inserts/updates problem under stressing !

I did some benchmarks of my Web site and notice I lost some hits
which I accumulate in postgres (6.5.1) database on Linux 2.0.36 system

Here is what I had before testing - 181 hits for msg_id=1463

1463| 181|Sat 24 Jul 12:12:24 1999 MSD|Sat 24 Jul 12:12:34 1999 MSD
(11 rows)

12:12[zeus]:/usr/local/apache/bin>ab -c 20 -n 200 http://astronet.sai.msu.su/db/pubs.html\?msg_id=1463; psql discovery -c 'select * from hits where msg_id=1463;'

After running 20 concurent connections, total number requests of 200 I
expected hit count must be increased by 200, but some hits doesn't recorded.
test reports all requests completed successfully and there were nothing
wrong in apache error logs. It's interesting that sometimes I got even
*more* hits than expected ! I didn't noticed any problem if I use smaller
number of concurrent connections.
I didn't use explicit locking - just insert/update into table using
plpgsql function. Do I need something special to take care many concurrent
inserts/updates ?

Regards,

Oleg

Here is my test results:

This is ApacheBench, Version 1.3
Copyright (c) 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Copyright (c) 1998-1999 The Apache Group, http://www.apache.org/

Server Software: Apache/1.3.6
Server Hostname: astronet.sai.msu.su
Server Port: 80

Document Path: /db/pubs.html?msg_id=1463
Document Length: 3564 bytes

Concurrency Level: 20
Time taken for tests: 10.120 seconds
Complete requests: 200
Failed requests: 0
Total transferred: 769800 bytes
HTML transferred: 712800 bytes
Requests per second: 19.76
Transfer rate: 76.07 kb/s received

Connnection Times (ms)
min avg max
Connect: 0 58 380
Processing: 58 734 4919
Total: 58 792 5299
msg_id|count|first_access |last_access
------+-----+----------------------------+----------------------------
1463| 370|Sat 24 Jul 12:12:24 1999 MSD|Sat 24 Jul 12:13:24 1999 MSD
(1 row)
^^^^
must be 381

Here is a entry from apache config file:

--------------------------------
PerlModule Apache::HitsDBI
<Location /db>
PerlLogHandler Apache::HitsDBI
</Location>

---------------------------------
package Apache::HitsDBI;
use Apache::Constants qw(:common);

use strict;
# preloaded in startup.pl
#use DBI ();

sub handler {
my $orig = shift;
my $url = $orig->uri;
my $args = $orig->args();
if ( $url =~ /pubs\.html/ && $args =~ /msg_id=(\d+)/ ) {
my $dbh = DBI->connect("dbi:Pg:dbname=discovery") || die DBI->errstr;
my $sth = $dbh->do("SELECT acc_hits($1)") || die $dbh->errstr;
}
return OK;
}

1;
__END__

-------------------------------
create table hits (
msg_id int4 not null,
count int4 not null,
first_access datetime default now(),
last_access datetime
);
create index idx_hits on hits(msg_id);

CREATE FUNCTION "acc_hits" (int4) RETURNS int4 AS '
Declare
keyval Alias For $1;
cnt int4;
curtime datetime;
Begin
curtime := ''now'';
Select count into cnt from hits where msg_id = keyval;
if Not Found then
cnt := 1;
-- first_access inserted on default, last_access is NULL
Insert Into hits (msg_id,count) values (keyval, cnt);
else
cnt := cnt + 1;
Update hits set count = cnt,last_access = curtime where msg_id = keyval;
End If;
return cnt;
End;
' LANGUAGE 'plpgsql';

---------------------------------

_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg(at)sai(dot)msu(dot)su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83