Apache worker is crashing after 2500+ users

I activated Apache Worker on my VPS (vps.net) and I'm having problems, 'cause the website is crashing when 2500+ users are accessing the website concurrently. For each 5 webrequests, 3 returns 404 and 2 returns the website after ~= 10 seconds.
I'm using whos.amung.us as counter.



My Apache Worker configuration:

Code:
KeepAlive On
MaxKeepAliveRequests 0
KeepAliveTimeout 1


<IfModule mpm_worker_module>
MaxClients 4000
ThreadsPerChild 50
ServerLimit 80
</IfModule>

Code (markup):

The VPS have the SO: Debian 64 LAMP, memory: 7,3gb, 1 Gbps Lan and CPU: 24ghz
When 2500+ users are accessing the website concurrently, the memory usage is 1300mb max.
I'm not using mysql, only apache + php

What I could to do to solve this problem?

 

Give this configuration a try

 

you need to contact your provider about server crash.
Sometimes is because of the vps node setting itself at maximum allowable connection.

 

also check if there is a DDOS attack,