Apache Technologies 14 Bedienungsanleitung PDF herunterladen (Seite 38)

38 Chapter 3 Managing Websites

Enabling the performance cache does not always improve performance. For example,

when the amount of static web content exceeds the physical RAM of your server, using

a performance cache increases memory swapping, which degrades performance.

Note: If you turn on SSL, the performance cache is automatically turned off.

Also note that when your server is running other services that compete for physical

RAM, such as AFP, the web performance cache may be less effective or may impact the

performance of those other services.

Consider these points when determine whether to use the performance cache for a

website.

• When the performance cache is enabled for any virtual hosts, a process named

webperfcache runs and takes over the listener on the configured TCP port (usually

port 80). The web server is then configured to listen on a different TCP port (usually

port 16080). All requests are received by the webperfcache process, and they are

either served from the in-memory cache or relayed to the web server. In the latter

case, the performance cache then receives a response from the web server, saves it in

its in-memory cache (if it can be cached and caching is enabled for that virtual host),

and returns the response to the requester.

• The performance cache is not compatible with Apache’s connection Keep-Alive

mechanism. By default, the Keep-Alive mechanism is disabled; you can turn it on in

Server Admin by checking Allow Persistent Connections in the General pane for Web

Service.

• The performance cache is also incompatible with SSL (as noted above), cookies, and

WebDAV.

• If a request contains cookie headers, the performance cache considers the request

uncacheable and does not cache any responses for that request.

The performance cache respects the expiration times of cacheable items such as

images, audio, and video files. By default these are all configured to expire after one

hour. If you are running a busy website with the performance cache enabled and you

notice a heavier load on your web server once per hour, it may be the result of all the

image, audio, and video files expiring at once. You may be able to flatten the load

profile by staggering the expiration times of these file types. These are controlled by

Apache’s ExpiresByType directive. This directive is not configurable with the Server

Admin application, so you’d need to use a text editor to edit /etc/httpd/httpd.conf.

Additional information about the performance cache can be found in its configuration

file: /etc/webperfcache/webperfcache.conf.

To enable or disable the performance cache for a website:

1 In Server Admin, click Web in the list for the server you want.

2 Click Settings in the button bar.

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Apache Technologies 14 Bedienungsanleitung Seite 38