[gomp] Speed up and improve CPU number checking

[Jakub Jelinek <jakub at redhat dot com>]

Hi!

sysconf (_SC_NPROCESSORS_ONLN) is horribly expensive on Linux (kernel's fault)
and because of that e.g. http://openmp.org/pipermail/omp/2007/000713.html
test is 10 times slower with omp_set_dynamic (1) compared to just using
the same number of threads with omp_set_dynamic (0).
In addition to this, if an OpenMP program is started bound just to a subset
of CPUs (say taskset -c 2-6 ./omp_program)), libgomp will happily count
even with CPUs it can't really use.
The following patch fixes both of this by using pthread_getaffinity_np
and popcount on the returned CPU set.  While on my quadcore box
omp_set_dynamic(0); omp_set_num_threads(4); testcase takes roughly .5sec,
without this patch omp_set_dynamic(1); testcase which also uses 4 threads
takes roughly 5sec and with this patch .6sec.
I'll commit in a few days unless I hear objections.

2007-05-02  Jakub Jelinek  <jakub@redhat.com>

	* config/linux/proc.c: New file.

--- libgomp/config/linux/proc.c.jj	2007-05-02 13:50:37.000000000 +0200
+++ libgomp/config/linux/proc.c	2007-05-02 16:00:47.000000000 +0200
@@ -0,0 +1,179 @@
+/* Copyright (C) 2005, 2006, 2007 Free Software Foundation, Inc.
+   Contributed by Jakub Jelinek <jakub@redhat.com>.
+
+   This file is part of the GNU OpenMP Library (libgomp).
+
+   Libgomp is free software; you can redistribute it and/or modify it
+   under the terms of the GNU Lesser General Public License as published by
+   the Free Software Foundation; either version 2.1 of the License, or
+   (at your option) any later version.
+
+   Libgomp is distributed in the hope that it will be useful, but WITHOUT ANY
+   WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+   FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public License for
+   more details.
+
+   You should have received a copy of the GNU Lesser General Public License 
+   along with libgomp; see the file COPYING.LIB.  If not, write to the
+   Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
+   MA 02110-1301, USA.  */
+
+/* As a special exception, if you link this library with other files, some
+   of which are compiled with GCC, to produce an executable, this library
+   does not by itself cause the resulting executable to be covered by the
+   GNU General Public License.  This exception does not however invalidate
+   any other reasons why the executable file might be covered by the GNU
+   General Public License.  */
+
+/* This file contains system specific routines related to counting
+   online processors and dynamic load balancing.  */
+
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE 1
+#endif
+#include "libgomp.h"
+#include <sched.h>
+#include <stdlib.h>
+#include <unistd.h>
+#ifdef HAVE_GETLOADAVG
+# ifdef HAVE_SYS_LOADAVG_H
+#  include <sys/loadavg.h>
+# endif
+#endif
+
+#ifdef HAVE_PTHREAD_AFFINITY_NP
+static unsigned long
+cpuset_popcount (cpu_set_t *cpusetp)
+{
+#ifdef CPU_COUNT
+  /* glibc 2.6 and above provide a macro for this.  */
+  return CPU_COUNT (cpusetp);
+#else
+  size_t i;
+  unsigned long ret = 0;
+  extern int check[sizeof (cpusetp->__bits[0]) == sizeof (unsigned long int)];
+
+  (void) check;
+  for (i = 0; i < sizeof (*cpusetp) / sizeof (cpusetp->__bits[0]); i++)
+    {
+      unsigned long int mask = cpusetp->__bits[i];
+      if (mask == 0)
+	continue;
+      ret += __builtin_popcountl (mask);
+    }
+  return ret;
+#endif
+}
+#endif
+
+/* At startup, determine the default number of threads.  It would seem
+   this should be related to the number of cpus online.  */
+
+void
+gomp_init_num_threads (void)
+{
+#ifdef HAVE_PTHREAD_AFFINITY_NP
+  cpu_set_t cpuset;
+
+  if (pthread_getaffinity_np (pthread_self (), sizeof (cpuset), &cpuset) == 0)
+    {
+      /* Count only the CPUs this process can use.  */
+      gomp_nthreads_var = cpuset_popcount (&cpuset);
+      if (gomp_nthreads_var == 0)
+	gomp_nthreads_var = 1;
+      return;
+    }
+#endif
+#ifdef _SC_NPROCESSORS_ONLN
+  gomp_nthreads_var = sysconf (_SC_NPROCESSORS_ONLN);
+#endif
+}
+
+static int
+get_num_procs (void)
+{
+#ifdef HAVE_PTHREAD_AFFINITY_NP
+  cpu_set_t cpuset;
+
+  if (gomp_cpu_affinity == NULL)
+    {
+      /* Count only the CPUs this process can use.  */
+      if (pthread_getaffinity_np (pthread_self (), sizeof (cpuset),
+				  &cpuset) == 0)
+	{
+	  int ret = cpuset_popcount (&cpuset);
+	  return ret != 0 ? ret : 1;
+	}
+    }
+  else
+    {
+      size_t idx;
+      static int affinity_cpus;
+
+      /* We can't use pthread_getaffinity_np in this case
+	 (we have changed it ourselves, it binds to just one CPU).
+	 Count instead the number of different CPUs we are
+	 using.  */
+      CPU_ZERO (&cpuset);
+      if (affinity_cpus == 0)
+	{
+	  int cpus = 0;
+	  for (idx = 0; idx < gomp_cpu_affinity_len; idx++)
+	    if (! CPU_ISSET (gomp_cpu_affinity[idx], &cpuset))
+	      {
+		cpus++;
+		CPU_SET (gomp_cpu_affinity[idx], &cpuset);
+	      }
+	  affinity_cpus = cpus;
+	}
+      return affinity_cpus;
+    }
+#endif
+#ifdef _SC_NPROCESSORS_ONLN
+  return sysconf (_SC_NPROCESSORS_ONLN);
+#else
+  return gomp_nthreads_var;
+#endif
+}
+
+/* When OMP_DYNAMIC is set, at thread launch determine the number of
+   threads we should spawn for this team.  */
+/* ??? I have no idea what best practice for this is.  Surely some
+   function of the number of processors that are *still* online and
+   the load average.  Here I use the number of processors online
+   minus the 15 minute load average.  */
+
+unsigned
+gomp_dynamic_max_threads (void)
+{
+  unsigned n_onln, loadavg;
+
+  n_onln = get_num_procs ();
+  if (n_onln > gomp_nthreads_var)
+    n_onln = gomp_nthreads_var;
+
+  loadavg = 0;
+#ifdef HAVE_GETLOADAVG
+  {
+    double dloadavg[3];
+    if (getloadavg (dloadavg, 3) == 3)
+      {
+	/* Add 0.1 to get a kind of biased rounding.  */
+	loadavg = dloadavg[2] + 0.1;
+      }
+  }
+#endif
+
+  if (loadavg >= n_onln)
+    return 1;
+  else
+    return n_onln - loadavg;
+}
+
+int
+omp_get_num_procs (void)
+{
+  return get_num_procs ();
+}
+
+ialias (omp_get_num_procs)

	Jakub