gecko-dev/xpcom/ds/TimeStamp_posix.cpp

177 lines
4.6 KiB
C++

/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim:set ts=2 sw=2 sts=2 et cindent: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
//
// 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 uint64_t sResolution;
static uint64_t sResolutionSigDigs;
static const uint16_t kNsPerUs = 1000;
static const uint64_t kNsPerMs = 1000000;
static const uint64_t kNsPerSec = 1000000000;
static const double kNsPerMsd = 1000000.0;
static const double kNsPerSecd = 1000000000.0;
static uint64_t
TimespecToNs(const struct timespec& ts)
{
uint64_t baseNs = uint64_t(ts.tv_sec) * kNsPerSec;
return baseNs + uint64_t(ts.tv_nsec);
}
static uint64_t
ClockTimeNs()
{
struct timespec ts;
// this can't fail: we know &ts is valid, and TimeStamp::Startup()
// 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.
return TimespecToNs(ts);
}
static uint64_t
ClockResolutionNs()
{
// NB: why not rely on 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.
uint64_t start = ClockTimeNs();
uint64_t end = ClockTimeNs();
uint64_t 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();
uint64_t candidate = (start - end);
if (candidate < minres)
minres = candidate;
}
if (0 == minres) {
// measurable resolution is either incredibly low, ~1ns, or very
// high. fall back on clock_getres()
struct timespec ts;
if (0 == clock_getres(CLOCK_MONOTONIC, &ts)) {
minres = TimespecToNs(ts);
}
}
if (0 == minres) {
// clock_getres probably failed. fall back on NSPR's resolution
// assumption
minres = 1 * kNsPerMs;
}
return minres;
}
namespace mozilla {
double
TimeDuration::ToSeconds() const
{
return double(mValue) / kNsPerSecd;
}
double
TimeDuration::ToSecondsSigDigits() const
{
// don't report a value < mResolution ...
int64_t valueSigDigs = sResolution * (mValue / sResolution);
// and chop off insignificant digits
valueSigDigs = sResolutionSigDigs * (valueSigDigs / sResolutionSigDigs);
return double(valueSigDigs) / kNsPerSecd;
}
TimeDuration
TimeDuration::FromMilliseconds(double aMilliseconds)
{
return TimeDuration::FromTicks(aMilliseconds * kNsPerMsd);
}
TimeDuration
TimeDuration::Resolution()
{
return TimeDuration::FromTicks(int64_t(sResolution));
}
struct TimeStampInitialization
{
TimeStampInitialization() {
TimeStamp::Startup();
}
~TimeStampInitialization() {
TimeStamp::Shutdown();
}
};
static TimeStampInitialization initOnce;
static bool gInitialized = false;
nsresult
TimeStamp::Startup()
{
if (gInitialized)
return NS_OK;
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);
gInitialized = true;
return NS_OK;
}
void
TimeStamp::Shutdown()
{
}
TimeStamp
TimeStamp::Now()
{
return TimeStamp(ClockTimeNs());
}
}