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 strange nonmonotonic behavior of gettimeoftheday
I've got following problem with 2.2.17 (Redhat stock kernel)
Linux ***** 2.2.17-14 #1 Mon Feb 5 14:57:25 EST 2001 i586 unknown
on AMD K6,  VIA Technologies VT 82C586, Compaq Presario XL119.
Following C program
#include <stdio.h>
#include <sys/time.h>
#include <unistd.h>
#include <time.h>
#define ABS(x) (x < 0 ? -x : x)
#define TIME_T struct timeval
#define TIME_DIFF_T long
#define GET_TIME(x) gettimeofday(&x, NULL)
#define TIME_DIFF(x1, x2) ((x2.tv_sec - x1.tv_sec)*1000000 + (x2.tv_usec -
x1.tv_usec))
int main(int argc, char** argv)
{
   TIME_T t1, t2;
   TIME_DIFF_T d;

   GET_TIME(t2);
   while (1) {
     GET_TIME(t1);
     d = TIME_DIFF(t2, t1);
     if (d > 500000 || d < 0) {
             fprintf(stderr, "Leap found: %ld msec\n", d);
             return 0;
     }
     t2 = t1;
   }
return 1;

gives following result on box in question
root@******:# ./clo
Leap found: -1687 msec
and prints nothing on all other  my boxes.
This gives me bunch of troubles with occasional hang ups and I found nothing
in kernel archives at
http://www.**-**.com/
just some notes about smth like this for SMP boxes with ntp. Is this issue
known, and how can I fix it?

  Thanks.

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 Tue, 19 Aug 2003 13:40:03 GMT   
 strange nonmonotonic behavior of gettimeoftheday

I've run into non-monotonic gettimeofday() on a PPC system with 2.2.17, but it
always seemed to be almost exactly a jiffy out, as though it was getting
hundredths of a second from the old tick, and microseconds from the new tick.
Your leap seems to be more unusual, and the first one I've seen on an x86 box.

Have you considered storing the results to see what happens on the next call?
Does it make up the difference, or do you just lose that time?

Chris

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 Tue, 19 Aug 2003 23:20:04 GMT   
 strange nonmonotonic behavior of gettimeoftheday

(That should read "usec" since you are printing microseconds and not
milliseconds.)

I've seen behaviour like that on an ARM processor because the time code
was not considering missed (or rather, delayed response to) timer
interrupts.  The time jump in that case was slightly less than 1 jiffie
(jiffie = 10ms).  It's likely rather hardware specific; see if you can
get someone with the same hardware to run your test code.

Eli
-----------------------.           Rule of Accuracy: When working toward
Eli Carter             |            the solution of a problem, it always
eli.carter(at)inet.com `------------------ helps if you know the answer.
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 Tue, 19 Aug 2003 23:20:04 GMT   
 strange nonmonotonic behavior of gettimeoftheday

I think it's a math problem in the test code. Try this:

#include <stdio.h>
#include <sys/time.h>

#define DEB(f)

int main()
{
    struct timeval t;
    double start_us;
    double stop_us;
    for(;;)
    {
        gettimeofday(&t, NULL);
        start_us  = (double) t.tv_sec * 1e6;
        start_us += (double) t.tv_usec;
        gettimeofday(&t, NULL);
        stop_us  = (double) t.tv_sec * 1e6;
        stop_us += (double) t.tv_usec;
        if(stop_us <= start_us)
            break;
        DEB(fprintf(stdout, "Start = %f, Stop = %f\n", start_us, stop_us));
    }
    fprintf(stderr, "Start = %f, Stop = %f\n", start_us, stop_us);
    return 0;

Note that two subsequent calls to gettimeofday() must not return the
same time even if your CPU runs infinitely fast. I haven't seen any
kernel in the past few years that fails this test.

Cheers,
{*filter*} Johnson

Penguin : Linux version 2.4.1 on an i686 machine (799.53 BogoMips).

"Memory is like gasoline. You use it up when you are running. Of
course you get it all back when you reboot..."; Actual explanation
obtained from the Micro$oft help desk.

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 Tue, 19 Aug 2003 23:50:03 GMT   
 strange nonmonotonic behavior of gettimeoftheday

Perhaps APM or SMI problems?
Could you run the attached program?

--
        Manfred

[ ms.c < 1K ]
#include <stdio.h>
#include <sys/time.h>
#include <unistd.h>
#include <time.h>

static unsigned long long get_tsc(void)
{
        unsigned long v1;
        unsigned long v2;
        __asm__ __volatile__(
                "rdtsc\n\t"
                : "=a" (v1), "=d" (v2));
        return (((unsigned long long)v2)<<32)+v1;

int main(int argc, char** argv)
{
        unsigned long long t1;
        unsigned long long t2;

        printf("RDTSC tester\n");
        t1 = get_tsc();
        for(;;) {
                t2 = get_tsc();
                if(t1 > t2) {
                        printf("tsc jumped backwards: from %lld to %lld.\n",
                                        t1, t2);
                }
#if 0
                printf("diff is %lld-%lld=%d.\n",t2,t1,t2-t1);
#endif
                t1 = t2;

        }
        return 1;



 Wed, 20 Aug 2003 01:20:03 GMT   
 strange nonmonotonic behavior of gettimeoftheday

Oh!  With only micro second resolution how is this avoided?  The only
"legal" thing to do to avoid this is for the fast boxes to loop until
the requirement is satisfied.  Is this really done?

George
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 Wed, 20 Aug 2003 02:40:04 GMT   
 strange nonmonotonic behavior of gettimeoftheday

Yes and no. It takes microseconds to call the kernel for anything (time
getpid() ), so it seldom loops. All the kernel has to do is remember
the last value returned. If the time isn't past that time yet, bump
that value and return it instead of waiting.

Cheers,
{*filter*} Johnson

Penguin : Linux version 2.4.1 on an i686 machine (799.53 BogoMips).

"Memory is like gasoline. You use it up when you are running. Of
course you get it all back when you reboot..."; Actual explanation
obtained from the Micro$oft help desk.

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 Wed, 20 Aug 2003 03:20:04 GMT   
 strange nonmonotonic behavior of gettimeoftheday
~snip~

Well, "has to do" and "does" are two different animals.  My reading of
the code shows that it does not.  I have a bit of code that does
gettimeofday() calls as fast as possible and on some boxes (ix86) have
seen the difference as low as 1 micro second.  It is not beyond
imagination that a box might return the same time two times in a row
given the processors performance increases we are seeing.  I, for one,
don't find this objectionable.  I WILL take exception to time running
backward, however.  (I don't see how this is avoided on the leap second
delete, but I have just started looking at this issue.)  As to returning
a time in the future as you suggest, I think this is a bad policy.  If
the box can actually do two gettimeofdays in one micro second or less,
it SHOULD return the same time (given the resolution can not resolve the
difference).  If this becomes an issue, and it will, those that care
should use the clock_gettime() call which should return time in nano
seconds.  This is part of the POSIX standard code for which we are
working on at:

http://sourceforge.net/projects/high-res-timers/

George
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 Wed, 20 Aug 2003 04:20:05 GMT   
 strange nonmonotonic behavior of gettimeoftheday
OK, short status from the same box. It was up for about 2 weeks, but
yesterday due this problem it become unuseable, as X failed at startup with
message about failed select(). Before reboot I made some tests
and found:
- it triggered by starting of X (without X no backjumps)
- it has something with interrupts, at least when I run program above
    (it is correct, at least it can determine problem) as
    while [ 1 ]; do ./clo; done
    and pressed key, it printed much less strings
- jumps are about 300-2000 microseconds
- there are some cases of such behaviour on Usenet (mainly diagnosed as
screen  flickering due incorrect  screensaver startup)

After reboot problem goes away( nothing changed in config). Maybe it related
to APM (as I did several suspends before this problem appears). Program
testing RDTSC works OK now. If this problem appears again - I will run it.
  Thanks for help.

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 Wed, 20 Aug 2003 06:40:06 GMT   
 
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