Improved support for OSX timers.

opal/mca/timer/darwin/timer_darwin.h

@@ -27,17 +27,29 @@ typedef uint64_t opal_timer_t;
 /* frequency in mhz */
 OPAL_DECLSPEC extern opal_timer_t opal_timer_darwin_freq;
 OPAL_DECLSPEC extern mach_timebase_info_data_t opal_timer_darwin_info;
+OPAL_DECLSPEC extern opal_timer_t opal_timer_darwin_bias;
 
+/**
+ * Use the pragmatic solution proposed at
+ * http://stackoverflow.com/questions/23378063/how-can-i-use-mach-absolute-time-without-overflowing/23378064#23378064
+ */
 static inline opal_timer_t
 opal_timer_base_get_cycles(void)
 {
+    uint64_t now = mach_absolute_time();
+
     if( opal_timer_darwin_info.denom == 0 ) {
-        (void) mach_timebase_info(&opal_timer_darwin_info);
+        (void)mach_timebase_info(&opal_timer_darwin_info);
+        if( opal_timer_darwin_info.denom > 1024 ) {
+            double frac = (double)opal_timer_darwin_info.numer/opal_timer_darwin_info.denom;
+            opal_timer_darwin_info.denom = 1024;
+            opal_timer_darwin_info.numer = opal_timer_darwin_info.denom * frac + 0.5;
+        }
+        opal_timer_darwin_bias = now;
     }
-    /* this is basically a wrapper around the "right" assembly to get
-       the tick counter off the PowerPC Time Base.  I believe it's
-       something similar on x86 */
-    return mach_absolute_time() * opal_timer_darwin_info.numer / opal_timer_darwin_info.denom / 1000;
+    /* this is basically a wrapper around the "right" assembly to convert
+       the tick counter off the PowerPC Time Base into nanos. */
+    return (now - opal_timer_darwin_bias) * opal_timer_darwin_info.numer / opal_timer_darwin_info.denom;
 }
 
 
@@ -45,7 +57,7 @@ static inline opal_timer_t
 opal_timer_base_get_usec(void)
 {
     /* freq is in Hz, so this gives usec */
-    return mach_absolute_time() * 1000000  / opal_timer_darwin_freq;
+    return opal_timer_base_get_cycles() / 1000;
 }
 
 

opal/mca/timer/darwin/timer_darwin_component.c

@@ -26,6 +26,7 @@
 
 opal_timer_t opal_timer_darwin_freq;
 mach_timebase_info_data_t opal_timer_darwin_info = {.denom = 0};
+opal_timer_t opal_timer_darwin_bias;
 
 static int opal_timer_darwin_open(void);
 
@@ -51,50 +52,48 @@ const opal_timer_base_component_2_0_0_t mca_timer_darwin_component = {
     },
 };
 
+/* mach_timebase_info() returns a fraction that can be multiplied
+   by the difference between two calls to mach_absolute_time() to
+   get the number of nanoseconds that passed between the two
+   calls.  
 
+   On PPC, mach_timebase_info returns numer = 1000000000 and denom
+   = 33333335 (or possibly 25000000, depending on the machine).
+   mach_absolute_time() returns a cycle count from the global
+   clock, which runs at 25 - 33MHz, so dividing the cycle count by
+   the frequency gives you seconds between the interval, then
+   multiplying by 1000000000 gives you nanoseconds.  Of course,
+   you should do the multiply first, then the divide to reduce
+   arithmetic errors due to integer math.  But since we want the
+   least amount of math in the critical path as possible and
+   mach_absolute_time is already a cycle counter, we claim we have
+   native cycle count support and set the frequencey to be the
+   frequencey of the global clock, which is sTBI.denom *
+   (1000000000 / sTBI.numer), which is sTBI.denom * (1 / 1), or
+   sTBI.denom.
+
+   On Intel, mach_timebase_info returns numer = 1 nd denom = 1,
+   meaning that mach_absolute_time() returns some global clock
+   time in nanoseconds.  Because PPC returns a frequency and
+   returning a time in microseconds would still require math in
+   the critical path (a divide, at that), we pretend that the
+   nanosecond timer is instead a cycle counter for a 1GHz clock
+   and that we're returning a cycle count natively.  so sTBI.denom
+   * (1000000000 / sTBI.numer) gives us 1 * (1000000000 / 1), or
+   1000000000, meaning we have a 1GHz clock.
+
+   More generally, since mach_timebase_info() gives the "keys" to
+   transition the return from mach_absolute_time() into
+   nanoseconds, taking the reverse of that and multipling by
+   1000000000 will give you a frequency in cycles / second if you
+   think of mach_absolute_time() always returning a cycle count.
+*/
 int opal_timer_darwin_open(void)
 {
-    mach_timebase_info_data_t sTBI;
+    /* Call the opal_timer_base_get_cycles once to start the enging */
+    (void)opal_timer_base_get_cycles();
 
-    mach_timebase_info(&sTBI);
-
-    /* mach_timebase_info() returns a fraction that can be multiplied
-       by the difference between two calls to mach_absolute_time() to
-       get the number of nanoseconds that passed between the two
-       calls.  
-
-       On PPC, mach_timebase_info returns numer = 1000000000 and denom
-       = 33333335 (or possibly 25000000, depending on the machine).
-       mach_absolute_time() returns a cycle count from the global
-       clock, which runs at 25 - 33MHz, so dividing the cycle count by
-       the frequency gives you seconds between the interval, then
-       multiplying by 1000000000 gives you nanoseconds.  Of course,
-       you should do the multiply first, then the divide to reduce
-       arithmetic errors due to integer math.  But since we want the
-       least amount of math in the critical path as possible and
-       mach_absolute_time is already a cycle counter, we claim we have
-       native cycle count support and set the frequencey to be the
-       frequencey of the global clock, which is sTBI.denom *
-       (1000000000 / sTBI.numer), which is sTBI.denom * (1 / 1), or
-       sTBI.denom.
-
-       On Intel, mach_timebase_info returns numer = 1 nd denom = 1,
-       meaning that mach_absolute_time() returns some global clock
-       time in nanoseconds.  Because PPC returns a frequency and
-       returning a time in microseconds would still require math in
-       the critical path (a divide, at that), we pretend that the
-       nanosecond timer is instead a cycle counter for a 1GHz clock
-       and that we're returning a cycle count natively.  so sTBI.denom
-       * (1000000000 / sTBI.numer) gives us 1 * (1000000000 / 1), or
-       1000000000, meaning we have a 1GHz clock.
-
-       More generally, since mach_timebase_info() gives the "keys" to
-       transition the return from mach_absolute_time() into
-       nanoseconds, taking the reverse of that and multipling by
-       1000000000 will give you a frequency in cycles / second if you
-       think of mach_absolute_time() always returning a cycle count.
-    */
-    opal_timer_darwin_freq = sTBI.denom * (1000000000 / sTBI.numer);
+    opal_timer_darwin_freq = opal_timer_darwin_info.denom * (1000000000 / opal_timer_darwin_info.numer);
 
     return OPAL_SUCCESS;
 }