Reviewers: jochen,
Message:
I included the suggestion from the original CL to use NoBarrier_Load() and
NoBarrier_Store().
It's perhaps a little incongruent because there is still (OS X-specific)
code in
time.cc that uses unchecked loads and stores. I can clean that up in
another CL
if you think that's worthwhile.
Description:
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.
This is v2 of the patch. v1 can be found at [0] but was reverted in [1]
because of Chromium sandbox restrictions. The necessary changes have
been applied upstream in [2].
[0] https://codereview.chromium.org/1125003002
[1] https://codereview.chromium.org/1130083003
[2] https://codereview.chromium.org/1133653002
BUG=
LOG=N
Please review this at https://codereview.chromium.org/1151283005/
Base URL: https://chromium.googlesource.com/v8/v8.git@master
Affected files (+35, -0 lines):
M 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..b9c3c181ab038826599eb86e4943a6a9ccdfd183
100644
--- a/src/base/platform/time.cc
+++ b/src/base/platform/time.cc
@@ -20,10 +20,16 @@
#include "src/base/lazy-instance.h"
#include "src/base/win32-headers.h"
#endif
+#include "src/base/atomicops.h"
#include "src/base/cpu.h"
#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 +258,40 @@ FILETIME Time::ToFiletime() const {
#elif V8_OS_POSIX
Time Time::Now() {
+#ifdef CLOCK_REALTIME_COARSE
+ struct timespec ts;
+ static const clockid_t kInvalidClockId = static_cast<clockid_t>(-1);
+ static clockid_t cached_clock_id = kInvalidClockId;
+ clockid_t clock_id = NoBarrier_Load(&cached_clock_id);
+ if (V8_UNLIKELY(clock_id == kInvalidClockId)) {
+ // Use CLOCK_REALTIME_COARSE when available (linux >= 2.6.32) and
precise
+ // enough. Unlike CLOCK_REALTIME, its coarse cousin is normally
serviced
+ // from the vDSO, without making an actual system call.
+ //
+ // CLOCK_REALTIME_COARSE precision is determined by CONFIG_HZ, the
number
+ // of ticks per second that the kernel runs at. Granularity can be as
low
+ // as one update every few hundred milliseconds so we need to ensure
it is
+ // not _too_ coarse. Most kernels are built with CONFIG_HZ=1000,
providing
+ // a one millisecond precision 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;
+ }
+ NoBarrier_Store(&cached_clock_id, clock_id);
+ }
+ 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
}
--
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