add support for the BGQ to the gettime function

gettime.c

@@ -35,18 +35,38 @@
 double gettime(void) {
   double t;
 #if (defined BGL && !defined BGP)
+
   const double clockspeed=1.0e-6/700.0;
   t = rts_get_timebase() * clockspeed;
+
 #elif defined MPI
+
   t = MPI_Wtime();
+
   /* clock_gettime is detected on BGL/BGP but it is an unsupported system call so we can't use it! */
 #elif (defined HAVE_CLOCK_GETTIME && !defined BGL)
+
   struct timespec ts;
+
+  /* on the BGQ the monotonic clock is directly connected to the hardware counters
+  and reports process CPU time, that is not a good measurement for threaded applications */
+#  ifdef BGQ
+  clock_gettime(CLOCK_REALTIME,&ts);
+#  else
   clock_gettime(CLOCK_MONOTONIC,&ts);
+#  endif
   t = ts.tv_sec + 1.0e-9*ts.tv_nsec;
+
 #else
+  /* This number is completely unreliable because the operating system and other processes
+     make the clock tick too. This is especially true with multiple threads where the number
+     of clock ticks will be multiplied by roughly the number of threads, but not quite, making
+     the measurement useless!  */
+
   t = (double)clock()/(CLOCKS_PER_SEC);
+
 #endif
+
   return t;
 }