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The Apache HTTP Server Reference Manual
by Apache Software Foundation
Paperback (6"x9"), 862 pages
ISBN 9781906966034
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23.2  Run-Time Configuration Issues





Related Modules

Related Directives





mod_dir
mpm_common
mod_status

AllowOverride
DirectoryIndex
HostnameLookups
EnableMMAP
EnableSendfile
KeepAliveTimeout
MaxSpareServers
MinSpareServers
Options
StartServers




23.2.1  HostnameLookups and other DNS considerations

Prior to Apache 1.3, HostnameLookups defaulted to On. This adds latency to every request because it requires a DNS lookup to complete before the request is finished. In Apache 1.3 this setting defaults to Off. If you need to have addresses in your log files resolved to hostnames, use the logresolve program that comes with Apache, or one of the numerous log reporting packages which are available.

It is recommended that you do this sort of postprocessing of your log files on some machine other than the production web server machine, in order that this activity not adversely affect server performance.

If you use any Allow from domain or Deny from domain directives (i.e., using a hostname, or a domain name, rather than an IP address) then you will pay for two DNS lookups (a reverse, followed by a forward lookup to make sure that the reverse is not being spoofed). For best performance, therefore, use IP addresses, rather than names, when using these directives, if possible.

Note that it’s possible to scope the directives, such as within a <Location /server-status> section. In this case the DNS lookups are only performed on requests matching the criteria. Here’s an example which disables lookups except for .html and .cgi files:

HostnameLookups off
<Files ~ "\.(html|cgi)$">

HostnameLookups on

</Files>

But even still, if you just need DNS names in some CGIs you could consider doing the gethostbyname call in the specific CGIs that need it.

23.2.2  FollowSymLinks and SymLinksIfOwnerMatch

Wherever in your URL-space you do not have an Options FollowSymLinks, or you do have an Options SymLinksIfOwnerMatch Apache will have to issue extra system calls to check up on symlinks. One extra call per filename component. For example, if you had:

DocumentRoot /www/htdocs
<Directory />

Options SymLinksIfOwnerMatch

</Directory>

and a request is made for the URI /index.html. Then Apache will perform lstat(2) on /www, /www/htdocs, and /www/htdocs/index.html. The results of these lstats are never cached, so they will occur on every single request. If you really desire the symlinks security checking you can do something like this:

DocumentRoot /www/htdocs
<Directory />

Options FollowSymLinks

</Directory>

<Directory /www/htdocs>

Options -FollowSymLinks +SymLinksIfOwnerMatch

</Directory>

This at least avoids the extra checks for the DocumentRoot path. Note that you’ll need to add similar sections if you have any Alias or RewriteRule paths outside of your document root. For highest performance, and no symlink protection, set FollowSymLinks everywhere, and never set SymLinksIfOwnerMatch.

23.2.3  AllowOverride

Wherever in your URL-space you allow overrides (typically .htaccess files) Apache will attempt to open .htaccess for each filename component. For example,

DocumentRoot /www/htdocs
<Directory />

AllowOverride all

</Directory>

and a request is made for the URI /index.html. Then Apache will attempt to open /.htaccess, /www/.htaccess, and /www/htdocs/.htaccess. The solutions are similar to the previous case of Options FollowSymLinks. For highest performance use AllowOverride None everywhere in your filesystem.

23.2.4  Negotiation

If at all possible, avoid content-negotiation if you’re really interested in every last ounce of performance. In practice the benefits of negotiation outweigh the performance penalties. There’s one case where you can speed up the server. Instead of using a wildcard such as:

DirectoryIndex index

Use a complete list of options:

DirectoryIndex index.cgi index.pl index.shtml index.html

where you list the most common choice first.

Also note that explicitly creating a type-map file provides better performance than using MultiViews, as the necessary information can be determined by reading this single file, rather than having to scan the directory for files.

If your site needs content negotiation consider using type-map files, rather than the Options MultiViews directive to accomplish the negotiation. See the Content Negotiation (p. 1423) documentation for a full discussion of the methods of negotiation, and instructions for creating type-map files.

23.2.5  Memory-mapping

In situations where Apache 2.x needs to look at the contents of a file being delivered–for example, when doing server-side-include processing–it normally memory-maps the file if the OS supports some form of mmap(2).

On some platforms, this memory-mapping improves performance. However, there are cases where memory-mapping can hurt the performance or even the stability of the httpd:

For installations where either of these factors applies, you should use EnableMMAP off to disable the memory-mapping of delivered files. (Note: This directive can be overridden on a per-directory basis.)

23.2.6  Sendfile

In situations where Apache 2.x can ignore the contents of the file to be delivered – for example, when serving static file content – it normally uses the kernel sendfile support the file if the OS supports the sendfile(2) operation.

On most platforms, using sendfile improves performance by eliminating separate read and send mechanics. However, there are cases where using sendfile can harm the stability of the httpd:

For installations where either of these factors applies, you should use EnableSendfile off to disable sendfile delivery of file contents. (Note: This directive can be overridden on a per-directory basis.)

23.2.7  Process Creation

Prior to Apache 1.3 the MinSpareServers, MaxSpareServers, and StartServers settings all had drastic effects on benchmark results. In particular, Apache required a "ramp-up" period in order to reach a number of children sufficient to serve the load being applied. After the initial spawning of StartServers children, only one child per second would be created to satisfy the MinSpareServers setting. So a server being accessed by 100 simultaneous clients, using the default StartServers of 5 would take on the order 95 seconds to spawn enough children to handle the load. This works fine in practice on real-life servers, because they aren’t restarted frequently. But does really poorly on benchmarks which might only run for ten minutes.

The one-per-second rule was implemented in an effort to avoid swamping the machine with the startup of new children. If the machine is busy spawning children it can’t service requests. But it has such a drastic effect on the perceived performance of Apache that it had to be replaced. As of Apache 1.3, the code will relax the one-per-second rule. It will spawn one, wait a second, then spawn two, wait a second, then spawn four, and it will continue exponentially until it is spawning 32 children per second. It will stop whenever it satisfies the MinSpareServers setting.

This appears to be responsive enough that it’s almost unnecessary to twiddle the MinSpareServers, MaxSpareServers and StartServers knobs. When more than 4 children are spawned per second, a message will be emitted to the ErrorLog. If you see a lot of these errors then consider tuning these settings. Use the mod_status output as a guide.

Related to process creation is process death induced by the MaxRequestsPerChild setting. By default this is 0, which means that there is no limit to the number of requests handled per child. If your configuration currently has this set to some very low number, such as 30, you may want to bump this up significantly. If you are running SunOS or an old version of Solaris, limit this to 10000 or so because of memory leaks.

When keep-alives are in use, children will be kept busy doing nothing waiting for more requests on the already open connection. The default KeepAliveTimeout of 5 seconds attempts to minimize this effect. The tradeoff here is between network bandwidth and server resources. In no event should you raise this above about 60 seconds, as most of the benefits are lost1 .

ISBN 9781906966034The Apache HTTP Server Reference ManualSee the print edition