The problem

By default apache2 is configured to support 150 concurrent connections. This forces all parallel requests beyond that limit to wait. Especially if, for example, active sync clients maintain a permanent connection for push events to arrive.

To calculate the number of concurrent sessions each of your apache nodes must be able to provide, consider the total number of concurrent sessions in your OX cluster and divide it by the number of apache nodes involved in serving those sessions. Then add some room for statistical fluctuations.

The solution

This is an example configuration to provide 8000 concurrent connections. Please ensure that your apache is using the mpm_worker module. This allows us to serve lots of concurrent connections by using less RAM than with mpm_prefork as we are going to start much less processes.

<IfModule mpm_worker_module>
    ServerLimit             250
    StartServers             10
    MinSpareThreads          75
    MaxSpareThreads         250 
    ThreadLimit              64
    ThreadsPerChild          32
    MaxClients             8000
    MaxRequestsPerChild   10000

Short explanation of the parameters:

ServerLimitDeclares the maximum number of running apache processes. If you change this value you have to restart the daemon.
StartServersThe number of processes to start initially when starting the apache daemon.
MinSpareThreads/MaxSpareThreadsThis regulates how many threads may stay idle without being killed. Apache regulates this on its own very well with default values.
ThreadsPerChildHow many threads can be created per process. Can be changed during a reload.
ThreadLimitThreadsPerChild can be configured as high as this value during runtime. If you change this value you have to restart the daemon.
MaxClientsThis declares how many concurrent connections we provide. Devided by ThreadsPerChild you get the suitable ServerLimit value. May be less than ServerLimit * ThreadsPerChild to reserve some resources that can be engaged during runtime with increasing MaxClients and reloading the configuration.
MaxRequestsPerChildDefines the number of Connections that a process can handle during its lifetime (keep-alives are counted once). After that it will be killed. This can be used to prevent possible apache memory leaks. If set to 0 the lifetime is infinite.

For further information on these parameters see and

--Steffen.templin 12:10, 23 May 2011 (UTC)