[Zope] Running out of sockets

[Gaute Amundsen]

Coming out of lurk mode here with a really tricky one.

We had just moved our server from:
Zope Version (unreleased version, python 2.3.3, linux2)
Python Version 2.3.3 (#2, Feb 21 2004, 21:17:53) [GCC 3.2.2 20030222 (Red Hat 
Linux 3.2.2-5)] 

To:
Zope Version (Zope 2.7.5-1.fc3, python 2.3.4, linux2)
Python Version 2.3.4 (#1, Feb 2 2005, 11:44:49) [GCC 3.4.3 20041212 (Red Hat 
3.4.3-9.EL4)] 

The first thing we noticed was:

Proxy Error
The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request GET /xp/pub/topp/invest/.
Reason: Error reading from remote server
Apache/2.0.52 (Red Hat) Server at www.foo.bar Port 80

( We are running a homegrown virtual host product very similar to the 
"virtual host monster" behind apache via rewrites.  No ZEO.)

This seems to corespond to a lot of:
PROBLEM(100) ZServer warning: server accept() threw an exception
in the log.

Restarting zope has fixed it for now, but 
"lsof -a -U -uzope" and "lsof -uzope | wc -l"
Shows the number of open sockets steadily growing, 
and apparently none closing..

We have not seen the Proxy Error again yet, but it seems pretty certain 
sockets will run out at some point. What to do? Where to dig?

I have found quite a number of references to similar problems by googling 
parts of this, but nothing leading me closer to figuring it out.

"ulimit -a" shows the limit is 1024
( at 1004 now everything seems fine except the occasional 
PROBLEM(100) XPTopic
seemingly unrelated, not accept() )

"netstat -axp" gives me loads of lines like this:
unix  2      [ ]         STREAM     CONNECTED     2792969 572/python  

We have tried various combinations for config options, but without any luck:
"force-connection-close on" did not seem to make any difference
"max-listen-sockets 500" did not keep "lsof -a -U -uzope | wc -l" from 
exceeding 500.

maximum-number-of-session-objects 10000
session-timeout-minutes 90
This should not be related no?

I realize this is becoming a very rambling question, I just thought I would 
include as much info as possible right away :)

Regards

Gaute Amundsen