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mariadb/mysys/my_rdtsc.c
Vasilii Lakhin 5f7c2a617f Fix typos in C comments in miscellaneous files
2025-03-24 13:36:28 +11:00

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/* Copyright (c) 2008, 2010, Oracle and/or its affiliates. All rights reserved.
Copyright (c) 2019, MariaDB Corporation.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; version 2 of the License.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1335 USA */
/*
rdtsc3 -- multi-platform timer code
pgulutzan@mysql.com, 2005-08-29
modified 2008-11-02
Functions:
my_timer_cycles ulonglong cycles
my_timer_nanoseconds ulonglong nanoseconds
my_timer_microseconds ulonglong "microseconds"
my_timer_milliseconds ulonglong milliseconds
my_timer_ticks ulonglong ticks
my_timer_init initialization / test
We'll call the first 5 functions (the ones that return
a ulonglong) "my_timer_xxx" functions.
Each my_timer_xxx function returns a 64-bit timing value
since an arbitrary 'epoch' start. Since the only purpose
is to determine elapsed times, wall-clock time-of-day
is not known and not relevant.
The my_timer_init function is necessary for initializing.
It returns information (underlying routine name,
frequency, resolution, overhead) about all my_timer_xxx
functions. A program should call my_timer_init once,
use the information to decide what my_timer_xxx function
to use, and subsequently call that function by function
pointer.
A typical use would be:
my_timer_init() ... once, at program start
...
time1= my_timer_xxx() ... time before start
[code that's timed]
time2= my_timer_xxx() ... time after end
elapsed_time= (time2 - time1) - overhead
*/
#include "my_global.h"
#include "my_rdtsc.h"
#if defined(_WIN32)
#include <stdio.h>
#include "windows.h"
#else
#include <stdio.h>
#endif
#if !defined(_WIN32)
#if TIME_WITH_SYS_TIME
#include <sys/time.h>
#include <time.h> /* for clock_gettime */
#else
#if HAVE_SYS_TIME_H
#include <sys/time.h>
#elif defined(HAVE_TIME_H)
#include <time.h>
#endif
#endif
#endif
#if defined(HAVE_SYS_TIMEB_H) && defined(HAVE_FTIME)
#include <sys/timeb.h> /* for ftime */
#endif
#if defined(HAVE_SYS_TIMES_H) && defined(HAVE_TIMES)
#include <sys/times.h> /* for times */
#endif
#if defined(__APPLE__) && defined(__MACH__)
#include <mach/mach_time.h>
#endif
/*
For nanoseconds, most platforms have nothing available that
(a) doesn't require bringing in a 40-kb librt.so library
(b) really has nanosecond resolution.
*/
ulonglong my_timer_nanoseconds(void)
{
#if defined(HAVE_READ_REAL_TIME)
{
timebasestruct_t tr;
read_real_time(&tr, TIMEBASE_SZ);
return (ulonglong) tr.tb_high * 1000000000 + (ulonglong) tr.tb_low;
}
#elif defined(HAVE_SYS_TIMES_H) && defined(HAVE_GETHRTIME)
/* SunOS 5.10+, Solaris, HP-UX: hrtime_t gethrtime(void) */
return (ulonglong) gethrtime();
#elif defined(HAVE_CLOCK_GETTIME) && defined(CLOCK_REALTIME)
{
struct timespec tp;
clock_gettime(CLOCK_REALTIME, &tp);
return (ulonglong) tp.tv_sec * 1000000000 + (ulonglong) tp.tv_nsec;
}
#elif defined(__APPLE__) && defined(__MACH__)
{
ulonglong tm;
static mach_timebase_info_data_t timebase_info= {0,0};
if (timebase_info.denom == 0)
(void) mach_timebase_info(&timebase_info);
tm= mach_absolute_time();
return (tm * timebase_info.numer) / timebase_info.denom;
}
#else
return 0;
#endif
}
/*
For microseconds, gettimeofday() is available on
almost all platforms. On Windows we use
QueryPerformanceCounter which will usually tick over
3.5 million times per second, and we don't throw
away the extra precision. (On Windows Server 2003
the frequency is same as the cycle frequency.)
*/
ulonglong my_timer_microseconds(void)
{
#if defined(HAVE_GETTIMEOFDAY)
{
static ulonglong last_value= 0;
struct timeval tv;
if (gettimeofday(&tv, NULL) == 0)
last_value= (ulonglong) tv.tv_sec * 1000000 + (ulonglong) tv.tv_usec;
else
{
/*
There are reports that gettimeofday(2) can have intermittent failures
on some platform, see for example Bug#36819.
We are not trying again or looping, just returning the best value possible
under the circumstances ...
*/
last_value++;
}
return last_value;
}
#elif defined(_WIN32)
{
/* QueryPerformanceCounter usually works with about 1/3 microsecond. */
LARGE_INTEGER t_cnt;
QueryPerformanceCounter(&t_cnt);
return (ulonglong) t_cnt.QuadPart;
}
#else
return 0;
#endif
}
/*
For milliseconds, we use ftime() if it's supported
or time()*1000 if it's not. With modern versions of
Windows and with HP Itanium, resolution is 10-15
milliseconds.
*/
#if defined(HAVE_CLOCK_GETTIME)
#if defined(CLOCK_MONOTONIC_FAST)
/* FreeBSD */
#define MY_CLOCK_ID CLOCK_MONOTONIC_FAST
#elif defined(CLOCK_MONOTONIC_COARSE)
/* Linux */
#define MY_CLOCK_ID CLOCK_MONOTONIC_COARSE
#elif defined(CLOCK_MONOTONIC)
/* POSIX (includes OSX) */
#define MY_CLOCK_ID CLOCK_MONOTONIC
#elif defined(CLOCK_REALTIME)
/* Solaris (which doesn't seem to have MONOTONIC) */
#define MY_CLOCK_ID CLOCK_REALTIME
#endif
#endif
ulonglong my_timer_milliseconds(void)
{
#if defined(MY_CLOCK_ID)
struct timespec tp;
clock_gettime(MY_CLOCK_ID, &tp);
return (ulonglong)tp.tv_sec * 1000 + (ulonglong)tp.tv_nsec / 1000000;
#elif defined(HAVE_SYS_TIMEB_H) && defined(HAVE_FTIME)
/* ftime() is obsolete but maybe the platform is old */
struct timeb ft;
ftime(&ft);
return (ulonglong)ft.time * 1000 + (ulonglong)ft.millitm;
#elif defined(HAVE_TIME)
return (ulonglong) time(NULL) * 1000;
#elif defined(_WIN32)
FILETIME ft;
GetSystemTimeAsFileTime( &ft );
return ((ulonglong)ft.dwLowDateTime +
(((ulonglong)ft.dwHighDateTime) << 32))/10000;
#else
return 0;
#endif
}
/*
For ticks, which we handle with times(), the frequency
is usually 100/second and the overhead is surprisingly
bad, sometimes even worse than gettimeofday's overhead.
*/
ulonglong my_timer_ticks(void)
{
#if defined(HAVE_SYS_TIMES_H) && defined(HAVE_TIMES)
{
struct tms times_buf;
return (ulonglong) times(&times_buf);
}
#elif defined(_WIN32)
return (ulonglong) GetTickCount();
#else
return 0;
#endif
}
/*
The my_timer_init() function and its sub-functions
have several loops which call timers. If there's
something wrong with a timer -- which has never
happened in tests -- we want the loop to end after
an arbitrary number of iterations, and my_timer_info
will show a discouraging result. The arbitrary
number is 1,000,000.
*/
#define MY_TIMER_ITERATIONS 1000000
/*
Calculate overhead. Called from my_timer_init().
Usually best_timer_overhead = cycles.overhead or
nanoseconds.overhead, so returned amount is in
cycles or nanoseconds. We repeat the calculation
ten times, so that we can disregard effects of
caching or interrupts. Result is quite consistent
for cycles, at least. But remember it's a minimum.
*/
static void my_timer_init_overhead(ulonglong *overhead,
ulonglong (*cycle_timer)(void),
ulonglong (*this_timer)(void),
ulonglong best_timer_overhead)
{
ulonglong time1, time2;
int i;
/* *overhead, least of 20 calculations - cycles.overhead */
for (i= 0, *overhead= 1000000000; i < 20; ++i)
{
time1= cycle_timer();
this_timer(); /* rather than 'time_tmp= timer();' */
time2= cycle_timer() - time1;
if (*overhead > time2)
*overhead= time2;
}
*overhead-= best_timer_overhead;
}
/*
Calculate Resolution. Called from my_timer_init().
If a timer goes up by jumps, e.g. 1050, 1075, 1100, ...
then the best resolution is the minimum jump, e.g. 25.
If it's always divisible by 1000 then it's just a
result of multiplication of a lower-precision timer
result, e.g. nanoseconds are often microseconds * 1000.
If the minimum jump is less than an arbitrary passed
figure (a guess based on maximum overhead * 2), ignore.
Usually we end up with nanoseconds = 1 because it's too
hard to detect anything <= 100 nanoseconds.
Often GetTickCount() has resolution = 15.
We don't check with ticks because they take too long.
*/
static ulonglong my_timer_init_resolution(ulonglong (*this_timer)(void),
ulonglong overhead_times_2)
{
ulonglong time1, time2;
ulonglong best_jump;
int i, jumps, divisible_by_1000, divisible_by_1000000;
divisible_by_1000= divisible_by_1000000= 0;
best_jump= 1000000;
for (i= jumps= 0; jumps < 3 && i < MY_TIMER_ITERATIONS * 10; ++i)
{
time1= this_timer();
time2= this_timer();
time2-= time1;
if (time2)
{
++jumps;
if (!(time2 % 1000))
{
++divisible_by_1000;
if (!(time2 % 1000000))
++divisible_by_1000000;
}
if (best_jump > time2)
best_jump= time2;
/* For milliseconds, one jump is enough. */
if (overhead_times_2 == 0)
break;
}
}
if (jumps == 3)
{
if (jumps == divisible_by_1000000)
return 1000000;
if (jumps == divisible_by_1000)
return 1000;
}
if (best_jump > overhead_times_2)
return best_jump;
return 1;
}
/*
Calculate cycle frequency by seeing how many cycles pass
in a 200-microsecond period. I tried with 10-microsecond
periods originally, and the result was often very wrong.
*/
static ulonglong my_timer_init_frequency(MY_TIMER_INFO *mti)
{
int i;
ulonglong time1, time2, time3, time4, denominator;
time1= my_timer_cycles();
time2= my_timer_microseconds();
time3= time2; /* Avoids a Microsoft/IBM compiler warning */
for (i= 0; i < MY_TIMER_ITERATIONS; ++i)
{
time3= my_timer_microseconds();
if (time3 - time2 > 200) break;
}
time4= my_timer_cycles() - mti->cycles.overhead;
time4-= mti->microseconds.overhead;
denominator = ((time3 - time2) == 0) ? 1 : time3 - time2;
return (mti->microseconds.frequency * (time4 - time1)) / denominator;
}
/*
Call my_timer_init before the first call to my_timer_xxx().
If something must be initialized, it happens here.
Set: what routine is being used e.g. "rdtsc"
Set: function, overhead, actual frequency, resolution.
*/
void my_timer_init(MY_TIMER_INFO *mti)
{
ulonglong (*best_timer)(void);
ulonglong best_timer_overhead;
ulonglong time1, time2;
int i;
/* cycles */
mti->cycles.frequency= 1000000000;
mti->cycles.routine= MY_TIMER_ROUTINE_CYCLES;
if (!mti->cycles.routine || !my_timer_cycles())
{
mti->cycles.routine= 0;
mti->cycles.resolution= 0;
mti->cycles.frequency= 0;
mti->cycles.overhead= 0;
}
/* nanoseconds */
mti->nanoseconds.frequency= 1000000000; /* initial assumption */
#if defined(HAVE_READ_REAL_TIME)
mti->nanoseconds.routine= MY_TIMER_ROUTINE_READ_REAL_TIME;
#elif defined(HAVE_SYS_TIMES_H) && defined(HAVE_GETHRTIME)
mti->nanoseconds.routine= MY_TIMER_ROUTINE_GETHRTIME;
#elif defined(HAVE_CLOCK_GETTIME)
mti->nanoseconds.routine= MY_TIMER_ROUTINE_CLOCK_GETTIME;
#elif defined(__APPLE__) && defined(__MACH__)
mti->nanoseconds.routine= MY_TIMER_ROUTINE_MACH_ABSOLUTE_TIME;
#else
mti->nanoseconds.routine= 0;
#endif
if (!mti->nanoseconds.routine || !my_timer_nanoseconds())
{
mti->nanoseconds.routine= 0;
mti->nanoseconds.resolution= 0;
mti->nanoseconds.frequency= 0;
mti->nanoseconds.overhead= 0;
}
/* microseconds */
mti->microseconds.frequency= 1000000; /* initial assumption */
#if defined(HAVE_GETTIMEOFDAY)
mti->microseconds.routine= MY_TIMER_ROUTINE_GETTIMEOFDAY;
#elif defined(_WIN32)
{
LARGE_INTEGER li;
/* Windows: typical frequency = 3579545, actually 1/3 microsecond. */
if (!QueryPerformanceFrequency(&li))
mti->microseconds.routine= 0;
else
{
mti->microseconds.frequency= li.QuadPart;
mti->microseconds.routine= MY_TIMER_ROUTINE_QUERYPERFORMANCECOUNTER;
}
}
#else
mti->microseconds.routine= 0;
#endif
if (!mti->microseconds.routine || !my_timer_microseconds())
{
mti->microseconds.routine= 0;
mti->microseconds.resolution= 0;
mti->microseconds.frequency= 0;
mti->microseconds.overhead= 0;
}
/* milliseconds */
mti->milliseconds.frequency= 1000; /* initial assumption */
#ifdef MY_CLOCK_ID
mti->milliseconds.routine= MY_TIMER_ROUTINE_CLOCK_GETTIME;
#elif defined(HAVE_SYS_TIMEB_H) && defined(HAVE_FTIME)
mti->milliseconds.routine= MY_TIMER_ROUTINE_FTIME;
#elif defined(_WIN32)
mti->milliseconds.routine= MY_TIMER_ROUTINE_GETSYSTEMTIMEASFILETIME;
#elif defined(HAVE_TIME)
mti->milliseconds.routine= MY_TIMER_ROUTINE_TIME;
#else
mti->milliseconds.routine= 0;
#endif
if (!mti->milliseconds.routine || !my_timer_milliseconds())
{
mti->milliseconds.routine= 0;
mti->milliseconds.resolution= 0;
mti->milliseconds.frequency= 0;
mti->milliseconds.overhead= 0;
}
/* ticks */
mti->ticks.frequency= 100; /* permanent assumption */
#if defined(HAVE_SYS_TIMES_H) && defined(HAVE_TIMES)
mti->ticks.routine= MY_TIMER_ROUTINE_TIMES;
#elif defined(_WIN32)
mti->ticks.routine= MY_TIMER_ROUTINE_GETTICKCOUNT;
#else
mti->ticks.routine= 0;
#endif
if (!mti->ticks.routine || !my_timer_ticks())
{
mti->ticks.routine= 0;
mti->ticks.resolution= 0;
mti->ticks.frequency= 0;
mti->ticks.overhead= 0;
}
/*
Calculate overhead in terms of the timer that
gives the best resolution: cycles or nanoseconds.
I doubt it ever will be as bad as microseconds.
*/
if (mti->cycles.routine)
best_timer= &my_timer_cycles;
else
{
if (mti->nanoseconds.routine)
{
best_timer= &my_timer_nanoseconds;
}
else
best_timer= &my_timer_microseconds;
}
/* best_timer_overhead = least of 20 calculations */
for (i= 0, best_timer_overhead= 1000000000; i < 20; ++i)
{
time1= best_timer();
time2= best_timer() - time1;
if (best_timer_overhead > time2)
best_timer_overhead= time2;
}
if (mti->cycles.routine)
my_timer_init_overhead(&mti->cycles.overhead,
best_timer,
&my_timer_cycles,
best_timer_overhead);
if (mti->nanoseconds.routine)
my_timer_init_overhead(&mti->nanoseconds.overhead,
best_timer,
&my_timer_nanoseconds,
best_timer_overhead);
if (mti->microseconds.routine)
my_timer_init_overhead(&mti->microseconds.overhead,
best_timer,
&my_timer_microseconds,
best_timer_overhead);
if (mti->milliseconds.routine)
my_timer_init_overhead(&mti->milliseconds.overhead,
best_timer,
&my_timer_milliseconds,
best_timer_overhead);
if (mti->ticks.routine)
my_timer_init_overhead(&mti->ticks.overhead,
best_timer,
&my_timer_ticks,
best_timer_overhead);
/*
Calculate resolution for nanoseconds or microseconds
or milliseconds, by seeing if it's always divisible
by 1000, and by noticing how much jumping occurs.
For ticks, just assume the resolution is 1.
*/
if (mti->cycles.routine)
mti->cycles.resolution= 1;
if (mti->nanoseconds.routine)
mti->nanoseconds.resolution=
my_timer_init_resolution(&my_timer_nanoseconds, 20000);
if (mti->microseconds.routine)
mti->microseconds.resolution=
my_timer_init_resolution(&my_timer_microseconds, 20);
if (mti->milliseconds.routine)
{
if (mti->milliseconds.routine == MY_TIMER_ROUTINE_TIME)
mti->milliseconds.resolution= 1000;
else
mti->milliseconds.resolution=
my_timer_init_resolution(&my_timer_milliseconds, 0);
}
if (mti->ticks.routine)
mti->ticks.resolution= 1;
/*
Calculate cycles frequency,
if we have both a cycles routine and a microseconds routine.
In tests, this usually results in a figure within 2% of
what "cat /proc/cpuinfo" says.
If the microseconds routine is QueryPerformanceCounter
(i.e. it's Windows), and the microseconds frequency is >
500,000,000 (i.e. it's Windows Server so it uses RDTSC)
and the microseconds resolution is > 100 (i.e. dreadful),
then calculate cycles frequency = microseconds frequency.
*/
if (mti->cycles.routine
&& mti->microseconds.routine)
{
if (mti->microseconds.routine ==
MY_TIMER_ROUTINE_QUERYPERFORMANCECOUNTER
&& mti->microseconds.frequency > 500000000
&& mti->microseconds.resolution > 100)
mti->cycles.frequency= mti->microseconds.frequency;
else
{
time1= my_timer_init_frequency(mti);
/* Repeat once in case there was an interruption. */
time2= my_timer_init_frequency(mti);
if (time1 < time2) mti->cycles.frequency= time1;
else mti->cycles.frequency= time2;
}
}
/*
Calculate milliseconds frequency =
(cycles-frequency/#-of-cycles) * #-of-milliseconds,
if we have both a milliseconds routine and a cycles
routine.
This will be inaccurate if milliseconds resolution > 1.
This is probably only useful when testing new platforms.
*/
if (mti->milliseconds.routine
&& mti->milliseconds.resolution < 1000
&& mti->microseconds.routine
&& mti->cycles.routine)
{
ulonglong time3, time4, denominator;
time1= my_timer_cycles();
time2= my_timer_milliseconds();
time3= time2; /* Avoids a Microsoft/IBM compiler warning */
for (i= 0; i < MY_TIMER_ITERATIONS * 1000; ++i)
{
time3= my_timer_milliseconds();
if (time3 - time2 > 10) break;
}
time4= my_timer_cycles();
denominator = ((time4 - time1) == 0) ? 1 : time4 - time1;
mti->milliseconds.frequency=
(mti->cycles.frequency * (time3 - time2)) / denominator;
}
/*
Calculate ticks.frequency =
(cycles-frequency/#-of-cycles * #-of-ticks,
if we have both a ticks routine and a cycles
routine,
This is probably only useful when testing new platforms.
*/
if (mti->ticks.routine
&& mti->microseconds.routine
&& mti->cycles.routine)
{
ulonglong time3, time4, denominator;
time1= my_timer_cycles();
time2= my_timer_ticks();
time3= time2; /* Avoids a Microsoft/IBM compiler warning */
#if defined(HAVE_SYS_TIMES_H) && defined(HAVE_TIMES)
for (i= 0; i < 1000; ++i)
#else
for (i= 0; i < MY_TIMER_ITERATIONS * 1000; ++i)
#endif
{
time3= my_timer_ticks();
if (time3 - time2 > 10) break;
}
time4= my_timer_cycles();
denominator = ((time4 - time1) == 0) ? 1 : time4 - time1;
mti->ticks.frequency=
(mti->cycles.frequency * (time3 - time2)) / denominator;
}
}
/*
Additional Comments
-------------------
This is for timing, i.e. finding out how long a piece of code
takes. If you want time of day matching a wall clock, the
my_timer_xxx functions won't help you.
The best timer is the one with highest frequency, lowest
overhead, and resolution=1. The my_timer_info() routine will tell
you at runtime which timer that is. Usually it will be
my_timer_cycles() but be aware that, although it's best,
it has possible flaws and dangers. Depending on platform:
- The frequency might change. We don't test for this. It
happens on laptops for power saving, and on blade servers
for avoiding overheating.
- The overhead that my_timer_init() returns is the minimum.
In fact it could be slightly greater because of caching or
because you call the routine by address, as recommended.
It could be hugely greater if there's an interrupt.
- The x86 cycle counter, RDTSC doesn't "serialize". That is,
if there is out-of-order execution, rdtsc might be processed
after an instruction that logically follows it.
(We could force serialization, but that would be slower.)
- It is possible to set a flag which renders RDTSC
inoperative. Somebody responsible for the kernel
of the operating system would have to make this
decision. For the platforms we've tested with, there's
no such problem.
- With a multi-processor arrangement, it's possible
to get the cycle count from one processor in
thread X, and the cycle count from another processor
in thread Y. They may not always be in synch.
- You can't depend on a cycle counter being available for
all platforms. On Alphas, the
cycle counter is only 32-bit, so it would overflow quickly,
so we don't bother with it. On platforms that we haven't
tested, there might be some if/endif combination that we
didn't expect, or some assembler routine that we didn't
supply.
The recommended way to use the timer routines is:
1. Somewhere near the beginning of the program, call
my_timer_init(). This should only be necessary once,
although you can call it again if you think that the
frequency has changed.
2. Determine the best timer based on frequency, resolution,
overhead -- all things that my_timer_init() returns.
Preserve the address of the timer and the my_timer_into
results in an easily-accessible place.
3. Instrument the code section that you're monitoring, thus:
time1= my_timer_xxx();
Instrumented code;
time2= my_timer_xxx();
elapsed_time= (time2 - time1) - overhead;
If the timer is always on, then overhead is always there,
so don't subtract it.
4. Save the elapsed time, or add it to a totaller.
5. When all timing processes are complete, transfer the
saved / totalled elapsed time to permanent storage.
Optionally you can convert cycles to microseconds at
this point. (Don't do so every time you calculate
elapsed_time! That would waste time and lose precision!)
For converting cycles to microseconds, use the frequency
that my_timer_init() returns. You'll also need to convert
if the my_timer_microseconds() function is the Windows
function QueryPerformanceCounter(), since that's sometimes
a counter with precision slightly better than microseconds.
Since we recommend calls by function pointer, we supply
no inline functions.
Some comments on the many candidate routines for timing ...
clock() -- We don't use because it would overflow frequently.
clock_gettime() -- In tests, clock_gettime often had
resolution = 1000.
ftime() -- A "man ftime" says: "This function is obsolete.
Don't use it." On every platform that we tested, if ftime()
was available, then so was gettimeofday(), and gettimeofday()
overhead was always at least as good as ftime() overhead.
gettimeofday() -- available on most platforms, though not
on Windows. There is a hardware timer (sometimes a Programmable
Interrupt Timer or "PIT") (sometimes a "HPET") used for
interrupt generation. When it interrupts (a "tick" or "jiffy",
typically 1 centisecond) it sets xtime. For gettimeofday, a
Linux kernel routine usually gets xtime and then gets rdtsc
to get elapsed nanoseconds since the last tick. On Red Hat
Enterprise Linux 3, there was once a bug which caused the
resolution to be 1000, i.e. one centisecond. We never check
for time-zone change.
getnstimeofday() -- something to watch for in future Linux
do_gettimeofday() -- exists on Linux but not for "userland"
get_cycles() -- a multi-platform function, worth watching
in future Linux versions. But we found platform-specific
functions which were better documented in operating-system
manuals. And get_cycles() can fail or return a useless
32-bit number. It might be available on some platforms,
such as arm, which we didn't test. Using
"include <linux/timex.h>" or "include <asm/timex.h>"
can lead to autoconf or compile errors, depending on system.
__rdtsc(): available for IA-32 and AMD64.
See "possible flaws and dangers" comments.
times(): what we use for ticks. Should just read the last
(xtime) tick count, therefore should be fast, but usually
isn't.
GetTickCount(): we use this for my_timer_ticks() on
Windows. Actually it really is a tick counter, so resolution
>= 10 milliseconds unless you have a very old Windows version.
With Windows 95 or 98 or ME, timeGetTime() has better resolution than
GetTickCount (1ms rather than 55ms). But with Windows NT or XP or 2000,
they're both getting from a variable in the Process Environment Block
(PEB), and the variable is set by the programmable interrupt timer, so
the resolution is the same (usually 10-15 milliseconds). Also timeGetTime
is slower on old machines:
http://www.doumo.jp/aon-java/jsp/postgretips/tips.jsp?tips=74.
Also timeGetTime requires linking winmm.lib,
Therefore we use GetTickCount.
It will overflow every 49 days because the return is 32-bit.
There is also a GetTickCount64 but it requires Vista or Windows Server 2008.
(As for GetSystemTimeAsFileTime, its precision is spurious, it
just reads the tick variable like the other functions do.
However, we don't expect it to overflow every 49 days, so we
will prefer it for my_timer_milliseconds().)
QueryPerformanceCounter() we use this for my_timer_microseconds()
on Windows. 1-PIT-tick (often 1/3-microsecond). Usually reads
the PIT so it's slow. On some Windows variants, uses RDTSC.
GetLocalTime() this is available on Windows but we don't use it.
getclock(): documented for Alpha, but not found during tests.
mach_absolute_time() and UpTime() are recommended for Apple.
Initially they weren't tried, because ppc_get_timebase seems to do the job.
But now we use mach_absolute_time for nanoseconds.
Any clock-based timer can be affected by NPT (ntpd program),
which means:
- full-second correction can occur for leap second
- tiny corrections can occur approximately every 11 minutes
(but I think they only affect the RTC which isn't the PIT).
We define "precision" as "frequency" and "high precision" is
"frequency better than 1 microsecond". We define "resolution"
as a synonym for "granularity". We define "accuracy" as
"closeness to the truth" as established by some authoritative
clock, but we can't measure accuracy.
Do not expect any of our timers to be monotonic; we
won't guarantee that they return constantly-increasing
unique numbers.
We tested with AIX, Solaris (x86 + Sparc), Linux (x86 +
Itanium), Windows, 64-bit Windows, QNX, FreeBSD, HPUX,
Irix, Mac. We didn't test with SCO.
*/
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