Feb 19, 2012

Apache performance tuning: directives (II)

Let's carry on with the second part of the article Apache performance tuning: directives (I).

If you take a look at the previous output, you may identify that the first process is the parent and the other ones, its children. The most important column is RSS (Resident Set Size), whereby you can see that the size of memory taken up by a child process is around 2 MB. Thus, you might work out the maximum number of Apache processes which can be simultaneously running on your system:

MaxClients = 338 MB / 2 MB = 169

This value is relative and never should be taken into account exactly. You have to consider that at any given time, other applications running on the system (mail, FTP, database, etc.) can ask for free memory. So in the preceding case, it would be reasonable to grab 200 MB instead of 338 MB. Furthermore, you must think that is not the same task to serve dynamic than static content.

If all processes are busy, Apache launches a series of spare processes (MinSpareServers). If a given time many processes are idle, these are removed with the aim of not exceeding a maximum number of spare processes (MaxSpareServers). In general, it must be satisfied that MinSpareServers ≤ StartServer ≤ MaxSpareServers, and MaxSpareServers should be 50% higher than MinSpareServers.

When a process has handled too many requests (MaxRequestsPerChild), this is deleted and another one is created again. An optimum value for MaxRequestsPerChild is 4000, although it is appropriate to fit it based on the data served.

What happens if a connection does not respond or keeps inactive indefinitely? A connection is another resource for the system and as such, must be freed up in case of not being used. The default timeout is usually very high (300 seconds). In order to enhance the server performance is useful to turn down this value to 20 (nowadays, the speed of the networks is huge, and it does not make sense to set such a high Timeout).

Lastly, we are going to treat an important issue for a web server: the persistence. When a web page is loaded, it is normal that the web browser establishes an only TCP connection and downloads all web elements, by carrying out multiple requests over the same connection.

So as to manage this feature, Apache provides three different parameters in its configuration file.

  • KeepAlive: enables (On) or disables (Off) persistent connections.

  • MaxKeepAliveRequests: maximum number of requests allowed on a persistent connection.

  • KeepAliveTimeout: amount of time the server will keep an inactive persistent connection before closing it.

It is recommended to use a very high value for the KeepAliveTimeout parameter (2 or 3 seconds). In respect of MaxKeepAliveRequests, will depend on the amount of content that web pages have on average, but it should not usually exceed 1000.

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