Return configured processors in NumberOfProcessors() on posix platforms

base/sys_info_posix.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/sys_info.h"
 
 #include <errno.h>
 #include <string.h>
 #include <sys/param.h>
 #include <sys/resource.h>
@@ skipping 12 matching lines @@
 #include <sys/vfs.h>
 #define statvfs statfs  // Android uses a statvfs-like statfs struct and call.
 #else
 #include <sys/statvfs.h>
 #endif
 
 namespace {
 
 #if !defined(OS_OPENBSD)
 int NumberOfProcessors() {
-  // It seems that sysconf returns the number of "logical" processors on both
-  // Mac and Linux.  So we get the number of "online logical" processors.
-  long res = sysconf(_SC_NPROCESSORS_ONLN);
+  // sysconf returns the number of "logical" (not "physical") processors on both
+  // Mac and Linux.  So we get the number of max available "logical" processors.
+  //
+  // Note that the number of "currently online" processors may be fewer than the
+  // returned value of NumberOfProcessors(). On some platforms, the kernel may
+  // make some processors offline intermittently, to save power when system
+  // loading is low.
+  //
+  // One common use case that needs to know the processor count is to create
+  // optimal number of threads for optimization. It should make plan according
+  // to the number of "max available" processors instead of "currently online"
+  // ones. The kernel should be smart enough to make all processors online when
+  // it has sufficient number of threads waiting to run.
+  long res = sysconf(_SC_NPROCESSORS_CONF);
   if (res == -1) {
     NOTREACHED();
     return 1;
   }
 
   return static_cast<int>(res);
 }
 
 base::LazyInstance<
     base::internal::LazySysInfoValue<int, NumberOfProcessors> >::Leaky
@@ skipping 78 matching lines @@
   }
   return arch;
 }
 
 // static
 size_t SysInfo::VMAllocationGranularity() {
   return getpagesize();
 }
 
 }  // namespace base