This is issue 122 in the bug tracker.
In the FastAGI_log.pl the values in the astguiclient.conf file are not used. I figure this out on one of our beefier servers when I was trying to increase the number of child processes for handling events. No matter what I set the min_servers value there would only be 5 servers when nothing was running. I also tried looking for the log file that FastAGI_log.pl says it is logging to when you connect to it's screen.
What I figured out is that the variables in FastAGI_log.pl are declared before the call to make it into a package. Basically all of the code that comes after this call is in a different scope. This means that these variables are undefined. This in turn means that by the time the code calls the run command all the run commands variables are undefined and Net::Server::Prefork uses it's defaults which are:
min_servers 5
min_spare_servers 2
max_spare_servers 10
max_servers 50
max_requests 1000
These are not bad values but they are not great. This also means that no logging is preformed on the prefork, which explains why there is no log file.
A temp. fix for this is to replace the variables in the run command with the values you would like to use.