How to raise the open file descriptor limit for slapd?

[David Landgren <david@landgren.net>]

List,

I have a fairly heavily used openldap-2.2.28 server in production and I 
am running into the 1024 fd descriptor limit. From time to time I get

  1024 beyond descriptor table limit 1024

on my syslog host. I found the following message in the archives:

http://www.openldap.org/lists/openldap-software/200303/msg00858.html
beyond descriptor table size error

One of the suggestions was to keep an eye on open sockets. So I wrote 
the following script:

netstat --inet --numeric-hosts --numeric-ports --numeric-users \
    | tail +3          \
    | awk '{print $6}' \
    | sort             \
    | uniq -c          \
    | awk '{n+=$1; print} END {printf("%7d\n", n)}'

which I run every five minutes. Within minutes of restarting slapd this 
morning, I was seeing:

Tue Oct  4 09:09:02 CEST 2005
     40 CLOSE_WAIT
    242 ESTABLISHED
     60 TIME_WAIT
    342
Tue Oct  4 09:14:03 CEST 2005
     37 CLOSE_WAIT
    293 ESTABLISHED
     65 TIME_WAIT
    395
Tue Oct  4 09:19:03 CEST 2005
     46 CLOSE_WAIT
    335 ESTABLISHED
     37 TIME_WAIT
    418

Now during the heavy activity during the morning I see

Tue Oct  4 11:29:11 CEST 2005
    131 CLOSE_WAIT
    664 ESTABLISHED
      2 FIN_WAIT1
      1 FIN_WAIT2
     32 TIME_WAIT
    830
Tue Oct  4 11:34:12 CEST 2005
    130 CLOSE_WAIT
    647 ESTABLISHED
      2 FIN_WAIT1
     14 TIME_WAIT
    793
Tue Oct  4 11:39:12 CEST 2005
    131 CLOSE_WAIT
    660 ESTABLISHED
      2 FIN_WAIT1
      1 FIN_WAIT2
     40 TIME_WAIT
    834
Tue Oct  4 11:44:13 CEST 2005
    130 CLOSE_WAIT
    659 ESTABLISHED
      2 FIN_WAIT1
     17 TIME_WAIT
    808


So the close_wait values are getting higher, and I may be able to tune 
this down by clamping the idle_timeout setting, but as it is, I don't 
think I have enough headroom.

One of the suggestions was to increase FD_SETSIZE, which seems to me to 
be the right thing to do, but I'm not quite sure what the recommended 
practice is. In the CHANGES file I see a mention of OPENLDAP_FD_SETSIZE 
but a quick grep through the source does not turn up any other reference.

Do I define this as an environment variable, or should I define 
CFLAGS=-DOPENLDAP_FD_SETSIZE=2048 , or something else?

Your suggestions are appreciated,
David
--
"It's overkill of course, but you can never have too much overkill."