Bug 522126, part 2: Add a POSIX clock (CLOCK_MONOTONIC) implementation of TimeStamp/TimeDuration. r=roc

xpcom/ds/TimeStamp_posix.cpp

@@ -0,0 +1,185 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* ***** BEGIN LICENSE BLOCK *****
+ * Version: MPL 1.1/GPL 2.0/LGPL 2.1
+ *
+ * The contents of this file are subject to the Mozilla Public License Version
+ * 1.1 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ * http://www.mozilla.org/MPL/
+ *
+ * Software distributed under the License is distributed on an "AS IS" basis,
+ * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
+ * for the specific language governing rights and limitations under the
+ * License.
+ *
+ * The Original Code is Mozilla code.
+ *
+ * The Initial Developer of the Original Code is the Mozilla Corporation.
+ * Portions created by the Initial Developer are Copyright (C) 2009
+ * the Initial Developer. All Rights Reserved.
+ *
+ * Contributor(s):
+ *  Chris Jones <jones.chris.g@gmail.com>
+ *
+ * Alternatively, the contents of this file may be used under the terms of
+ * either the GNU General Public License Version 2 or later (the "GPL"), or
+ * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
+ * in which case the provisions of the GPL or the LGPL are applicable instead
+ * of those above. If you wish to allow use of your version of this file only
+ * under the terms of either the GPL or the LGPL, and not to allow others to
+ * use your version of this file under the terms of the MPL, indicate your
+ * decision by deleting the provisions above and replace them with the notice
+ * and other provisions required by the GPL or the LGPL. If you do not delete
+ * the provisions above, a recipient may use your version of this file under
+ * the terms of any one of the MPL, the GPL or the LGPL.
+ *
+ * ***** END LICENSE BLOCK ***** */
+
+//
+// Implement TimeStamp::Now() with POSIX clocks.
+//
+// The "tick" unit for POSIX clocks is simply a nanosecond, as this is
+// the smallest unit of time representable by struct timespec.  That
+// doesn't mean that a nanosecond is the resolution of TimeDurations
+// obtained with this API; see TimeDuration::Resolution;
+//
+
+#include <time.h>
+
+#include "mozilla/TimeStamp.h"
+
+// Estimate of the smallest duration of time we can measure.
+static PRUint64 sResolution;
+static PRUint64 sResolutionSigDigs;
+
+static const PRUint16 kNsPerUs   =       1000;
+static const PRUint64 kNsPerMs   =    1000000;
+static const PRUint64 kNsPerSec  = 1000000000; 
+static const double kNsPerSecd   = 1000000000.0;
+
+
+static PRUint64
+ClockTimeNs()
+{
+  struct timespec ts;
+  // this can't fail: we know &ts is valid, and TimeStamp::Init()
+  // checks that CLOCK_MONOTONIC is supported (and aborts if not)
+  clock_gettime(CLOCK_MONOTONIC, &ts);
+
+  // tv_sec is defined to be relative to an arbitrary point in time,
+  // but it would be madness for that point in time to be earlier than
+  // the Epoch.  So we can safely assume that even if time_t is 32
+  // bits, tv_sec won't overflow while the browser is open.  Revisit
+  // this argument if we're still building with 32-bit time_t around
+  // the year 2037.
+  PRUint64 baseNs = PRUint64(ts.tv_sec) * kNsPerSec;
+
+  return baseNs + PRUint64(ts.tv_nsec);
+}
+
+static PRUint64
+ClockResolutionNs()
+{
+  // NB: why not use clock_getres()?  Two reasons: (i) it might lie,
+  // and (ii) it might return an "ideal" resolution that while
+  // theoretically true, could never be measured in practice.  Since
+  // clock_gettime() likely involves a system call on your platform,
+  // the "actual" timing resolution shouldn't be lower than syscall
+  // overhead.
+
+  PRUint64 start = ClockTimeNs();
+  PRUint64 end = ClockTimeNs();
+  PRUint64 minres = (end - start);
+
+  // 10 total trials is arbitrary: what we're trying to avoid by
+  // looping is getting unlucky and being interrupted by a context
+  // switch or signal, or being bitten by paging/cache effects
+  for (int i = 0; i < 9; ++i) {
+    start = ClockTimeNs();
+    end = ClockTimeNs();
+
+    PRUint64 candidate = (start - end);
+    if (candidate < minres)
+      minres = candidate;
+  }
+
+  if (0 == minres) {
+    NS_WARNING("the clock resolution is *not* 1ns, something's wrong");
+    minres = 1;                 // to avoid /0
+  }
+  if (minres / kNsPerMs)
+    NS_WARNING("the clock resolution is *not* >=1ms, something's wrong");
+
+  return minres;
+}
+
+
+namespace mozilla {
+
+double
+TimeDuration::ToSeconds() const
+{
+  return double(mValue) / kNsPerSecd;
+}
+
+double
+TimeDuration::ToSecondsSigDigits() const
+{
+  // don't report a value < mResolution ...
+  PRInt64 valueSigDigs = sResolution * (mValue / sResolution);
+  // and chop off insignificant digits
+  valueSigDigs = sResolutionSigDigs * (valueSigDigs / sResolutionSigDigs);
+  return double(valueSigDigs) / kNsPerSecd;
+}
+
+TimeDuration
+TimeDuration::FromSeconds(PRInt32 aSeconds)
+{
+  return TimeDuration::FromTicks((PRInt64(aSeconds) * PRInt64(kNsPerSec)));
+}
+
+TimeDuration
+TimeDuration::FromMilliseconds(PRInt32 aMilliseconds)
+{
+  return TimeDuration::FromTicks(PRInt64(aMilliseconds) * PRInt64(kNsPerMs));
+}
+
+TimeDuration
+TimeDuration::Resolution()
+{
+  return TimeDuration::FromTicks(sResolution);
+}
+
+
+nsresult
+TimeStamp::Startup()
+{
+  struct timespec dummy;
+  if (0 != clock_gettime(CLOCK_MONOTONIC, &dummy))
+      NS_RUNTIMEABORT("CLOCK_MONOTONIC is absent!");
+
+  sResolution = ClockResolutionNs();
+
+  // find the number of significant digits in sResolution, for the
+  // sake of ToSecondsSigDigits()
+  for (sResolutionSigDigs = 1;
+       !(sResolutionSigDigs == sResolution
+         || 10*sResolutionSigDigs > sResolution);
+       sResolutionSigDigs *= 10);
+
+  return NS_OK;
+}
+
+void
+TimeStamp::Shutdown()
+{
+}
+
+TimeStamp
+TimeStamp::Now()
+{
+  return TimeStamp(ClockTimeNs());
+}
+
+}