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..a6c426f62db48c4e71f18b4b5789744baf1c597a 100644
--- a/src/platform/time.cc
+++ b/src/platform/time.cc
@@ -28,6 +28,7 @@
 #include "platform/time.h"
 
 #if V8_OS_POSIX
+#include <errno.h>
 #include <sys/time.h>
 #endif
 #if V8_OS_MACOSX
@@ -270,6 +271,8 @@ FILETIME Time::ToFiletime() const {
 
 #elif V8_OS_POSIX
 
+// OS X doesn't support POSIX high-res timers and Solaris uses gethrtime().
+#if V8_OS_MACOSX || V8_OS_SOLARIS
 Time Time::Now() {
   struct timeval tv;
   int result = gettimeofday(&tv, NULL);
@@ -277,6 +280,36 @@ Time Time::Now() {
   USE(result);
   return FromTimeval(tv);
 }
+#else
+inline int GetBestMonotonicClock(struct timespec* ts) {
+#if V8_OS_LINUX
+  // Prefer CLOCK_MONOTONIC_COARSE.  It's a high-res clock that's available as
+  // of 2.6.32.  It can be fully serviced from the vDSO, even on virtualized
+  // systems, which saves the overhead of a system call.  The coarseness is not
+  // an issue for us because it still has sub-microsecond resolution.
+  static bool no_clock_monotonic_coarse;
+  if (no_clock_monotonic_coarse == false) {

[Sven Panne, 2013/10/30 12:54:11]

http://refactoring.com/catalog/removeDoubleNegative.html :-)

Furthermore, using a static will introduce locking behind the scenes IIRC, I am
not 100% sure if this is a problem here or not.

[bnoordhuis, 2013/10/30 13:38:56]

Agree on the double negative but I wrote it this way to ensure that the flag
ends up in the .bss rather than the .data section.

Not sure what you mean by locking.  There is no mutex involved, if that is what
you mean.  At worst, you'll have some cache line dirtying at the start.  But
that's a one-time cost and it only happens when CLOCK_MONOTONIC_COARSE is not
supported.

[Sven Panne, 2013/10/30 14:26:50]

OK, I was a bit confused. What I meant was the fact that GCC protects the
initialization of a non-POD static with __cxa_guard_acquire /
__cxa_guard_release unless you specify-fno-threadsafe-statics. That's not the
case here, but our thread sanitizer tool will be unhappy about this
nevertheless, I guess...

+    if (clock_gettime(6 /* CLOCK_MONOTONIC_COARSE */, ts) == 0) {

[Benedikt Meurer, 2013/10/30 12:49:15]

I don't like hardcoding the constant here. Better add some

#if V8_OS_LINUX
# ifnfdef CLOCK_MONOTONIC_COARSE
#  define CLOCK_MONOTONIC_COARSE 6
# endif
#endif  // V8_OS_LINUX

at the beginning of the file.

+      return 0;
+    }
+    if (errno != EINVAL) {
+      return -1;
+    }
+    no_clock_monotonic_coarse = true;
+  }
+#endif  // V8_OS_LINUX
+  return clock_gettime(CLOCK_MONOTONIC, ts);
+}
+
+
+Time Time::Now() {

[Benedikt Meurer, 2013/10/30 12:49:15]

Time::Now() has to use gettimeofday() on POSIX systems or CLOCK_REALTIME. It
should return a Time value not a TimeTicks value.

[bnoordhuis, 2013/10/30 13:38:56]

You mean CLOCK_REALTIME_COARSE is not allowed?  If so, why is that?
Not sure what you mean.  Time::FromTimespec() returns a Time, right?

[Benedikt Meurer, 2013/10/30 13:50:04]

CLOCK_MONOTONIC != CLOCK_REALTIME. Time values have to use the realtime clock.

[bnoordhuis, 2013/10/30 14:18:40]

Trying to deepen my understanding here.  Is that because the return value of
Date.now() is backed by Time::Now()?

+  struct timespec ts;
+  int result = GetBestMonotonicClock(&ts);
+  ASSERT_EQ(0, result);
+  USE(result);
+  return FromTimespec(ts);
+}
+#endif  // V8_OS_MACOSX || V8_OS_SOLARIS
 
 
 Time Time::NowFromSystemTime() {
@@ -570,7 +603,7 @@ TimeTicks TimeTicks::HighResolutionNow() {
   ticks = (tv.tv_sec * Time::kMicrosecondsPerSecond + tv.tv_usec);
 #elif V8_OS_POSIX
   struct timespec ts;
-  int result = clock_gettime(CLOCK_MONOTONIC, &ts);
+  int result = GetBestMonotonicClock(&ts);

[Benedikt Meurer, 2013/10/30 12:49:15]

Please take a look at the Xorg GetTimeInMillis() function in os/utils.c. I think
we should use something similar here; can be safely inlined into the
HighResolutionNow() function, IMHO.

[bnoordhuis, 2013/10/30 13:38:56]

What in particular do you want to steal^Wcopy?  The extra check for
CLOCK_MONOTONIC?

[Benedikt Meurer, 2013/10/30 13:50:04]

    static clockid_t clockid;
    if (!clockid) {
#ifdef CLOCK_MONOTONIC_COARSE
        if (clock_getres(CLOCK_MONOTONIC_COARSE, &tp) == 0 &&
            (tp.tv_nsec / 1000) <= 1000 &&
            clock_gettime(CLOCK_MONOTONIC_COARSE, &tp) == 0)
            clockid = CLOCK_MONOTONIC_COARSE;
        else
#endif
            clockid = CLOCK_MONOTONIC;
    }
    int result = clock_gettime(clockid, &tp);

[bnoordhuis, 2013/10/30 14:18:40]

I'm a man of many qualities but mind reading is not one of them.  I know the
code but what in particular are the relevant bits?  The call to clock_getres()?

   ASSERT_EQ(0, result);
   USE(result);
   ticks = (ts.tv_sec * Time::kMicrosecondsPerSecond +