Use CLOCK_REALTIME_COARSE when available.

On systems that have CLOCK_REALTIME_COARSE with good enough resolution, we can
avoid making a system call to get the current time; it's serviced from the vDSO.

BUG=
LOG=N

Review URL: https://codereview.chromium.org/1125003002

Cr-Commit-Position: refs/heads/master@{#28280}

src/base/platform/time.cc

@@ -24,6 +24,11 @@
 #include "src/base/logging.h"
 #include "src/base/platform/platform.h"
 
+// CLOCK_REALTIME_COARSE was added in Linux 2.6.32.
+#if V8_OS_LINUX && !defined(CLOCK_REALTIME_COARSE)
+#define CLOCK_REALTIME_COARSE 5
+#endif
+
 namespace v8 {
 namespace base {
 
@@ -252,11 +257,44 @@ FILETIME Time::ToFiletime() const {
 #elif V8_OS_POSIX
 
 Time Time::Now() {
+#ifdef CLOCK_REALTIME_COARSE
+  struct timespec ts;
+  // clockid_t is an int32_t; loads and stores are atomic.
+  static const clockid_t kInvalidClockId = static_cast<clockid_t>(-1);
+  static clockid_t clock_id = kInvalidClockId;
+  if (V8_UNLIKELY(clock_id == kInvalidClockId)) {
+    // CLOCK_REALTIME_COARSE is not supported on Linux kernels < 2.6.32.
+    // Probe the kernel to see if it's available and has <= 1 ms resolution.
+    //
+    // CLOCK_REALTIME_COARSE, unlike CLOCK_REALTIME, can often be serviced
+    // entirely from the vDSO without the need to make a system call.
+    // It can have a dramatic impact on applications that frequently
+    // query the current time.
+    //
+    // One caveat is that CLOCK_REALTIME_COARSE is tied to CONFIG_HZ, the
+    // number of ticks per second that the kernel runs at.  Its granularity
+    // can be as low as one update every 300 ms so we need to make sure that
+    // it is accurate enough.  Fortunately, many if not most kernels are built
+    // with CONFIG_HZ=1000, giving it a one millisecond precision and that is
+    // good enough for our purposes.
+    if (clock_getres(CLOCK_REALTIME_COARSE, &ts) < 0 || ts.tv_sec > 0 ||
+        ts.tv_nsec > 1000 * 1000) {
+      clock_id = CLOCK_REALTIME;  // Not available or not suitable.
+    } else {
+      clock_id = CLOCK_REALTIME_COARSE;
+    }
+  }
+  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
 }