2004-11-03 13:51:07 +00:00
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.\" Copyright 7/93 by Darren Senn <sinster@scintilla.santa-clara.ca.us>
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.\" Based on a similar page Copyright 1992 by Rick Faith
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.\" May be freely distributed
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.\" Modified Tue Oct 22 00:22:35 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
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2005-04-06 15:29:32 +00:00
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.\" 2005-04-06 mtk, Matthias Lang <matthias@corelatus.se>
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.\" Noted MAX_SEC_IN_JIFFIES ceiling
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2008-06-23 10:28:23 +00:00
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.TH GETITIMER 2 2008-04-24 "Linux" "Linux Programmer's Manual"
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2004-11-03 13:51:07 +00:00
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.SH NAME
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getitimer, setitimer \- get or set value of an interval timer
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.SH SYNOPSIS
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2006-03-08 02:48:53 +00:00
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.nf
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2004-11-03 13:51:07 +00:00
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.B #include <sys/time.h>
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.sp
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2006-03-08 02:48:53 +00:00
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.BI "int getitimer(int " which ", struct itimerval *" value );
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.br
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.BI "int setitimer(int " which ", const struct itimerval *" value ,
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.BI " struct itimerval *" ovalue );
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.fi
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2004-11-03 13:51:07 +00:00
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.SH DESCRIPTION
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2007-04-12 22:42:49 +00:00
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The system provides each process with three interval timers,
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each decrementing in a distinct time domain.
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When any timer expires, a signal is sent to the
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2004-11-03 13:51:07 +00:00
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process, and the timer (potentially) restarts.
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.TP 1.5i
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.B ITIMER_REAL
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decrements in real time, and delivers
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.B SIGALRM
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upon expiration.
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.TP
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.B ITIMER_VIRTUAL
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decrements only when the process is executing, and delivers
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.B SIGVTALRM
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upon expiration.
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.TP
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.B ITIMER_PROF
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decrements both when the process executes and when the system is executing
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2007-04-12 22:42:49 +00:00
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on behalf of the process.
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Coupled with
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2004-11-03 13:51:07 +00:00
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.BR ITIMER_VIRTUAL ,
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2007-04-12 22:42:49 +00:00
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this timer is usually used to profile the time spent by the
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application in user and kernel space.
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2004-11-03 13:51:07 +00:00
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.B SIGPROF
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is delivered upon expiration.
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.LP
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Timer values are defined by the following structures:
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.PD 0
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2007-12-23 14:14:04 +00:00
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.in +4n
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2004-11-03 13:51:07 +00:00
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.nf
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Salut Olivier (and Nishanth),
Regarding man page documentation of the problem of short sleeps
for setiteimer(2)...
> > -- pointers to those threads
>
> http://bugzilla.kernel.org/show_bug.cgi?id=4569
> http://lkml.org/lkml/2005/4/29/163
>
> > -- indications of which kernel versions show this bahaviour
>
> AFAIK, all versions as far as x86 is concerned.
> Dunno if it is hardware specific.
>
> > -- a (short) test program to demonstrate it, if you have one.
>
> See the bugzilla bug's attachments
Sorry for the long delay in following this up, but I've got to
it now. I tweaked your suggestions slightly:
{{
Timers will never expire before the requested time,
-instead expiring some short, constant time afterwards, dependent
-on the system timer resolution (currently 10ms).
+but may expire some (short) time afterwards, which depends
+on the system timer resolution and on the system load.
+Upon expiration, a signal will be generated and the timer reset.
+If the timer expires while the process is active (always true for
+On certain systems (including x86), the Linux kernel has a bug which will
+produce premature timer expirations of up to one jiffy under some
+circumstances.
}}
Thanks for this bug reporet,
Nishanth: if and when your changes are accepted, and the problem
is thus fixed, could you please send me a notification of that
fact, and I can then further amend the manual pages.
Cheers,
Michael
/* itimer_short_interval_bug.c
June 2005
In current Linux kernels, an interval timer set using setitimer()
can sometimes sleep *less* than the specified interval.
This program demonstrates the behaviour by looping through all
itimer values from 1 microsecond upwards, in one microsecond steps.
*/
/* Adapted from a program by Olivier Croquette, June 2005 */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/wait.h>
typedef unsigned long long int u_time_t; /* in microsecs */
static int handler_flag;
/* return time as a number of microsecs */
static u_time_t
gettime(void )
{
struct timeval tv;
if ( gettimeofday(&tv, NULL) == -1) {
perror("gettimeofday()");
return 0;
}
return (tv.tv_usec + tv.tv_sec * 1000000LL);
}
static void
handler (int sig, siginfo_t *siginfo, void *context)
{
handler_flag++;
return ;
}
/* Sleep for 'time' microsecs. */
static int
isleep(u_time_t time)
{
struct itimerval newtv;
sigset_t sigset;
struct sigaction sigact;
if (time == 0)
return 0;
/* block SIGALRM */
sigemptyset (&sigset);
sigaddset (&sigset, SIGALRM);
sigprocmask (SIG_BLOCK, &sigset, NULL);
/* set up our handler */
sigact.sa_sigaction = handler;
sigemptyset(&sigact.sa_mask);
sigact.sa_flags = SA_SIGINFO;
sigaction (SIGALRM, &sigact, NULL);
newtv.it_interval.tv_sec = 0;
newtv.it_interval.tv_usec = 0;
newtv.it_value.tv_sec = time / 1000000;
newtv.it_value.tv_usec = time % 1000000;
if (setitimer(ITIMER_REAL,&newtv,NULL) == -1) {
perror("setitimer(set)");
return 1;
}
sigemptyset (&sigset);
sigsuspend (&sigset);
return 0;
}
int
main(int argc, char *argv[]) {
u_time_t wait;
int loop, numLoops;
u_time_t t1, t2;
u_time_t actual;
long long minDiff, maxDiff, totDiff, diff;
int numFail = 0;
if (argc != 2) {
fprintf(stderr, "Usage: %s num-loops\n", argv[0]);
exit(EXIT_FAILURE);
} /* if */
numLoops = atoi(argv[1]);
setbuf(stdout, NULL);
for (wait = 1; ; wait++) {
maxDiff = 0;
numFail = 0;
totDiff = 0;
minDiff = -wait;
if (wait % 10000 == 0)
printf("%llu\n", wait);
for (loop = 0; loop < numLoops; loop++) {
t1 = gettime();
handler_flag = 0;
isleep(wait);
if ( handler_flag != 1 )
printf("Problem with the handler flag (%d)!\n", handler_flag);
t2 = gettime();
actual = t2 - t1;
if ( actual < wait ) {
diff = actual - wait;
if (diff < maxDiff)
maxDiff = diff;
if (diff > minDiff)
minDiff = diff;
totDiff += diff;
numFail++;
} /* if */
} /* for */
if (numFail > 0)
printf("%llu: %3d fail (%4lld %4lld; avg=%6.1f)\n",
wait, numFail, minDiff, maxDiff,
(double) totDiff / numFail);
} /* for */
return 0;
} /* main */
2005-06-13 09:01:49 +00:00
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2004-11-03 13:51:07 +00:00
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struct itimerval {
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struct timeval it_interval; /* next value */
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struct timeval it_value; /* current value */
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};
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2007-12-23 14:14:04 +00:00
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2004-11-03 13:51:07 +00:00
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struct timeval {
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long tv_sec; /* seconds */
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long tv_usec; /* microseconds */
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};
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.fi
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2007-12-23 14:14:04 +00:00
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.in
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2004-11-03 13:51:07 +00:00
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.PD
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.LP
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The function
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2005-06-23 07:16:55 +00:00
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.BR getitimer ()
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2004-11-03 13:51:07 +00:00
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fills the structure indicated by
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.I value
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with the current setting for the timer indicated by
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.I which
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(one of
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.BR ITIMER_REAL ,
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.BR ITIMER_VIRTUAL ,
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or
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.BR ITIMER_PROF ).
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The element
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2005-04-06 15:29:32 +00:00
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.I it_value
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2004-11-03 13:51:07 +00:00
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is set to the amount of time remaining on the timer, or zero if the timer
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2007-04-12 22:42:49 +00:00
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is disabled.
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Similarly,
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2005-04-06 15:29:32 +00:00
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.I it_interval
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2004-11-03 13:51:07 +00:00
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is set to the reset value.
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The function
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2005-06-23 07:16:55 +00:00
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.BR setitimer ()
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2004-11-03 13:51:07 +00:00
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sets the indicated timer to the value in
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.IR value .
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If
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.I ovalue
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2008-06-23 10:28:23 +00:00
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is non-NULL, the old value of the timer is stored there.
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2004-11-03 13:51:07 +00:00
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.LP
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Timers decrement from
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.I it_value
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to zero, generate a signal, and reset to
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.IR it_interval .
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A timer which is set to zero
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.RI ( it_value
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is zero or the timer expires and
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.I it_interval
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is zero) stops.
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.LP
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Both
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.I tv_sec
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and
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.I tv_usec
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are significant in determining the duration of a timer.
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.LP
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Timers will never expire before the requested time,
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Salut Olivier (and Nishanth),
Regarding man page documentation of the problem of short sleeps
for setiteimer(2)...
> > -- pointers to those threads
>
> http://bugzilla.kernel.org/show_bug.cgi?id=4569
> http://lkml.org/lkml/2005/4/29/163
>
> > -- indications of which kernel versions show this bahaviour
>
> AFAIK, all versions as far as x86 is concerned.
> Dunno if it is hardware specific.
>
> > -- a (short) test program to demonstrate it, if you have one.
>
> See the bugzilla bug's attachments
Sorry for the long delay in following this up, but I've got to
it now. I tweaked your suggestions slightly:
{{
Timers will never expire before the requested time,
-instead expiring some short, constant time afterwards, dependent
-on the system timer resolution (currently 10ms).
+but may expire some (short) time afterwards, which depends
+on the system timer resolution and on the system load.
+Upon expiration, a signal will be generated and the timer reset.
+If the timer expires while the process is active (always true for
+On certain systems (including x86), the Linux kernel has a bug which will
+produce premature timer expirations of up to one jiffy under some
+circumstances.
}}
Thanks for this bug reporet,
Nishanth: if and when your changes are accepted, and the problem
is thus fixed, could you please send me a notification of that
fact, and I can then further amend the manual pages.
Cheers,
Michael
/* itimer_short_interval_bug.c
June 2005
In current Linux kernels, an interval timer set using setitimer()
can sometimes sleep *less* than the specified interval.
This program demonstrates the behaviour by looping through all
itimer values from 1 microsecond upwards, in one microsecond steps.
*/
/* Adapted from a program by Olivier Croquette, June 2005 */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/wait.h>
typedef unsigned long long int u_time_t; /* in microsecs */
static int handler_flag;
/* return time as a number of microsecs */
static u_time_t
gettime(void )
{
struct timeval tv;
if ( gettimeofday(&tv, NULL) == -1) {
perror("gettimeofday()");
return 0;
}
return (tv.tv_usec + tv.tv_sec * 1000000LL);
}
static void
handler (int sig, siginfo_t *siginfo, void *context)
{
handler_flag++;
return ;
}
/* Sleep for 'time' microsecs. */
static int
isleep(u_time_t time)
{
struct itimerval newtv;
sigset_t sigset;
struct sigaction sigact;
if (time == 0)
return 0;
/* block SIGALRM */
sigemptyset (&sigset);
sigaddset (&sigset, SIGALRM);
sigprocmask (SIG_BLOCK, &sigset, NULL);
/* set up our handler */
sigact.sa_sigaction = handler;
sigemptyset(&sigact.sa_mask);
sigact.sa_flags = SA_SIGINFO;
sigaction (SIGALRM, &sigact, NULL);
newtv.it_interval.tv_sec = 0;
newtv.it_interval.tv_usec = 0;
newtv.it_value.tv_sec = time / 1000000;
newtv.it_value.tv_usec = time % 1000000;
if (setitimer(ITIMER_REAL,&newtv,NULL) == -1) {
perror("setitimer(set)");
return 1;
}
sigemptyset (&sigset);
sigsuspend (&sigset);
return 0;
}
int
main(int argc, char *argv[]) {
u_time_t wait;
int loop, numLoops;
u_time_t t1, t2;
u_time_t actual;
long long minDiff, maxDiff, totDiff, diff;
int numFail = 0;
if (argc != 2) {
fprintf(stderr, "Usage: %s num-loops\n", argv[0]);
exit(EXIT_FAILURE);
} /* if */
numLoops = atoi(argv[1]);
setbuf(stdout, NULL);
for (wait = 1; ; wait++) {
maxDiff = 0;
numFail = 0;
totDiff = 0;
minDiff = -wait;
if (wait % 10000 == 0)
printf("%llu\n", wait);
for (loop = 0; loop < numLoops; loop++) {
t1 = gettime();
handler_flag = 0;
isleep(wait);
if ( handler_flag != 1 )
printf("Problem with the handler flag (%d)!\n", handler_flag);
t2 = gettime();
actual = t2 - t1;
if ( actual < wait ) {
diff = actual - wait;
if (diff < maxDiff)
maxDiff = diff;
if (diff > minDiff)
minDiff = diff;
totDiff += diff;
numFail++;
} /* if */
} /* for */
if (numFail > 0)
printf("%llu: %3d fail (%4lld %4lld; avg=%6.1f)\n",
wait, numFail, minDiff, maxDiff,
(double) totDiff / numFail);
} /* for */
return 0;
} /* main */
2005-06-13 09:01:49 +00:00
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but may expire some (short) time afterwards, which depends
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2008-06-23 10:28:23 +00:00
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on the system timer resolution and on the system load; see
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.BR time (7).
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2005-06-21 13:50:30 +00:00
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(But see BUGS below.)
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Salut Olivier (and Nishanth),
Regarding man page documentation of the problem of short sleeps
for setiteimer(2)...
> > -- pointers to those threads
>
> http://bugzilla.kernel.org/show_bug.cgi?id=4569
> http://lkml.org/lkml/2005/4/29/163
>
> > -- indications of which kernel versions show this bahaviour
>
> AFAIK, all versions as far as x86 is concerned.
> Dunno if it is hardware specific.
>
> > -- a (short) test program to demonstrate it, if you have one.
>
> See the bugzilla bug's attachments
Sorry for the long delay in following this up, but I've got to
it now. I tweaked your suggestions slightly:
{{
Timers will never expire before the requested time,
-instead expiring some short, constant time afterwards, dependent
-on the system timer resolution (currently 10ms).
+but may expire some (short) time afterwards, which depends
+on the system timer resolution and on the system load.
+Upon expiration, a signal will be generated and the timer reset.
+If the timer expires while the process is active (always true for
+On certain systems (including x86), the Linux kernel has a bug which will
+produce premature timer expirations of up to one jiffy under some
+circumstances.
}}
Thanks for this bug reporet,
Nishanth: if and when your changes are accepted, and the problem
is thus fixed, could you please send me a notification of that
fact, and I can then further amend the manual pages.
Cheers,
Michael
/* itimer_short_interval_bug.c
June 2005
In current Linux kernels, an interval timer set using setitimer()
can sometimes sleep *less* than the specified interval.
This program demonstrates the behaviour by looping through all
itimer values from 1 microsecond upwards, in one microsecond steps.
*/
/* Adapted from a program by Olivier Croquette, June 2005 */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/wait.h>
typedef unsigned long long int u_time_t; /* in microsecs */
static int handler_flag;
/* return time as a number of microsecs */
static u_time_t
gettime(void )
{
struct timeval tv;
if ( gettimeofday(&tv, NULL) == -1) {
perror("gettimeofday()");
return 0;
}
return (tv.tv_usec + tv.tv_sec * 1000000LL);
}
static void
handler (int sig, siginfo_t *siginfo, void *context)
{
handler_flag++;
return ;
}
/* Sleep for 'time' microsecs. */
static int
isleep(u_time_t time)
{
struct itimerval newtv;
sigset_t sigset;
struct sigaction sigact;
if (time == 0)
return 0;
/* block SIGALRM */
sigemptyset (&sigset);
sigaddset (&sigset, SIGALRM);
sigprocmask (SIG_BLOCK, &sigset, NULL);
/* set up our handler */
sigact.sa_sigaction = handler;
sigemptyset(&sigact.sa_mask);
sigact.sa_flags = SA_SIGINFO;
sigaction (SIGALRM, &sigact, NULL);
newtv.it_interval.tv_sec = 0;
newtv.it_interval.tv_usec = 0;
newtv.it_value.tv_sec = time / 1000000;
newtv.it_value.tv_usec = time % 1000000;
if (setitimer(ITIMER_REAL,&newtv,NULL) == -1) {
perror("setitimer(set)");
return 1;
}
sigemptyset (&sigset);
sigsuspend (&sigset);
return 0;
}
int
main(int argc, char *argv[]) {
u_time_t wait;
int loop, numLoops;
u_time_t t1, t2;
u_time_t actual;
long long minDiff, maxDiff, totDiff, diff;
int numFail = 0;
if (argc != 2) {
fprintf(stderr, "Usage: %s num-loops\n", argv[0]);
exit(EXIT_FAILURE);
} /* if */
numLoops = atoi(argv[1]);
setbuf(stdout, NULL);
for (wait = 1; ; wait++) {
maxDiff = 0;
numFail = 0;
totDiff = 0;
minDiff = -wait;
if (wait % 10000 == 0)
printf("%llu\n", wait);
for (loop = 0; loop < numLoops; loop++) {
t1 = gettime();
handler_flag = 0;
isleep(wait);
if ( handler_flag != 1 )
printf("Problem with the handler flag (%d)!\n", handler_flag);
t2 = gettime();
actual = t2 - t1;
if ( actual < wait ) {
diff = actual - wait;
if (diff < maxDiff)
maxDiff = diff;
if (diff > minDiff)
minDiff = diff;
totDiff += diff;
numFail++;
} /* if */
} /* for */
if (numFail > 0)
printf("%llu: %3d fail (%4lld %4lld; avg=%6.1f)\n",
wait, numFail, minDiff, maxDiff,
(double) totDiff / numFail);
} /* for */
return 0;
} /* main */
2005-06-13 09:01:49 +00:00
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Upon expiration, a signal will be generated and the timer reset.
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If the timer expires while the process is active (always true for
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2006-03-20 00:52:31 +00:00
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.BR ITIMER_VIRTUAL )
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2007-04-12 22:42:49 +00:00
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the signal will be delivered immediately when generated.
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Otherwise the
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2004-11-03 13:51:07 +00:00
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delivery will be offset by a small time dependent on the system loading.
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.SH "RETURN VALUE"
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2007-04-12 22:42:49 +00:00
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On success, zero is returned.
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On error, \-1 is returned, and
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2004-11-03 13:51:07 +00:00
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.I errno
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is set appropriately.
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.SH ERRORS
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.TP
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.B EFAULT
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.I value
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or
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.I ovalue
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are not valid pointers.
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.TP
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.B EINVAL
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.I which
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is not one of
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.BR ITIMER_REAL ,
|
2006-03-20 00:52:31 +00:00
|
|
|
.BR ITIMER_VIRTUAL ,
|
2004-11-03 13:51:07 +00:00
|
|
|
or
|
2007-08-28 13:15:35 +00:00
|
|
|
.BR ITIMER_PROF ;
|
2008-05-07 13:33:52 +00:00
|
|
|
or (since Linux 2.6.22) one of the
|
2007-08-28 13:15:35 +00:00
|
|
|
.I tv_usec
|
|
|
|
|
fields contains a value outside the range 0 to 999999.
|
2007-05-18 16:06:42 +00:00
|
|
|
.SH "CONFORMING TO"
|
|
|
|
|
POSIX.1-2001, SVr4, 4.4BSD (this call first appeared in 4.2BSD).
|
2008-03-19 15:09:59 +00:00
|
|
|
.\" FIXME . Mar 08: The next POSIX.1 revisions marks getitimer() and
|
|
|
|
|
.\" setitimer() obsolete.
|
2006-07-22 15:54:34 +00:00
|
|
|
.SH NOTES
|
|
|
|
|
A child created via
|
|
|
|
|
.BR fork (2)
|
|
|
|
|
does not inherit its parent's interval timers.
|
2007-04-12 22:42:49 +00:00
|
|
|
Interval timers are preserved across an
|
2006-07-22 15:54:34 +00:00
|
|
|
.BR execve (2).
|
2008-06-23 10:28:23 +00:00
|
|
|
|
2008-06-25 15:32:01 +00:00
|
|
|
POSIX.1 leaves the
|
2008-06-23 10:28:23 +00:00
|
|
|
interaction between
|
|
|
|
|
.BR setitimer()
|
2008-06-27 12:10:15 +00:00
|
|
|
and the three interfaces
|
2008-06-23 10:28:23 +00:00
|
|
|
.BR alarm (2),
|
|
|
|
|
.BR sleep (3),
|
|
|
|
|
and
|
|
|
|
|
.BR usleep (3)
|
|
|
|
|
unspecified.
|
2004-11-03 13:51:07 +00:00
|
|
|
.SH BUGS
|
Salut Olivier (and Nishanth),
Regarding man page documentation of the problem of short sleeps
for setiteimer(2)...
> > -- pointers to those threads
>
> http://bugzilla.kernel.org/show_bug.cgi?id=4569
> http://lkml.org/lkml/2005/4/29/163
>
> > -- indications of which kernel versions show this bahaviour
>
> AFAIK, all versions as far as x86 is concerned.
> Dunno if it is hardware specific.
>
> > -- a (short) test program to demonstrate it, if you have one.
>
> See the bugzilla bug's attachments
Sorry for the long delay in following this up, but I've got to
it now. I tweaked your suggestions slightly:
{{
Timers will never expire before the requested time,
-instead expiring some short, constant time afterwards, dependent
-on the system timer resolution (currently 10ms).
+but may expire some (short) time afterwards, which depends
+on the system timer resolution and on the system load.
+Upon expiration, a signal will be generated and the timer reset.
+If the timer expires while the process is active (always true for
+On certain systems (including x86), the Linux kernel has a bug which will
+produce premature timer expirations of up to one jiffy under some
+circumstances.
}}
Thanks for this bug reporet,
Nishanth: if and when your changes are accepted, and the problem
is thus fixed, could you please send me a notification of that
fact, and I can then further amend the manual pages.
Cheers,
Michael
/* itimer_short_interval_bug.c
June 2005
In current Linux kernels, an interval timer set using setitimer()
can sometimes sleep *less* than the specified interval.
This program demonstrates the behaviour by looping through all
itimer values from 1 microsecond upwards, in one microsecond steps.
*/
/* Adapted from a program by Olivier Croquette, June 2005 */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/wait.h>
typedef unsigned long long int u_time_t; /* in microsecs */
static int handler_flag;
/* return time as a number of microsecs */
static u_time_t
gettime(void )
{
struct timeval tv;
if ( gettimeofday(&tv, NULL) == -1) {
perror("gettimeofday()");
return 0;
}
return (tv.tv_usec + tv.tv_sec * 1000000LL);
}
static void
handler (int sig, siginfo_t *siginfo, void *context)
{
handler_flag++;
return ;
}
/* Sleep for 'time' microsecs. */
static int
isleep(u_time_t time)
{
struct itimerval newtv;
sigset_t sigset;
struct sigaction sigact;
if (time == 0)
return 0;
/* block SIGALRM */
sigemptyset (&sigset);
sigaddset (&sigset, SIGALRM);
sigprocmask (SIG_BLOCK, &sigset, NULL);
/* set up our handler */
sigact.sa_sigaction = handler;
sigemptyset(&sigact.sa_mask);
sigact.sa_flags = SA_SIGINFO;
sigaction (SIGALRM, &sigact, NULL);
newtv.it_interval.tv_sec = 0;
newtv.it_interval.tv_usec = 0;
newtv.it_value.tv_sec = time / 1000000;
newtv.it_value.tv_usec = time % 1000000;
if (setitimer(ITIMER_REAL,&newtv,NULL) == -1) {
perror("setitimer(set)");
return 1;
}
sigemptyset (&sigset);
sigsuspend (&sigset);
return 0;
}
int
main(int argc, char *argv[]) {
u_time_t wait;
int loop, numLoops;
u_time_t t1, t2;
u_time_t actual;
long long minDiff, maxDiff, totDiff, diff;
int numFail = 0;
if (argc != 2) {
fprintf(stderr, "Usage: %s num-loops\n", argv[0]);
exit(EXIT_FAILURE);
} /* if */
numLoops = atoi(argv[1]);
setbuf(stdout, NULL);
for (wait = 1; ; wait++) {
maxDiff = 0;
numFail = 0;
totDiff = 0;
minDiff = -wait;
if (wait % 10000 == 0)
printf("%llu\n", wait);
for (loop = 0; loop < numLoops; loop++) {
t1 = gettime();
handler_flag = 0;
isleep(wait);
if ( handler_flag != 1 )
printf("Problem with the handler flag (%d)!\n", handler_flag);
t2 = gettime();
actual = t2 - t1;
if ( actual < wait ) {
diff = actual - wait;
if (diff < maxDiff)
maxDiff = diff;
if (diff > minDiff)
minDiff = diff;
totDiff += diff;
numFail++;
} /* if */
} /* for */
if (numFail > 0)
printf("%llu: %3d fail (%4lld %4lld; avg=%6.1f)\n",
wait, numFail, minDiff, maxDiff,
(double) totDiff / numFail);
} /* for */
return 0;
} /* main */
2005-06-13 09:01:49 +00:00
|
|
|
The generation and delivery of a signal are distinct, and
|
|
|
|
|
only one instance of each of the signals listed above may be pending
|
|
|
|
|
for a process.
|
2007-12-19 06:29:23 +00:00
|
|
|
Under very heavy loading, an
|
2004-11-03 13:51:07 +00:00
|
|
|
.B ITIMER_REAL
|
Salut Olivier (and Nishanth),
Regarding man page documentation of the problem of short sleeps
for setiteimer(2)...
> > -- pointers to those threads
>
> http://bugzilla.kernel.org/show_bug.cgi?id=4569
> http://lkml.org/lkml/2005/4/29/163
>
> > -- indications of which kernel versions show this bahaviour
>
> AFAIK, all versions as far as x86 is concerned.
> Dunno if it is hardware specific.
>
> > -- a (short) test program to demonstrate it, if you have one.
>
> See the bugzilla bug's attachments
Sorry for the long delay in following this up, but I've got to
it now. I tweaked your suggestions slightly:
{{
Timers will never expire before the requested time,
-instead expiring some short, constant time afterwards, dependent
-on the system timer resolution (currently 10ms).
+but may expire some (short) time afterwards, which depends
+on the system timer resolution and on the system load.
+Upon expiration, a signal will be generated and the timer reset.
+If the timer expires while the process is active (always true for
+On certain systems (including x86), the Linux kernel has a bug which will
+produce premature timer expirations of up to one jiffy under some
+circumstances.
}}
Thanks for this bug reporet,
Nishanth: if and when your changes are accepted, and the problem
is thus fixed, could you please send me a notification of that
fact, and I can then further amend the manual pages.
Cheers,
Michael
/* itimer_short_interval_bug.c
June 2005
In current Linux kernels, an interval timer set using setitimer()
can sometimes sleep *less* than the specified interval.
This program demonstrates the behaviour by looping through all
itimer values from 1 microsecond upwards, in one microsecond steps.
*/
/* Adapted from a program by Olivier Croquette, June 2005 */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/wait.h>
typedef unsigned long long int u_time_t; /* in microsecs */
static int handler_flag;
/* return time as a number of microsecs */
static u_time_t
gettime(void )
{
struct timeval tv;
if ( gettimeofday(&tv, NULL) == -1) {
perror("gettimeofday()");
return 0;
}
return (tv.tv_usec + tv.tv_sec * 1000000LL);
}
static void
handler (int sig, siginfo_t *siginfo, void *context)
{
handler_flag++;
return ;
}
/* Sleep for 'time' microsecs. */
static int
isleep(u_time_t time)
{
struct itimerval newtv;
sigset_t sigset;
struct sigaction sigact;
if (time == 0)
return 0;
/* block SIGALRM */
sigemptyset (&sigset);
sigaddset (&sigset, SIGALRM);
sigprocmask (SIG_BLOCK, &sigset, NULL);
/* set up our handler */
sigact.sa_sigaction = handler;
sigemptyset(&sigact.sa_mask);
sigact.sa_flags = SA_SIGINFO;
sigaction (SIGALRM, &sigact, NULL);
newtv.it_interval.tv_sec = 0;
newtv.it_interval.tv_usec = 0;
newtv.it_value.tv_sec = time / 1000000;
newtv.it_value.tv_usec = time % 1000000;
if (setitimer(ITIMER_REAL,&newtv,NULL) == -1) {
perror("setitimer(set)");
return 1;
}
sigemptyset (&sigset);
sigsuspend (&sigset);
return 0;
}
int
main(int argc, char *argv[]) {
u_time_t wait;
int loop, numLoops;
u_time_t t1, t2;
u_time_t actual;
long long minDiff, maxDiff, totDiff, diff;
int numFail = 0;
if (argc != 2) {
fprintf(stderr, "Usage: %s num-loops\n", argv[0]);
exit(EXIT_FAILURE);
} /* if */
numLoops = atoi(argv[1]);
setbuf(stdout, NULL);
for (wait = 1; ; wait++) {
maxDiff = 0;
numFail = 0;
totDiff = 0;
minDiff = -wait;
if (wait % 10000 == 0)
printf("%llu\n", wait);
for (loop = 0; loop < numLoops; loop++) {
t1 = gettime();
handler_flag = 0;
isleep(wait);
if ( handler_flag != 1 )
printf("Problem with the handler flag (%d)!\n", handler_flag);
t2 = gettime();
actual = t2 - t1;
if ( actual < wait ) {
diff = actual - wait;
if (diff < maxDiff)
maxDiff = diff;
if (diff > minDiff)
minDiff = diff;
totDiff += diff;
numFail++;
} /* if */
} /* for */
if (numFail > 0)
printf("%llu: %3d fail (%4lld %4lld; avg=%6.1f)\n",
wait, numFail, minDiff, maxDiff,
(double) totDiff / numFail);
} /* for */
return 0;
} /* main */
2005-06-13 09:01:49 +00:00
|
|
|
timer may expire before the signal from a previous expiration
|
|
|
|
|
has been delivered.
|
2004-11-03 13:51:07 +00:00
|
|
|
The second signal in such an event will be lost.
|
2005-04-06 15:29:32 +00:00
|
|
|
|
2008-06-23 10:28:23 +00:00
|
|
|
On Linux kernels before 2.6.16, timer values are represented in jiffies.
|
2007-04-12 22:42:49 +00:00
|
|
|
If a request is made set a timer with a value whose jiffies
|
2007-06-22 17:16:20 +00:00
|
|
|
representation exceeds
|
|
|
|
|
.B MAX_SEC_IN_JIFFIES
|
2005-04-06 15:29:32 +00:00
|
|
|
(defined in
|
|
|
|
|
.IR include/linux/jiffies.h ),
|
|
|
|
|
then the timer is silently truncated to this ceiling value.
|
2008-05-07 13:33:52 +00:00
|
|
|
On Linux/i386 (where, since Linux 2.6.13,
|
2006-04-26 04:42:14 +00:00
|
|
|
the default jiffy is 0.004 seconds),
|
2005-04-06 15:29:32 +00:00
|
|
|
this means that the ceiling value for a timer is
|
2006-04-26 04:42:14 +00:00
|
|
|
approximately 99.42 days.
|
2008-06-23 10:28:23 +00:00
|
|
|
Since Linux 2.6.16,
|
|
|
|
|
the kernel uses a different internal representation for times,
|
|
|
|
|
and this ceiling is removed.
|
2005-04-06 15:29:32 +00:00
|
|
|
|
2007-12-24 17:31:35 +00:00
|
|
|
On certain systems (including i386),
|
2007-04-12 22:42:49 +00:00
|
|
|
Linux kernels before version 2.6.12 have a bug which will produce
|
2005-06-23 07:16:55 +00:00
|
|
|
premature timer expirations of up to one jiffy under some circumstances.
|
|
|
|
|
This bug is fixed in kernel 2.6.12.
|
Von: "Michael Kerrisk" <mtk-manpages@gmx.net>
An: Olivier Croquette <ocroquette@free.fr>
Betreff: Re: 2.6.12 and setitimer
Datum: Mon, 4 Jul 2005 08:36:35 +0200 (MEST)
Hi Olivier,
> You will probably consider adding also a note to point out that the bug
> will stay a known bug of the 2.4 serie:
>
> http://lkml.org/lkml/2005/7/1/165
First off, I _very_ much appreciate the fact that you keep
informing me of the progress of this bug! Thank you.
At the moment, I'm inlined yo leave the manual page as it is.
It currently reads:
On certain systems (including x86), Linux ker‐
nels before version 2.6.12 have a bug which
will produce premature timer expirations of up
to one jiffy under some circumstances. This
bug is fixed in kernel 2.6.12.
To me that implies that the bug also affects kernels before
2.4 -- e.g., 2.4.x. Now, what would be interesting is if the
bug *does* get fixed in 2.4, then I could also add a note
about the 2.4.x version where it is fixed.
In the meantime, I have added a note to myself (i.e., a comment
in the man page source) about this point.
If the bug *does* eventually get fixed in 2.4.x, and you
hear of it, please do let me know.
Thanks,
Michael
2005-07-04 06:44:50 +00:00
|
|
|
.\" 4 Jul 2005: It looks like this bug may remain in 2.4.x.
|
|
|
|
|
.\" http://lkml.org/lkml/2005/7/1/165
|
Salut Olivier (and Nishanth),
Regarding man page documentation of the problem of short sleeps
for setiteimer(2)...
> > -- pointers to those threads
>
> http://bugzilla.kernel.org/show_bug.cgi?id=4569
> http://lkml.org/lkml/2005/4/29/163
>
> > -- indications of which kernel versions show this bahaviour
>
> AFAIK, all versions as far as x86 is concerned.
> Dunno if it is hardware specific.
>
> > -- a (short) test program to demonstrate it, if you have one.
>
> See the bugzilla bug's attachments
Sorry for the long delay in following this up, but I've got to
it now. I tweaked your suggestions slightly:
{{
Timers will never expire before the requested time,
-instead expiring some short, constant time afterwards, dependent
-on the system timer resolution (currently 10ms).
+but may expire some (short) time afterwards, which depends
+on the system timer resolution and on the system load.
+Upon expiration, a signal will be generated and the timer reset.
+If the timer expires while the process is active (always true for
+On certain systems (including x86), the Linux kernel has a bug which will
+produce premature timer expirations of up to one jiffy under some
+circumstances.
}}
Thanks for this bug reporet,
Nishanth: if and when your changes are accepted, and the problem
is thus fixed, could you please send me a notification of that
fact, and I can then further amend the manual pages.
Cheers,
Michael
/* itimer_short_interval_bug.c
June 2005
In current Linux kernels, an interval timer set using setitimer()
can sometimes sleep *less* than the specified interval.
This program demonstrates the behaviour by looping through all
itimer values from 1 microsecond upwards, in one microsecond steps.
*/
/* Adapted from a program by Olivier Croquette, June 2005 */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/wait.h>
typedef unsigned long long int u_time_t; /* in microsecs */
static int handler_flag;
/* return time as a number of microsecs */
static u_time_t
gettime(void )
{
struct timeval tv;
if ( gettimeofday(&tv, NULL) == -1) {
perror("gettimeofday()");
return 0;
}
return (tv.tv_usec + tv.tv_sec * 1000000LL);
}
static void
handler (int sig, siginfo_t *siginfo, void *context)
{
handler_flag++;
return ;
}
/* Sleep for 'time' microsecs. */
static int
isleep(u_time_t time)
{
struct itimerval newtv;
sigset_t sigset;
struct sigaction sigact;
if (time == 0)
return 0;
/* block SIGALRM */
sigemptyset (&sigset);
sigaddset (&sigset, SIGALRM);
sigprocmask (SIG_BLOCK, &sigset, NULL);
/* set up our handler */
sigact.sa_sigaction = handler;
sigemptyset(&sigact.sa_mask);
sigact.sa_flags = SA_SIGINFO;
sigaction (SIGALRM, &sigact, NULL);
newtv.it_interval.tv_sec = 0;
newtv.it_interval.tv_usec = 0;
newtv.it_value.tv_sec = time / 1000000;
newtv.it_value.tv_usec = time % 1000000;
if (setitimer(ITIMER_REAL,&newtv,NULL) == -1) {
perror("setitimer(set)");
return 1;
}
sigemptyset (&sigset);
sigsuspend (&sigset);
return 0;
}
int
main(int argc, char *argv[]) {
u_time_t wait;
int loop, numLoops;
u_time_t t1, t2;
u_time_t actual;
long long minDiff, maxDiff, totDiff, diff;
int numFail = 0;
if (argc != 2) {
fprintf(stderr, "Usage: %s num-loops\n", argv[0]);
exit(EXIT_FAILURE);
} /* if */
numLoops = atoi(argv[1]);
setbuf(stdout, NULL);
for (wait = 1; ; wait++) {
maxDiff = 0;
numFail = 0;
totDiff = 0;
minDiff = -wait;
if (wait % 10000 == 0)
printf("%llu\n", wait);
for (loop = 0; loop < numLoops; loop++) {
t1 = gettime();
handler_flag = 0;
isleep(wait);
if ( handler_flag != 1 )
printf("Problem with the handler flag (%d)!\n", handler_flag);
t2 = gettime();
actual = t2 - t1;
if ( actual < wait ) {
diff = actual - wait;
if (diff < maxDiff)
maxDiff = diff;
if (diff > minDiff)
minDiff = diff;
totDiff += diff;
numFail++;
} /* if */
} /* for */
if (numFail > 0)
printf("%llu: %3d fail (%4lld %4lld; avg=%6.1f)\n",
wait, numFail, minDiff, maxDiff,
(double) totDiff / numFail);
} /* for */
return 0;
} /* main */
2005-06-13 09:01:49 +00:00
|
|
|
|
2006-08-04 12:39:17 +00:00
|
|
|
POSIX.1-2001 says that
|
2005-06-23 07:16:55 +00:00
|
|
|
.BR setitimer ()
|
2005-04-06 15:29:32 +00:00
|
|
|
should fail if a
|
|
|
|
|
.I tv_usec
|
|
|
|
|
value is specified that is outside of the range 0 to 999999.
|
2007-08-28 13:15:35 +00:00
|
|
|
However, in kernels up to and including 2.6.21,
|
|
|
|
|
Linux does not give an error, but instead silently
|
2005-04-06 15:29:32 +00:00
|
|
|
adjusts the corresponding seconds value for the timer.
|
2007-08-28 13:15:35 +00:00
|
|
|
From kernel 2.6.22 onwards,
|
|
|
|
|
this non-conformance has been repaired:
|
|
|
|
|
an improper
|
2006-04-26 19:39:33 +00:00
|
|
|
.I tv_usec
|
2007-08-28 13:15:35 +00:00
|
|
|
value results in an
|
|
|
|
|
.B EINVAL
|
|
|
|
|
error.
|
|
|
|
|
.\" Bugzilla report 25 Apr 2006:
|
2006-04-26 04:42:14 +00:00
|
|
|
.\" http://bugzilla.kernel.org/show_bug.cgi?id=6443
|
|
|
|
|
.\" "setitimer() should reject non-canonical arguments"
|
2007-05-16 18:25:50 +00:00
|
|
|
.SH "SEE ALSO"
|
|
|
|
|
.BR gettimeofday (2),
|
|
|
|
|
.BR sigaction (2),
|
|
|
|
|
.BR signal (2),
|
2008-02-12 14:40:53 +00:00
|
|
|
.BR timerfd_create (2),
|
2007-05-16 18:25:50 +00:00
|
|
|
.BR time (7)
|