Use CLOCK_MONOTONIC_COARSE and CLOCK_REALTIME_COARSE on Linux if available.

src/platform/time.cc

Index: src/platform/time.cc
diff --git a/src/platform/time.cc b/src/platform/time.cc
index de0ca16473f6b5106485508653cfe797f6632c37..6d95ed2a3f2641edccb86ac8a2b4b4535cbdd653 100644
--- a/src/platform/time.cc
+++ b/src/platform/time.cc
@@ -43,6 +43,10 @@
 #include "win32-headers.h"
 #endif
 
+#if V8_OS_LINUX && !defined(CLOCK_MONOTONIC_COARSE)
+#define CLOCK_MONOTONIC_COARSE 6  // 2.6.32 and up.
+#endif
+
 namespace v8 {
 namespace internal {
 
@@ -570,7 +574,23 @@ TimeTicks TimeTicks::HighResolutionNow() {
   ticks = (tv.tv_sec * Time::kMicrosecondsPerSecond + tv.tv_usec);
 #elif V8_OS_POSIX
   struct timespec ts;
+#if V8_OS_LINUX
+  // Prefer CLOCK_MONOTONIC_COARSE, a high-res clock that's available in
+  // 2.6.32 and can be fully serviced from the vDSO, even on virtualized
+  // systems.  Check that it's not *too* coarse though.
+  static clock_t clock_id = -1;

[Benedikt Meurer, 2013/10/31 06:44:50]

I think we can safely use zero here, i.e. no explicit initialization (as Xorg
does).

[bnoordhuis, 2013/10/31 12:22:24]

I wrote it that way because 0 is a valid clock (CLOCK_REALTIME.)  I'm fine with
either though, just say the word.

[Benedikt Meurer, 2013/11/04 09:09:57]

Please just use 0 here.

+  if (clock_id == -1) {
+    if (clock_getres(CLOCK_MONOTONIC_COARSE, &ts) == 0 &&

[Benedikt Meurer, 2013/10/31 06:44:50]

This won't compile on non-Linux systems that miss the CLOCK_MONOTONIC_COARSE
define.

[bnoordhuis, 2013/10/31 12:22:24]

If it weren't for the #if V8_OS_LINUX a few lines up, you mean? :-)

[Benedikt Meurer, 2013/11/04 09:09:57]

Ups, right. Ok, that's fine.

+        ts.tv_nsec <= 1 * 1000 * 1000) {  // 1 ms granularity or less.
+      clock_id = CLOCK_MONOTONIC_COARSE;
+    } else {
+      clock_id = CLOCK_MONOTONIC;
+    }
+  }
+  int result = clock_gettime(clock_id, &ts);
+#else
   int result = clock_gettime(CLOCK_MONOTONIC, &ts);
+#endif  // V8_OS_LINUX
   ASSERT_EQ(0, result);
   USE(result);
   ticks = (ts.tv_sec * Time::kMicrosecondsPerSecond +