Autodetect the number of CPUs when possible if NumCPUs==0

This is needed for IOCP, since telling the IOCP backend about all
your CPUs is a good idea.  It'll also come in handy with asn's
multithreaded crypto stuff, and for people who run servers without
reading the manual.

changes/cpudetect

@@ -0,0 +1,3 @@
+  o Minor features
+    - If you set the NumCPUs option to 0, Tor will try to detect how many
+      CPUs you have.  This is the new default behavior.

configure.in

@@ -226,7 +226,7 @@ dnl -------------------------------------------------------------------
 dnl Check for functions before libevent, since libevent-1.2 apparently
 dnl exports strlcpy without defining it in a header.
 
-AC_CHECK_FUNCS(gettimeofday ftime socketpair uname inet_aton strptime getrlimit strlcat strlcpy strtoull getaddrinfo localtime_r gmtime_r memmem strtok_r flock prctl vasprintf)
+AC_CHECK_FUNCS(gettimeofday ftime socketpair uname inet_aton strptime getrlimit strlcat strlcpy strtoull getaddrinfo localtime_r gmtime_r memmem strtok_r flock prctl vasprintf sysconf)
 
 using_custom_malloc=no
 if test x$enable_openbsd_malloc = xyes ; then

doc/tor.1.txt

@@ -860,7 +860,9 @@ is non-zero):
     characters inclusive, and must contain only the characters [a-zA-Z0-9].
 
 **NumCPUs** __num__::
-    How many processes to use at once for decrypting onionskins. (Default: 1)
+    How many processes to use at once for decrypting onionskins and other
+    parallelizable operations.  If this is set to 0, Tor will try to detect
+    how many CPUs you have, defaulting to 1 if it can't tell.  (Default: 0)
 
 **ORPort** __PORT__::
     Advertise this port to listen for connections from Tor clients and servers.

src/common/compat.c

@@ -1898,6 +1898,52 @@ spawn_exit(void)
 #endif
 }
 
+/** Implementation logic for compute_num_cpus(). */
+static int
+compute_num_cpus_impl(void)
+{
+#ifdef MS_WINDOWS
+  SYSTEM_INFO info;
+  memset(&info, 0, sizeof(info));
+  GetSystemInfo(&info);
+  if (info.dwNumberOfProcessors >= 1 && info.dwNumberOfProcessors < INT_MAX)
+    return (int)info.dwNumberOfProcessors;
+  else
+    return -1;
+#elif defined(HAVE_SYSCONF) && defined(_SC_NPROCESSORS_CONF)
+  long cpus = sysconf(_SC_NPROCESSORS_CONF);
+  if (cpus >= 1 && cpus < INT_MAX)
+    return (int)cpus;
+  else
+    return -1;
+#else
+  return -1;
+#endif
+}
+
+#define MAX_DETECTABLE_CPUS 16
+
+/** Return how many CPUs we are running with.  We assume that nobody is
+ * using hot-swappable CPUs, so we don't recompute this after the first
+ * time.  Return -1 if we don't know how to tell the number of CPUs on this
+ * system.
+ */
+int
+compute_num_cpus(void)
+{
+  static int num_cpus = -2;
+  if (num_cpus == -2) {
+    num_cpus = compute_num_cpus_impl();
+    tor_assert(num_cpus != -2);
+    if (num_cpus > MAX_DETECTABLE_CPUS)
+      log_notice(LD_GENERAL, "Wow!  I detected that you have %d CPUs. I "
+                 "will not autodetect any more than %d, though.  If you "
+                 "want to configure more, set NumCPUs in your torrc",
+                 num_cpus, MAX_DETECTABLE_CPUS);
+  }
+  return num_cpus;
+}
+
 /** Set *timeval to the current time of day.  On error, log and terminate.
  * (Same as gettimeofday(timeval,NULL), but never returns -1.)
  */

src/common/compat.h

@@ -522,6 +522,8 @@ void spawn_exit(void) ATTR_NORETURN;
 #undef TOR_IS_MULTITHREADED
 #endif
 
+int compute_num_cpus(void);
+
 /* Because we use threads instead of processes on most platforms (Windows,
  * Linux, etc), we need locking for them.  On platforms with poor thread
  * support or broken gethostbyname_r, these functions are no-ops. */

src/or/config.c

@@ -307,7 +307,7 @@ static config_var_t _option_vars[] = {
   V(WarnUnsafeSocks,              BOOL,     "1"),
   V(NoPublish,                   BOOL,     "0"),
   VAR("NodeFamily",              LINELIST, NodeFamilies,         NULL),
-  V(NumCpus,                     UINT,     "1"),
+  V(NumCpus,                     UINT,     "0"),
   V(NumEntryGuards,              UINT,     "3"),
   V(ORListenAddress,             LINELIST, NULL),
   V(ORPort,                      UINT,     "0"),
@@ -4892,6 +4892,19 @@ config_parse_interval(const char *s, int *ok)
   return (int)r;
 }
 
+/** Return the number of cpus configured in <b>options</b>.  If we are
+ * told to auto-detect the number of cpus, return the auto-detected number. */
+int
+get_num_cpus(const or_options_t *options)
+{
+  if (options->NumCpus == 0) {
+    int n = compute_num_cpus();
+    return (n >= 1) ? n : 1;
+  } else {
+    return options->NumCpus;
+  }
+}
+
 /**
  * Initialize the libevent library.
  */
@@ -4913,7 +4926,7 @@ init_libevent(const or_options_t *options)
 
   memset(&cfg, 0, sizeof(cfg));
   cfg.disable_iocp = options->DisableIOCP;
-  cfg.num_cpus = options->NumCpus;
+  cfg.num_cpus = get_num_cpus(options);
 
   tor_libevent_initialize(&cfg);
 

src/or/config.h

@@ -57,6 +57,8 @@ char *options_get_datadir_fname2_suffix(or_options_t *options,
 #define get_datadir_fname_suffix(sub1, suffix) \
   get_datadir_fname2_suffix((sub1), NULL, (suffix))
 
+int get_num_cpus(const or_options_t *options);
+
 or_state_t *get_or_state(void);
 int or_state_save(time_t now);
 

src/or/cpuworker.c

@@ -366,7 +366,7 @@ spawn_cpuworker(void)
 static void
 spawn_enough_cpuworkers(void)
 {
-  int num_cpuworkers_needed = get_options()->NumCpus;
+  int num_cpuworkers_needed = get_num_cpus(get_options());
 
   if (num_cpuworkers_needed < MIN_CPUWORKERS)
     num_cpuworkers_needed = MIN_CPUWORKERS;