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new floor(), better periodic recalc

This commit is contained in:
Marc Alexander Lehmann 2011-02-20 02:56:23 +00:00
parent 4a748881d0
commit ed4a1b5f25
4 changed files with 106 additions and 32 deletions
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2
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@ -3,7 +3,7 @@ Revision history for libev, a high-performance and full-featured event loop.
TODO: ev_loop_prefork hint?
- change the default periodic reschedule function to hopefully be more
exact and correct.
exact and correct even in corner cases or in the far future.
- document reasonable ranges for interval and offset.
- do not rely on -lm anymore: use it when available but use our
own floor () if it is missing. This should make it easier to embed.

111
ev.c
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@ -45,6 +45,12 @@
# include "config.h"
# endif
#if HAVE_FLOOR
# ifndef EV_USE_FLOOR
# define EV_USE_FLOOR 1
# endif
#endif
# if HAVE_CLOCK_SYSCALL
# ifndef EV_USE_CLOCK_SYSCALL
# define EV_USE_CLOCK_SYSCALL 1
@ -158,7 +164,6 @@
#endif
#include <math.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
@ -234,6 +239,10 @@ EV_CPP(extern "C" {)
# define EV_NSIG 65
#endif
#ifndef EV_USE_FLOOR
# define EV_USE_FLOOR 0
#endif
#ifndef EV_USE_CLOCK_SYSCALL
# if __linux && __GLIBC__ >= 2
# define EV_USE_CLOCK_SYSCALL EV_FEATURE_OS
@ -445,14 +454,11 @@ struct signalfd_siginfo
#endif
/*
* This is used to avoid floating point rounding problems.
* It is added to ev_rt_now when scheduling periodics
* to ensure progress, time-wise, even when rounding
* errors are against us.
* This is used to work around floating point rounding problems.
* This value is good at least till the year 4000.
* Better solutions welcome.
*/
#define TIME_EPSILON 0.0001220703125 /* 1/8192 */
#define MIN_INTERVAL 0.0001220703125 /* 1/2**13, good till 4000 */
/*#define MIN_INTERVAL 0.00000095367431640625 /* 1/2**20, good till 2200 */
#define MIN_TIMEJUMP 1. /* minimum timejump that gets detected (if monotonic clock available) */
#define MAX_BLOCKTIME 59.743 /* never wait longer than this time (to detect time jumps) */
@ -525,6 +531,54 @@ static EV_ATOMIC_T have_monotonic; /* did clock_gettime (CLOCK_MONOTONIC) work?
/*****************************************************************************/
/* define a suitable floor function (only used by periodics atm) */
#if EV_USE_FLOOR
# include <math.h>
# define ev_floor(v) floor (v)
#else
#include <float.h>
/* a floor() replacement function, should be independent of ev_tstamp type */
static ev_tstamp noinline
ev_floor (ev_tstamp v)
{
/* the choice of shift factor is not terribly important */
#if FLT_RADIX != 2 /* assume FLT_RADIX == 10 */
const ev_tstamp shift = sizeof (unsigned long) >= 8 ? 10000000000000000000. : 1000000000.;
#else
const ev_tstamp shift = sizeof (unsigned long) >= 8 ? 18446744073709551616. : 4294967296.;
#endif
/* argument too large for an unsigned long? */
if (expect_false (v >= shift))
{
ev_tstamp f;
if (v == v - 1.)
return v; /* very large number */
f = shift * ev_floor (v * (1. / shift));
return f + ev_floor (v - f);
}
/* special treatment for negative args? */
if (expect_false (v < 0.))
{
ev_tstamp f = -ev_floor (-v);
return f - (f == v ? 0 : 1);
}
/* fits into an unsigned long */
return (unsigned long)v;
}
#endif
/*****************************************************************************/
#ifdef __linux
# include <sys/utsname.h>
#endif
@ -2210,12 +2264,28 @@ timers_reify (EV_P)
#if EV_PERIODIC_ENABLE
inline_speed void
static void noinline
periodic_recalc (EV_P_ ev_periodic *w)
{
/* TODO: use slow but potentially more correct incremental algo, */
/* also do not rely on ceil */
ev_at (w) = w->offset + ceil ((ev_rt_now - w->offset) / w->interval) * w->interval;
ev_tstamp interval = w->interval > MIN_INTERVAL ? w->interval : MIN_INTERVAL;
ev_tstamp at = w->offset + interval * ev_floor ((ev_rt_now - w->offset) / interval);
/* the above almost always errs on the low side */
while (at <= ev_rt_now)
{
ev_tstamp nat = at + w->interval;
/* when resolution fails us, we use ev_rt_now */
if (expect_false (nat == at))
{
at = ev_rt_now;
break;
}
at = nat;
}
ev_at (w) = at;
}
/* make periodics pending */
@ -2247,20 +2317,6 @@ periodics_reify (EV_P)
else if (w->interval)
{
periodic_recalc (EV_A_ w);
/* if next trigger time is not sufficiently in the future, put it there */
/* this might happen because of floating point inexactness */
if (ev_at (w) - ev_rt_now < TIME_EPSILON)
{
ev_at (w) += w->interval;
/* if interval is unreasonably low we might still have a time in the past */
/* so correct this. this will make the periodic very inexact, but the user */
/* has effectively asked to get triggered more often than possible */
if (ev_at (w) < ev_rt_now)
ev_at (w) = ev_rt_now;
}
ANHE_at_cache (periodics [HEAP0]);
downheap (periodics, periodiccnt, HEAP0);
}
@ -2348,9 +2404,12 @@ time_update (EV_P_ ev_tstamp max_block)
*/
for (i = 4; --i; )
{
ev_tstamp diff;
rtmn_diff = ev_rt_now - mn_now;
if (expect_true (fabs (odiff - rtmn_diff) < MIN_TIMEJUMP))
diff = odiff - rtmn_diff;
if (expect_true ((diff < 0. ? -diff : diff) < MIN_TIMEJUMP))
return; /* all is well */
ev_rt_now = ev_time ();

18
ev.pod
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@ -2153,9 +2153,12 @@ Another way to think about it (for the mathematically inclined) is that
C<ev_periodic> will try to run the callback in this mode at the next possible
time where C<time = offset (mod interval)>, regardless of any time jumps.
For numerical stability it is preferable that the C<offset> value is near
C<ev_now ()> (the current time), but there is no range requirement for
this value, and in fact is often specified as zero.
The C<interval> I<MUST> be positive, and for numerical stability, the
interval value should be higher than C<1/8192> (which is around 100
microseconds) and C<offset> should be higher than C<0> and should have
at most a similar magnitude as the current time (say, within a factor of
ten). Typical values for offset are, in fact, C<0> or something between
C<0> and C<interval>, which is also the recommended range.
Note also that there is an upper limit to how often a timer can fire (CPU
speed for example), so if C<interval> is very small then timing stability
@ -4206,6 +4209,15 @@ F<event.h> that are not directly supported by the libev core alone.
In standalone mode, libev will still try to automatically deduce the
configuration, but has to be more conservative.
=item EV_USE_FLOOR
If defined to be C<1>, libev will use the C<floor ()> function for its
periodic reschedule calculations, otherwise libev will fall back on a
portable (slower) implementation. If you enable this, you usually have to
link against libm or something equivalent. Enabling this when the C<floor>
function is not available will fail, so the safe default is to not enable
this.
=item EV_USE_MONOTONIC
If defined to be C<1>, libev will try to detect the availability of the

7
libev.m4
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@ -16,7 +16,7 @@ AC_CHECK_FUNCS(clock_gettime, [], [
#include <time.h>],
[struct timespec ts; int status = syscall (SYS_clock_gettime, CLOCK_REALTIME, &ts)])],
[ac_have_clock_syscall=1
AC_DEFINE(HAVE_CLOCK_SYSCALL, 1, "use syscall interface for clock_gettime")
AC_DEFINE(HAVE_CLOCK_SYSCALL, 1, Define to 1 to use the syscall interface for clock_gettime)
AC_MSG_RESULT(yes)],
[AC_MSG_RESULT(no)])
fi
@ -35,5 +35,8 @@ AC_CHECK_FUNCS(nanosleep, [], [
fi
])
AC_CHECK_LIB(m, ceil)
if test -z "$LIBEV_M4_AVOID_LIBM"; then
LIBM=m
fi
AC_SEARCH_LIBS(floor, $LIBM, [AC_DEFINE(HAVE_FLOOR, 1, Define to 1 if the floor function is available)])