsert.c
View File

@@ -19,6 +19,7 @@
#include "access/spgist_private.h"
#include "access/spgxlog.h"
#include "access/xloginsert.h"
#include "common/pg_prng.h"
#include "miscadmin.h"
#include "storage/bufmgr.h"
#include "utils/rel.h"
@@ -2210,7 +2211,9 @@ spgdoinsert(Relation index, SpGistState *state,
if (out.resultType == spgAddNode)
elog(ERROR, "cannot add a node to an allTheSame inner tuple");
else if (out.resultType == spgMatchNode)
out.result.matchNode.nodeN = random() % innerTuple->nNodes;
out.result.matchNode.nodeN =
pg_prng_uint64_range(&pg_global_prng_state,
0, innerTuple->nNodes - 1);
}
switch (out.resultType)

3
src/backend/access/transam/xact.c
View File

@@ -37,6 +37,7 @@
#include "commands/async.h"
#include "commands/tablecmds.h"
#include "commands/trigger.h"
#include "common/pg_prng.h"
#include "executor/spi.h"
#include "libpq/be-fsstubs.h"
#include "libpq/pqsignal.h"
@@ -1990,7 +1991,7 @@ StartTransaction(void)
/* Determine if statements are logged in this transaction */
xact_is_sampled = log_xact_sample_rate != 0 &&
(log_xact_sample_rate == 1 ||
random() <= log_xact_sample_rate * MAX_RANDOM_VALUE);
pg_prng_double(&pg_global_prng_state) <= log_xact_sample_rate);
/*
* initialize current transaction state fields