need load balancing advice

[Ray Zimmerman]

We have a web app which consists of some very expensive dynamic 
pages. Network bandwidth isn't an issue, but web-server CPU load is. 
We used to run a front-end webserver, back-end webserver and database 
server on the same machine. Recently we tried moving the back-end 
webserver to two separate machines which the front-end server proxies 
to in round-robin style. With our typical load this has solved the 
problem quite nicely.

However, we find that under extremely high load the system is 
unstable and "falls over" onto one or the other of the two backend 
servers. In other words, the load on one of them gets really high, 
while the other looks basically idle (while serving the same number 
of requests, mind you) and it tends to stay that way. I think it's 
due to the way our app works, where the slow running requests end up 
with more cleanup work to do, and the fast requests typically end up 
doing less work.

Any suggestions for a simple method of allocating a higher number of 
requests to the machine with the lower load?  I've looked a little 
bit at mod_backhand, which looks like it might do what we want ... 
are there any other simple tools available to handle this?

Right now we're simply using a PerlTransHandler on the front end 
server. I wouldn't mind a customized approach like this, but I don't 
know of a good, efficient way to get the info on the system load of 
the two backend servers.

--
 Ray Zimmerman  / e-mail: [EMAIL PROTECTED] / 428-B Phillips Hall
  Sr Research  /   phone: (607) 255-9645  /  Cornell University
   Associate  /      FAX: (815) 377-3932 /   Ithaca, NY  14853