Use CLOCK_REALTIME_COARSE when available.

src/base/platform/time.cc

Index: src/base/platform/time.cc
diff --git a/src/base/platform/time.cc b/src/base/platform/time.cc
index b6a11cff34f313661536eb0d20b43cf7d598e479..ad60270d47954e71806cdd1bee353a2f9ea77c2c 100644
--- a/src/base/platform/time.cc
+++ b/src/base/platform/time.cc
@@ -252,11 +252,31 @@ FILETIME Time::ToFiletime() const {
 #elif V8_OS_POSIX
 
 Time Time::Now() {
+#if V8_OS_LINUX

[Benedikt Meurer, 2015/05/06 09:40:34]

How about using #ifdef CLOCK_REALTIME_COARSE here?
Amd ise CLOCK_REALTIME_COARSE below instead of the handwritten constant?

+  struct timespec ts;
+  static clockid_t clock_id = -1;

[Dean McNamee, 2015/05/06 09:09:34]

I would maybe make a note that clockid_t is defined as an int.  The concern
would otherwise be, for example, if it were an int64 on 32-bit platforms and
this code is multithreaded, there could be a race on writing to clockid_t.  In
theory they would be writing the same value so it wouldn't really be a problem
but I usually think it's just good practice to make a note so that it is clear
that you considered the situation.

+  if (clock_id == -1) {
+    // CLOCK_REALTIME_COARSE is not supported on kernels <= 2.6.31.
+    // Probe the kernel to see if it's available and has <= 1 ms resolution.
+    static const clockid_t kClockRealTimeCoarse = 5;

[Dean McNamee, 2015/05/06 09:09:34]

Is there a reason you can't use the CLOCK_REALTIME_COARSE define?  Just header
versions?

+    if (-1 == clock_getres(kClockRealTimeCoarse, &ts) ||
+        ts.tv_nsec > 1000 * 1000 || ts.tv_sec > 0) {
+      clock_id = CLOCK_REALTIME;  // Not available or not suitable.
+    } else {
+      clock_id = kClockRealTimeCoarse;

[Dean McNamee, 2015/05/06 09:09:34]

I would make a note, if possible, about what the difference in precision is. 
The man page just says:

   CLOCK_REALTIME_COARSE (since Linux 2.6.32; Linux-specific)
              A faster but less precise version of CLOCK_REALTIME.  Use when
              you need very fast, but not fine-grained timestamps.

Are we talking milliseconds, microseconds, etc?

+    }
+  }
+  int result = clock_gettime(clock_id, &ts);
+  DCHECK_EQ(0, result);
+  USE(result);
+  return FromTimespec(ts);
+#else
   struct timeval tv;
   int result = gettimeofday(&tv, NULL);
   DCHECK_EQ(0, result);
   USE(result);
   return FromTimeval(tv);
+#endif
 }