Re: [PERFORM] autovacuum daemon stops doing work after about an - Mailing list pgsql-hackers

--On Thursday, December 04, 2003 16:20:22 -0500 Vivek Khera
<khera@kcilink.com> wrote:

>>>>>> "MTO" == Matthew T O'Connor <matthew@zeut.net> writes:
>
> MTO> Could this be the recently reported bug where time goes backwards on
> MTO> FreeBSD?  Can anyone who knows more about this problem chime in, I
> know it MTO> was recently discussed on Hackers.
>
>
> Time does not go backwards -- the now and then variables are properly
> incrementing in time as you see from the debugging output.
>
> The error appears to be with the computation of the "diff".  It is
> either a C programming error, or a compiler error.  I'm not a C "cop"
> so I can't tell you which it is.
>
> Witness this program, below, compiled as "cc -g -o t t.c" and the
> output here:
>
> % ./t
> seconds = 3509
> seconds1 = 3509000000
> useconds = -452486
> stepped diff = 3508547514
> seconds2 = -785967296
> seconds3 = 3509000000
> diff = -786419782
> long long diff = 3508547514
> %
>
> apperantly, if you compute (now.tv_sec - then.tv_sec) * 1000000 all at
> once, it overflows since the RHS is all computed using longs rather
> than long longs.  Fix is to cast at least one of the values to long
> long on the RHS, as in the computation of seconds3 below.  compare
> that to the computation of seconds2 and you'll see that this is the
> cause.
>
> I'd be curious to see the output of this program on other platforms
> and other compilers.  I'm using gcc 2.95.4 as shipped with FreeBSD
> 4.8+.
this is with the UnixWare compiler:
$ cc -O -o testvk testvk.c
$ ./testvk
seconds = 3509
seconds1 = 3509000000
useconds = -452486
stepped diff = 3508547514
seconds2 = -785967296
seconds3 = 3509000000
diff = -786419782
long long diff = 3508547514
$


I think this is a C bug.



>
> That all being said, you should never sleep less than the base time,
> and never for more than a max amount, perhaps 1 hour?
>
>
> --cut here--
># include <sys/time.h>
># include <stdio.h>
>
> int
> main()
> {
>   struct timeval now, then;
>   long long diff = 0;
>   long long seconds, seconds1, seconds2, seconds3, useconds;
>
>   now.tv_sec = 1070565077L;
>   now.tv_usec = 216477L;
>
>   then.tv_sec = 1070561568L;
>   then.tv_usec = 668963L;
>
>   seconds = now.tv_sec - then.tv_sec;
>   printf("seconds = %lld\n",seconds);
>   seconds1 = seconds * 1000000;
>   printf("seconds1 = %lld\n",seconds1);
>   useconds = now.tv_usec - then.tv_usec;
>   printf("useconds = %lld\n",useconds);
>
>   diff = seconds1 + useconds;
>   printf("stepped diff = %lld\n",diff);
>
>   /* this appears to be the culprit... it should be same as seconds1 */
>   seconds2 = (now.tv_sec - then.tv_sec) * 1000000;
>   printf("seconds2 = %lld\n",seconds2);
>
>   /* seems we need to cast long's to long long's for this computation */
>   seconds3 = ((long long)now.tv_sec - (long long)then.tv_sec) * 1000000;
>   printf("seconds3 = %lld\n",seconds3);
>
>
>   diff = (now.tv_sec - then.tv_sec) * 1000000 + (now.tv_usec -
> then.tv_usec);   printf ("diff = %lld\n",diff);
>
>   diff = ((long long)now.tv_sec - (long long)then.tv_sec) * 1000000 +
> (now.tv_usec - then.tv_usec);   printf ("long long diff = %lld\n",diff);
>
>   exit(0);
> }
>
>
> --cut here--
>
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--
Larry Rosenman                     http://www.lerctr.org/~ler
Phone: +1 972-414-9812                 E-Mail: ler@lerctr.org
US Mail: 1905 Steamboat Springs Drive, Garland, TX 75044-6749