gecko/xpcom/ds/TimeStamp_darwin.cpp
Jeff Muizelaar f5865f36e6 Bug 539093. TimeStamp implementation for OS X using mach_absolute_time. r=cjones
--HG--
extra : rebase_source : 66c407448f94ae1b9efa8c7d0d7291e22b5e6191
2011-07-20 14:27:10 -04:00

193 lines
5.7 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: */
/* ***** 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 Foundation.
* Portions created by the Initial Developer are Copyright (C) 2011
* the Initial Developer. All Rights Reserved.
*
* Contributor(s):
* Jeff Muizelaar <jmuizelaar@mozilla.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
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* ***** END LICENSE BLOCK ***** */
//
// Implement TimeStamp::Now() with mach_absolute_time
//
// The "tick" unit for mach_absolute_time is defined using mach_timebase_info() which
// gives a conversion ratio to nanoseconds. For more information see Apple's QA1398.
//
// This code is inspired by Chromium's time_mac.cc. The biggest
// differences are that we explicitly initialize using
// TimeStamp::Initialize() instead of lazily in Now() and that
// we store the time value in ticks and convert when needed instead
// of storing the time value in nanoseconds.
#include <mach/mach_time.h>
#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 kNsPerMsd = 1000000.0;
static const double kNsPerSecd = 1000000000.0;
static double sNsPerTick;
static PRUint64
ClockTime()
{
// mach_absolute_time is it when it comes to ticks on the Mac. Other calls
// with less precision (such as TickCount) just call through to
// mach_absolute_time.
//
// At the time of writing mach_absolute_time returns the number of nanoseconds
// since boot. This won't overflow 64bits for 500+ years so we aren't going
// to worry about that possiblity
return mach_absolute_time();
}
static PRUint64
ClockResolutionNs()
{
PRUint64 start = ClockTime();
PRUint64 end = ClockTime();
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 = ClockTime();
end = ClockTime();
PRUint64 candidate = (start - end);
if (candidate < minres)
minres = candidate;
}
if (0 == minres) {
// measurable resolution is either incredibly low, ~1ns, or very
// high. fall back on NSPR's resolution assumption
minres = 1 * kNsPerMs;
}
return minres;
}
namespace mozilla {
double
TimeDuration::ToSeconds() const
{
return (mValue * sNsPerTick) / 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 (valueSigDigs * sNsPerTick) / kNsPerSecd;
}
TimeDuration
TimeDuration::FromMilliseconds(double aMilliseconds)
{
return TimeDuration::FromTicks(PRInt64((aMilliseconds * kNsPerMsd) / sNsPerTick));
}
TimeDuration
TimeDuration::Resolution()
{
return TimeDuration::FromTicks(PRInt64(sResolution));
}
struct TimeStampInitialization
{
TimeStampInitialization() {
TimeStamp::Startup();
}
~TimeStampInitialization() {
TimeStamp::Shutdown();
}
};
static TimeStampInitialization initOnce;
static PRBool gInitialized = PR_FALSE;
nsresult
TimeStamp::Startup()
{
if (gInitialized)
return NS_OK;
mach_timebase_info_data_t timebaseInfo;
// Apple's QA1398 suggests that the output from mach_timebase_info
// will not change while a program is running, so it should be safe
// to cache the result.
kern_return_t kr = mach_timebase_info(&timebaseInfo);
if (kr != KERN_SUCCESS)
NS_RUNTIMEABORT("mach_timebase_info failed");
sNsPerTick = double(timebaseInfo.numer) / timebaseInfo.denom;
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 = PR_TRUE;
return NS_OK;
}
void
TimeStamp::Shutdown()
{
}
TimeStamp
TimeStamp::Now()
{
return TimeStamp(ClockTime());
}
}