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.

2 files changed, 48 insertions, 0 deletions

diff --git a/src/common/compat.c b/src/common/compat.c
index 20394b4c5d..b7f4f17cae 100644
--- a/src/common/compat.c
+++ b/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.)
  */
diff --git a/src/common/compat.h b/src/common/compat.h
index 7d59501e2b..5b6cfc40b5 100644
--- a/src/common/compat.h
+++ b/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. */