Re: autovacuum daemon stops doing work after about an hour - Mailing list pgsql-performance

>>>>> "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+.

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--