Apache2 Optimization Tips for High Volume of Lightweight connections

We have a very lightweight tracking script written in PHP, running on Apache/2.2.14 (Ubuntu). The script will receive a high number of concurrent connections, but each connection will be fast. Currently, we are using prefork, configured as follows:

StartServers          5
MinSpareServers       5
MaxSpareServers      10
ServerLimit         600
MaxClients          600
MaxRequestsPerChild   0

We also have KeepAlive Off

I've played around with these setting quite a bit, and have been testing with apache benchmark. Anytime I raise the concurrent connections with ab, I get "apr_socket_recv: Connection reset by peer (104)." I've also raised the max number of file descriptors.

I'm wondering if any apache gurus out there can point me in the right direction for this type of setup (high number of lightweight connections). What are the optimum values for StartServers, Min/MaxSpareServers, etc? Is the worker MPM worth looking into? Any thoughts are welcome.

Solution

I can give you some hints:

try to use Apache in worker mode instead of prefork. To do that either put PHP in fastcgi mode (php-fpm) or take the risk to keep it in mod_php inside a threaded apache worker (the risk is that siome external libraries may conflict, like locale settings, but if your PHp tracking code is small you may control that everything there is multi-thread enabled -- PHP5 without any external lib is multi-thread enabled)
If you MaxClient is 600 then put 600 in StartServers, MinSpareServers and MaxSpareServers. else Apache is creating a new fork at a very low speed :

the parent process creates new children at a maximum rate of 1 per second.

if you think your server can handle 600 forks then take the RAM, create the 600 forks, and maybe alter the MaxRequestsPerChild setting to something like 3000, so that sometimes old forks are removed and recreated (avoiding memleaks). You will not losse any time in the fork creation rate and Apache will not loose any time managing creation and deletion of childrens.

Disabling KeepAlive is a good thing in your case, as you did
To known what is the right value for MaxLients, either in prefork or worker mode, just test it, track the memory used by one fork and divide the siez of your available RAM by this number. Be carefull, php will use some RAM as well, with mod_php this RAM will be in the apache fork memory usage, in php-fpm it will be in the php-fpm processes, check the memory_limit setting in PHP for the max size of one PHP process.
Reduce your PHP RAM usage, so that you will be able to run more PHP scripts in parallel. Do not build big arrays, keep the session light, etc. Using APC opcode may reduce your memory footprint (and do other nice things as well), using PHP 5.3 instead of 5.2 as well.