diff --git a/src/util/stopwatch.h b/src/util/stopwatch.h
index 661d3762b..b2a397b6e 100644
--- a/src/util/stopwatch.h
+++ b/src/util/stopwatch.h
@@ -105,7 +105,7 @@ public:
             mach_timespec_t _stop;
             clock_get_time(m_host_clock, &_stop);
             m_time += (_stop.tv_sec - m_start.tv_sec) * 1000000000ull;
-            m_time += (_stop.tv_nsec - m_start.tv_nsec);
+	    m_time += (_stop.tv_nsec - m_start.tv_nsec);
             m_running = false;
         }
     }
@@ -146,7 +146,7 @@ public:
     
     void start() {
         if (!m_running) {
-            clock_gettime(CLOCK_THREAD_CPUTIME_ID, &m_start);
+            clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &m_start);
             m_running = true;
         }
     }
@@ -154,9 +154,10 @@ public:
     void stop() {
 	if (m_running) {
             struct timespec _stop;
-            clock_gettime(CLOCK_THREAD_CPUTIME_ID, &_stop);
+            clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &_stop);
             m_time += (_stop.tv_sec - m_start.tv_sec) * 1000000000ull;
-            m_time += (_stop.tv_nsec - m_start.tv_nsec);
+	    if (m_time != 0 || _stop.tv_nsec >= m_start.tv_nsec)
+	      m_time += (_stop.tv_nsec - m_start.tv_nsec);
             m_running = false;
         }
     }