Prefork is the Apache default Multi-Processing Module. When you start working as a System administrator, You generally deal with low traffic servers. You don’t need to know about how Apache handles multiple processes at one time. Even I was don’t know all about it. But after some days when you start working with high load servers and you need to optimize Apache. That time its important to know about Multi-Processing Modules in Apache and how it works
Apache Prefork Default Configuration
Below is the default Prefork Multi-Processing module configuration as below for Apache versions.
For Apache 2.4
StartServers 3 MinSpareServers 5 MaxSpareServers 10 MaxRequestWorkers 400 MaxConnectionsPerChild 0
For Apache 2.2
StartServers 5 MinSpareServers 5 MaxSpareServers 10 MaxClients 150 MaxRequestsPerChild 0
Prefork Configuration Directives:
Keep this value as high as your hardware allows you. See below details, to how to set this value.
a. Find of total number of RAM on your system.
b. Find out how much RAM is available for Apache server.
c. The average memory used by a single Apache process.
Value <= ( Total Memory available for Apache) / ( Memory used by one process )5.
We recommend to keep this value higher ( At least greater than 0 ). Setting this parameter 0 means process will never expire, that may create memory leak issue and consume high memory.
When to change StartServers, MinSpareServers and MinSpareServers Directives Values :
You never need to change these directive on low traffic servers. If you are handling a server with thousands of requests per minute, Then only you need to change these values. But first you must know about frequency of new processes generation in Apache
Apache 2.2can generate one processes per second. Apache 2.4can generate up to 32 processes in on second. When minimum idle spare processes are less than MinSpareServers value, Apache start 1 process and wait for one second, If it’s still less, Apache start 2 processes and wait a second, If it’s still less Apache starts 4 processes and wait for a second, similarly Apache can start-up to 32 spare child processes per second and it repeated until minimum spare process exceeded MinSpareServers value.
For example if your server is running with Apache 2.2, then Apache can only start one spare processes per second. It means if your server is getting more than 1 connection per second, then You need to adjust all these parameters to always keep running some idle processes. But it also depends of how much time a processes takes to complete.
Similarly, If server is running with Apache 2.4, Your may need to change these parameters very rarely, because Apache 2.4 can start-up to 32 chiles processes per second.