xpcom/ds/TimeStamp_posix.cpp

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  22  * Contributor(s):
  23  *  Chris Jones <jones.chris.g@gmail.com>
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  38
  39 //
  40 // Implement TimeStamp::Now() with POSIX clocks.
  41 //
  42 // The "tick" unit for POSIX clocks is simply a nanosecond, as this is
  43 // the smallest unit of time representable by struct timespec.  That
  44 // doesn't mean that a nanosecond is the resolution of TimeDurations
  45 // obtained with this API; see TimeDuration::Resolution;
  46 //
  47
  48 #include <time.h>
  49
  50 #include "mozilla/TimeStamp.h"
  51
  52 // Estimate of the smallest duration of time we can measure.
  53 static PRUint64 sResolution;
  54 static PRUint64 sResolutionSigDigs;
  55
  56 static const PRUint16 kNsPerUs   =       1000;
  57 static const PRUint64 kNsPerMs   =    1000000;
  58 static const PRUint64 kNsPerSec  = 1000000000;
  59 static const double kNsPerSecd   = 1000000000.0;
  60
  61 static PRUint64
  62 TimespecToNs(const struct timespec& ts)
  63 {
  64   PRUint64 baseNs = PRUint64(ts.tv_sec) * kNsPerSec;
  65   return baseNs + PRUint64(ts.tv_nsec);
  66 }
  67
  68 static PRUint64
  69 ClockTimeNs()
  70 {
  71   struct timespec ts;
  72   // this can't fail: we know &ts is valid, and TimeStamp::Init()
  73   // checks that CLOCK_MONOTONIC is supported (and aborts if not)
  74   clock_gettime(CLOCK_MONOTONIC, &ts);
  75
  76   // tv_sec is defined to be relative to an arbitrary point in time,
  77   // but it would be madness for that point in time to be earlier than
  78   // the Epoch.  So we can safely assume that even if time_t is 32
  79   // bits, tv_sec won't overflow while the browser is open.  Revisit
  80   // this argument if we're still building with 32-bit time_t around
  81   // the year 2037.
  82   return TimespecToNs(ts);
  83 }
  84
  85 static PRUint64
  86 ClockResolutionNs()
  87 {
  88   // NB: why not rely on clock_getres()?  Two reasons: (i) it might
  89   // lie, and (ii) it might return an "ideal" resolution that while
  90   // theoretically true, could never be measured in practice.  Since
  91   // clock_gettime() likely involves a system call on your platform,
  92   // the "actual" timing resolution shouldn't be lower than syscall
  93   // overhead.
  94
  95   PRUint64 start = ClockTimeNs();
  96   PRUint64 end = ClockTimeNs();
  97   PRUint64 minres = (end - start);
  98
  99   // 10 total trials is arbitrary: what we're trying to avoid by
 100   // looping is getting unlucky and being interrupted by a context
 101   // switch or signal, or being bitten by paging/cache effects
 102   for (int i = 0; i < 9; ++i) {
 103     start = ClockTimeNs();
 104     end = ClockTimeNs();
 105
 106     PRUint64 candidate = (start - end);
 107     if (candidate < minres)
 108       minres = candidate;
 109   }
 110
 111   if (0 == minres) {
 112     // measurable resolution is either incredibly low, ~1ns, or very
 113     // high.  fall back on clock_getres()
 114     struct timespec ts;
 115     clock_getres(CLOCK_MONOTONIC, &ts);
 116
 117     minres = TimespecToNs(ts);
 118   }
 119
 120   if (0 == minres) {
 121     // clock_getres probably failed.  fall back on NSPR's resolution
 122     // assumption
 123     minres = 1 * kNsPerMs;
 124   }
 125
 126   return minres;
 127 }
 128
 129
 130 namespace mozilla {
 131
 132 double
 133 TimeDuration::ToSeconds() const
 134 {
 135   return double(mValue) / kNsPerSecd;
 136 }
 137
 138 double
 139 TimeDuration::ToSecondsSigDigits() const
 140 {
 141   // don't report a value < mResolution ...
 142   PRInt64 valueSigDigs = sResolution * (mValue / sResolution);
 143   // and chop off insignificant digits
 144   valueSigDigs = sResolutionSigDigs * (valueSigDigs / sResolutionSigDigs);
 145   return double(valueSigDigs) / kNsPerSecd;
 146 }
 147
 148 TimeDuration
 149 TimeDuration::FromSeconds(PRInt32 aSeconds)
 150 {
 151   return TimeDuration::FromTicks((PRInt64(aSeconds) * PRInt64(kNsPerSec)));
 152 }
 153
 154 TimeDuration
 155 TimeDuration::FromMilliseconds(PRInt32 aMilliseconds)
 156 {
 157   return TimeDuration::FromTicks(PRInt64(aMilliseconds) * PRInt64(kNsPerMs));
 158 }
 159
 160 TimeDuration
 161 TimeDuration::Resolution()
 162 {
 163   return TimeDuration::FromTicks(sResolution);
 164 }
 165
 166 struct TimeStampInitialization
 167 {
 168   TimeStampInitialization() {
 169     TimeStamp::Startup();
 170   }
 171   ~TimeStampInitialization() {
 172     TimeStamp::Shutdown();
 173   }
 174 };
 175
 176 static TimeStampInitialization initOnce;
 177 static PRBool gInitialized = PR_FALSE;
 178
 179 nsresult
 180 TimeStamp::Startup()
 181 {
 182   if (gInitialized)
 183     return NS_OK;
 184
 185   struct timespec dummy;
 186   if (0 != clock_gettime(CLOCK_MONOTONIC, &dummy))
 187       NS_RUNTIMEABORT("CLOCK_MONOTONIC is absent!");
 188
 189   sResolution = ClockResolutionNs();
 190
 191   // find the number of significant digits in sResolution, for the
 192   // sake of ToSecondsSigDigits()
 193   for (sResolutionSigDigs = 1;
 194        !(sResolutionSigDigs == sResolution
 195          || 10*sResolutionSigDigs > sResolution);
 196        sResolutionSigDigs *= 10);
 197
 198   gInitialized = PR_TRUE;
 199   return NS_OK;
 200 }
 201
 202 void
 203 TimeStamp::Shutdown()
 204 {
 205 }
 206
 207 TimeStamp
 208 TimeStamp::Now()
 209 {
 210   return TimeStamp(ClockTimeNs());
 211 }
 212
 213 }