NOTE: pexists/C is only marginally (5-12%) faster than pexists/PP 
in this case, but there are other cases where the C implementation is
a factor of 2 faster (scroll down)... This test shows the advantage
of a straight pexists call to parsing Proc::ProcessTable output.

all tests were done 3 times and averaged (test scripts below)
      pexists/C   pexists/PurePerl   ptable/cache_ttys   ptable/uncached
lap:   15830806           13940630                3803              3769
imac:   1137529            1046094                 557               552
hub:    2308826            2189815                1517              1512

pexists/C / pexists/PurePerl (module is rebuilt for the two tests)
sleep 60 & \
   ( sleep .5 ; pid=$! ; time perl -e \
      'my $wpid="'$pid'"; use Proc::Exists qw(pexists); $i=0;
      while(pexists($wpid)) { ++$i }; print "$i\n" '
   )

ptable/cache_ttys
sleep 60 & \
   ( sleep .5 ; pid=$! ; time perl -e \
      'my $wpid="'$pid'"; use Proc::ProcessTable; $i=0;
      $p = new Proc::ProcessTable( "cache_ttys" => 1 );
      while(grep { $_->pid eq $wpid } @{$p->table}) { ++$i };
      print "$i\n" '
   )

ptable/uncached
sleep 60 & \
   ( sleep .5 ; pid=$! ; time perl -e \
      'my $wpid="'$pid'"; use Proc::ProcessTable; $i=0;
      $p = new Proc::ProcessTable();
      while(grep { $_->pid eq $wpid } @{$p->table}) { ++$i };
      print "$i\n" '
   )

... and now as promised, tests highlighting the difference between the 
C and pureperl implementations...

      1yes-1no/C   1yes-1no/PurePerl    2yes/C   2yes/PurePerl
lap:    13411479             6077823  12475458         5800595
imac:     989762              477005    931743          465103
hub:     1926110             1160990   1803996         1138647

1yes-1no:
sleep 60 & \
   ( sleep .5 ; pid=$! ; time perl -e \
      'my $wpid="'$pid'"; use Proc::Exists qw(pexists); $i=0;
      while(pexists($wpid, 99999)) { ++$i }; print "$i\n" '
   )

2yes:
#don't check $wpid twice in a row, b/c then you have to back to back
#signal latency to the same process to deal with - ie you have to
#wait for the other process to eat the signal before you send another
#numbers via XS: lap: 12539305, imac: 908734, hub: 1720115
sleep 60 & \
   ( sleep .5 ; pid=$! ; time perl -e \
      'my $wpid="'$pid'"; use Proc::Exists qw(pexists); $i=0;
      while(2==pexists($wpid, 1)) { ++$i }; print "$i\n" '
   )


NOTE: the behavior of perl/select is essentially the following:
For sleep values down to 1/Hz setting, sleep for approximately the time 
inicated. For values below that, sleep 1/Hz seconds instead. For values 
*WAY* below that, don't even give up control! So if you use this to 
poll if another process is ready (not to be used as a substitute for a 
proper lock, only as an adjunct/optimization!), don't make the mistake
of sleeping for 0 or even 0.0000001 seconds, figuring that will give up 
control to another process. it won't! this behavior may or may not be 
different on other unix variants.

INTERVAL=0.01; sleep 60 &
   ( sleep .5 ; pid=$! ; time perl -e \
      'my $wpid="'$pid'"; use Proc::Exists qw(pexists); $i=0;
      while(pexists($wpid)) {
         ++$i; select(undef, undef, undef, '$INTERVAL');
      } print "$i\n" '
   )

interval\iter/cpu%: imac         hub           lap
1                   60/<0.01%    60/<0.01%     60/<0.01%
0.1                 599/<0.01%   600/<0.01%    600/<0.01%
0.01                4974/0.01%   5999/0.01%    4995/<0.01%
0.001               14909/0.02%  5998/0.01%    14965/<0.01%
0.0001              14898/0.02%  5998/0.01%    14963/<0.01%
0.00001             14904/0.02%  5999/0.01%    14980/<0.01%
0.000001            14911/0.02%  2023672/100%  14955/<0.01%
0.0000001           881039/100%  2024237/100%  12431092/100%
imac = 300MHz G3 PPC,       linux 2.6.20, Hz=250
hub  = 750MHz P3 x86,       linux 2.4.26, Hz=100
lap  = 2.3GHz core2 x64_64, linux 2.6.22, Hz=250
TODO: compare a tickless kernel