Tuning mod_wsgi in daemon mode

I'm running wsgi application on apache mod_wsgi in daemon mode. I have these lines in the configuration

WSGIDaemonProcess app processes=2 threads=3 display-name=%{GROUP} 
WSGIProcessGroup app

How do I find the optimal combination/tuning of processes and threads?

EDIT: This link [given in answer bellow] was quite usefull: http://serverfault.com/questions/145617/apache-2-2-mpm-worker-more-threads-or-more-processes/146382#146382

Now, my question is this: If my server gives quite good performance for my needs, should I reduce the number of threads to increase stability / reliability? Can I even set it to 1?

php can’t connect to mysql with error 13 (but command line can)

I have weird situation in newly installed server, and it seems that Google can't help me this time. I can't connect to (remote) mysql from my php-code. When I try to connect from command line on the same server the connection succseds.

Could not connect: Can't connect to MySQL server on 'MYSQL.SERVER' (13)

Here is the code and the connect attempt from the command line

[[email protected] httpdocs]$ cat  test.php

$link = mysql_connect('MYSQL.SERVER', 'testusersimon', '123456');
if (!$link) {
    die('Could not connect: ' .  mysql_error());
echo 'Connected successfully';

[[email protected] httpdocs]$ mysql -h MYSQL.SERVER -utestusersimon --password=123456
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 352108
Server version: 5.0.45-community-nt-log MySQL Community Edition (GPL)

Copyright (c) 2000, 2010, Oracle and/or its affiliates. All rights reserved.
This software comes with ABSOLUTELY NO WARRANTY. This is free software,
and you are welcome to modify and redistribute it under the GPL v2 license

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> quit

I tried running the php script both in mod_php mode and in FastCGI, check that "/etc/php.d/mysql.ini" shows up in the phpinfo() as well as mysql,mysqli and pdo_mysql sections.

but the result was the same, I know its something simple but I just can't . Please help :)

Edit: The problem was with SElinux

setsebool -P httpd_can_network_connect_db=1

Was the solution.