Make pgbench use erand48() rather than random().

glibc renders random() thread-safe by wrapping a futex lock around it;
testing reveals that this limits the performance of pgbench on machines
with many CPU cores.  Rather than switching to random_r(), which is
only available on GNU systems and crashes unless you use undocumented
alchemy to initialize the random state properly, switch to our built-in
implementation of erand48(), which is both thread-safe and concurrent.

Since the list of reasons not to use the operating system's erand48()
is getting rather long, rename ours to pg_erand48() (and similarly
for our implementations of lrand48() and srand48()) and just always
use those.  We were already doing this on Cygwin anyway, and the
glibc implementation is not quite thread-safe, so pgbench wouldn't
be able to use that either.

Per discussion with Tom Lane.

src/backend/optimizer/geqo/geqo_random.c

@@ -36,5 +36,5 @@ geqo_rand(PlannerInfo *root)
 {
 	GeqoPrivateData *private = (GeqoPrivateData *) root->join_search_private;
 
-	return erand48(private->random_state);
+	return pg_erand48(private->random_state);
 }

src/backend/optimizer/geqo/geqo_selection.c

@@ -64,9 +64,9 @@ geqo_selection(PlannerInfo *root, Chromosome *momma, Chromosome *daddy,
 	 * Ensure we have selected different genes, except if pool size is only
 	 * one, when we can't.
 	 *
-	 * This code has been observed to hang up in an infinite loop when the
-	 * platform's implementation of erand48() is broken.  We consider that a
-	 * feature: it lets you know you'd better fix the random-number generator.
+	 * This code was observed to hang up in an infinite loop when the
+	 * platform's implementation of erand48() was broken.  We now always
+	 * use our own version.
 	 */
 	if (pool->size > 1)
 	{

src/include/optimizer/geqo.h

@@ -73,7 +73,7 @@ extern double Geqo_seed;		/* 0 .. 1 */
 typedef struct
 {
 	List	   *initial_rels;	/* the base relations we are joining */
-	unsigned short random_state[3];		/* state for erand48() */
+	unsigned short random_state[3];		/* state for pg_erand48() */
 } GeqoPrivateData;
 
 

src/include/pg_config.h.in

@@ -149,9 +149,6 @@
 /* Define to 1 if you have the <editline/readline.h> header file. */
 #undef HAVE_EDITLINE_READLINE_H
 
-/* Define to 1 if you have the `erand48' function. */
-#undef HAVE_ERAND48
-
 /* Define to 1 if you have the `ERR_set_mark' function. */
 #undef HAVE_ERR_SET_MARK
 

src/include/port.h

@@ -380,12 +380,9 @@ extern off_t ftello(FILE *stream);
 #endif
 #endif
 
-#ifndef HAVE_ERAND48
-/* we assume all of these are present or missing together */
-extern double erand48(unsigned short xseed[3]);
-extern long lrand48(void);
-extern void srand48(long seed);
-#endif
+extern double pg_erand48(unsigned short xseed[3]);
+extern long pg_lrand48(void);
+extern void pg_srand48(long seed);
 
 #ifndef HAVE_FSEEKO
 #define fseeko(a, b, c) fseek(a, b, c)

src/port/Makefile

@@ -30,8 +30,8 @@ include $(top_builddir)/src/Makefile.global
 override CPPFLAGS := -I$(top_builddir)/src/port -DFRONTEND $(CPPFLAGS)
 LIBS += $(PTHREAD_LIBS)
 
-OBJS = $(LIBOBJS) chklocale.o dirmod.o exec.o inet_net_ntop.o noblock.o \
-	path.o pgcheckdir.o pgmkdirp.o pgsleep.o pgstrcasecmp.o \
+OBJS = $(LIBOBJS) chklocale.o dirmod.o erand48.o exec.o inet_net_ntop.o \
+	noblock.o path.o pgcheckdir.o pgmkdirp.o pgsleep.o pgstrcasecmp.o \
 	qsort.o qsort_arg.o sprompt.o thread.o
 
 # foo_srv.o and foo.o are both built from foo.c, but only foo.o has -DFRONTEND

src/port/erand48.c

@@ -2,10 +2,14 @@
  *
  * erand48.c
  *
- * This file supplies versions of erand48(), lrand48(), and srand48()
- * for machines that lack them.  (These are all the members of the drand48
- * family that Postgres currently requires.  We name the file after erand48
- * because that is the one that configure tests for.)
+ * This file supplies pg_erand48(), pg_lrand48(), and pg_srand48(), which
+ * are just like erand48(), lrand48(), and srand48() except that we use
+ * our own implementation rather than the one provided by the operating
+ * system.  We used to test for an operating system version rather than
+ * unconditionally using our own, but (1) some versions of Cygwin have a
+ * buggy erand48() that always returns zero and (2) as of 2011, glibc's
+ * erand48() is strangely coded to be almost-but-not-quite thread-safe,
+ * which doesn't matter for the backend but is important for pgbench.
  *
  *
  * Copyright (c) 1993 Martin Birgmeier
@@ -72,7 +76,7 @@ _dorand48(unsigned short xseed[3])
 
 
 double
-erand48(unsigned short xseed[3])
+pg_erand48(unsigned short xseed[3])
 {
 	_dorand48(xseed);
 	return ldexp((double) xseed[0], -48) +
@@ -81,14 +85,14 @@ erand48(unsigned short xseed[3])
 }
 
 long
-lrand48(void)
+pg_lrand48(void)
 {
 	_dorand48(_rand48_seed);
 	return ((long) _rand48_seed[2] << 15) + ((long) _rand48_seed[1] >> 1);
 }
 
 void
-srand48(long seed)
+pg_srand48(long seed)
 {
 	_rand48_seed[0] = RAND48_SEED_0;
 	_rand48_seed[1] = (unsigned short) seed;

src/port/random.c

@@ -21,5 +21,5 @@
 long
 random()
 {
-	return lrand48();
+	return pg_lrand48();
 }

src/port/srandom.c

@@ -21,5 +21,5 @@
 void
 srandom(unsigned int seed)
 {
-	srand48((long int) seed);
+	pg_srand48((long int) seed);
 }