commit 2b53b3334fea4e6f64a3aec9e2487e711f9a77dd
Author: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Date:   Wed Oct 1 16:50:59 2014 -0400

    * ext/etc/etc.c (etc_nprocessors_affinity): use sched_getaffinity
      for getting precious number of available cpus.
    * ext/etc/etc.c (etc_nprocessors): use etc_nprocessors_affinity if
      possible.

diff --git a/ChangeLog b/ChangeLog
index 35a55ab..cfec9fe 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Thu Oct  2 05:50:59 2014  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
+
+	* ext/etc/etc.c (etc_nprocessors_affinity): use sched_getaffinity
+	  for getting precious number of available cpus.
+	* ext/etc/etc.c (etc_nprocessors): use etc_nprocessors_affinity if
+	  possible.
+
 Thu Oct  2 03:42:12 2014  Tanaka Akira  <akr@fsij.org>
 
 	* ext/etc/etc.c (etc_nprocessors_online): New method.
diff --git a/configure.in b/configure.in
index c6142a4..70de1a7 100644
--- a/configure.in
+++ b/configure.in
@@ -2033,6 +2033,7 @@ AC_CHECK_FUNCS(pread)
 AC_CHECK_FUNCS(qsort_r)
 AC_CHECK_FUNCS(readlink)
 AC_CHECK_FUNCS(round)
+AC_CHECK_FUNCS(sched_getaffinity)
 AC_CHECK_FUNCS(seekdir)
 AC_CHECK_FUNCS(select_large_fdset)
 AC_CHECK_FUNCS(sendfile)
diff --git a/ext/etc/etc.c b/ext/etc/etc.c
index 2546560..6a596cb 100644
--- a/ext/etc/etc.c
+++ b/ext/etc/etc.c
@@ -30,6 +30,10 @@
 #include <sys/utsname.h>
 #endif
 
+#ifdef HAVE_SCHED_GETAFFINITY
+#include <sched.h>
+#endif
+
 static VALUE sPasswd;
 #ifdef HAVE_GETGRENT
 static VALUE sGroup;
@@ -891,7 +895,35 @@ io_pathconf(VALUE io, VALUE arg)
 #define io_pathconf rb_f_notimplement
 #endif
 
+
 #if defined(HAVE_SYSCONF) && defined(_SC_NPROCESSORS_ONLN)
+#ifdef HAVE_SCHED_GETAFFINITY
+static VALUE
+etc_nprocessors_affinity(long ncpus)
+{
+    cpu_set_t *cpuset;
+    size_t size;
+    int ret;
+
+    cpuset = CPU_ALLOC((int)ncpus);
+    if (!cpuset) {
+	rb_sys_fail("CPU_ALLOC");
+    }
+    size = CPU_ALLOC_SIZE(ncpus);
+    CPU_ZERO_S(size, cpuset);
+
+    ret = sched_getaffinity(0, size, cpuset);
+    if (ret==-1) {
+	rb_sys_fail("sched_getaffinity");
+    }
+
+    ret = CPU_COUNT_S(size, cpuset);
+    CPU_FREE(cpuset);
+
+    return INT2NUM(ret);
+}
+#endif
+
 /*
  * Returns the number of online processors.
  *
@@ -900,12 +932,21 @@ io_pathconf(VALUE io, VALUE arg)
  *
  * This method is implemented as:
  * - sysconf(_SC_NPROCESSORS_ONLN): GNU/Linux, NetBSD, FreeBSD, OpenBSD, DragonFly BSD, OpenIndiana, Mac OS X, AIX
+ * - sched_getaffinity(): Linux
  *
  * Example:
  *
  *   require 'etc'
  *   p Etc.nprocessors #=> 4
  *
+ * The result might be smaller number than physical cpus especially when ruby
+ * process is bound to specific cpus. This is intended for getting better
+ * parallel processing.
+ *
+ * Example: (Linux)
+ *
+ *   $ taskset 0x3 ./ruby -retc -e "p Etc.nprocessors"  #=> 2
+ *
  */
 static VALUE
 etc_nprocessors(VALUE obj)
@@ -917,6 +958,11 @@ etc_nprocessors(VALUE obj)
     if (ret == -1) {
         rb_sys_fail("sysconf(_SC_NPROCESSORS_ONLN)");
     }
+
+#ifdef HAVE_SCHED_GETAFFINITY
+    return etc_nprocessors_affinity(ret);
+#endif
+
     return LONG2NUM(ret);
 }
 #else