Apache : Concurrent threads, Idle threads, max and min possible threads ( Max and Min connections )

Apache : Concurrent threads, Idle threads, max and min possible threads ( Max and Min connections )

Based on the MPM ( multi processing modules ) your apache web server is using, below are the important parameters those governs all these configurations

# StartServers                 : initial number of server processes to start

# MaxClients                   : maximum number of simultaneous client connections

# MinSpareThreads         : minimum number of worker threads which are kept spare

# MaxSpareThreads        : maximum number of worker threads which are kept spare

# ThreadsPerChild           : constant number of worker threads in each server process

# MaxRequestsPerChild   : maximum number of requests a server process serves

For example - 

ServerLimit 16

StartServers 2

MaxClients 200

MinSpareThreads 25

MaxSpareThreads 75

ThreadsPerChild 25

1. First of all, whenever an apache is started, it will start 2 child processes which is determined      by StartServers parameter. 

2. Then each process will start 25 threads determined by ThreadsPerChildparameter so this      means 2 process can service only 50 concurrent connections/clients i.e. 25x2=50. 

3. Now if more concurrent users comes, then another child process will start, that can service          another 25 users. 

4. But how many child processes can be started is controlled by ServerLimit parameter, this    means that in the configuration above, I can have 16 child processes in total, with each child       process can handle 25 thread, in total handling 16x25=400 concurrent users. 

5. But if number defined in MaxClients is less which is 200 here, then this means that after 8      child processes, no extra process will start since we have defined an upper cap                  

   of MaxClients. 

6. This also means that if I set MaxClients to 1000, after 16 child processes and 400     

    connections, no extra process will start and we cannot service more than 400 concurrent    

    clients even if we have increase the MaxClient parameter. 

In this case, we need to also increase ServerLimit to 1000/25 i.e. MaxClients/ThreadsPerChild=40 So this is the optmized configuration to server 1000 clients

So, the good configuration will be 

<IfModule mpm_worker_module>

    ServerLimit          40

    StartServers          2

    MaxClients          1000

    MinSpareThreads      25

    MaxSpareThreads      75 

    ThreadsPerChild      25

    MaxRequestsPerChild   0

</IfModule>