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WP Super Cache

A very fast caching engine for WordPress that produces static html files.

How do I know my blog is being cached?

Go to Settings->WP Super Cache and look for the "Cache Tester" form on the easy settings page. Click "Test Cache" and the plugin will request the front page of the site twice, comparing a timestamp on each to make sure they match.

If you want to do it manually, enable debugging in the plugin settings page and load the log file in a new browser tab. Then view your blog while logged in and logged out. You should see activity in the log. View the source of any page on your site. When a page is first created, you'll see the text "Dynamic page generated in XXXX seconds." and "Cached page generated by WP-Super-Cache on YYYY-MM-DD HH:MM:SS" at the end of the source code. On reload, a cached page will show the same timestamp so wait a few seconds before checking. In legacy caching mode, if you have compression enabled, the text "Compression = gzip" will be added. If compression is disabled and the page is served as a static html file, the text "super cache" will be added. The only other way to check if your cached file was served by PHP script or from the static cache is by looking at the HTTP headers. PHP cached pages will have the header "WP-Super-Cache: Served supercache file from PHP". Legacy cached files will have the header, "WP-Super-Cache: Served legacy cache file". I used the Live HTTP Headers extension for Firefox to examine the headers. You should also check your cache directory in wp-content/cache/supercache/hostname/ for static cache files. If the plugin rules are missing from your .htaccess file, the plugin will attempt to serve the super cached page if it's found. The header "WP-Super-Cache: Served supercache file from PHP" if this happens.

Legacy (WP-Cache) vs Supercache files

WP-Cache files are stored in wp-content/cache/ (or on MU sites in a blogs sub directory) and are named wp-cache-XXXXXXXXXXXXXXXXX.html. Associated meta files are stored in a meta sub directory. Those files contain information about the cached file. These files are generated by the "legacy caching" code in the plugin. Supercache files are stored in wp-content/cache/supercache/HOSTNAME/ where HOSTNAME is your domain name. The files are stored in directories matching your site's permalink structure.

Will comments and other dynamic parts of my blog update immediately?

Comments will show as soon as they are moderated, depending on the comment policy of the blog owner. Other dynamic elements on a page may not update unless they are written in Javascript, Flash, Java or another client side browser language. The plugin really produces static html pages. No PHP is executed when those pages are served. "Popularity Contest" is one such plugin that will not work.

Will the Super Cache compression slow down my server?

No, it will do the opposite. Super Cache files are compressed and stored that way so the heavy compression is done only once. These files are generally much smaller and are sent to a visitor's browser much more quickly than uncompressed html. As a result, your server spends less time talking over the network which saves CPU time and bandwidth, and can also serve the next request much more quickly.

How do I make certain parts of the page stay dynamic?

Note: this functionality is disabled by default. You will have to enable it on the Advanced Settings page.

There are 2 ways of doing this. You can use Javascript to draw the part of the page you want to keep dynamic. That's what Google Adsense and many widgets from external sites do and is the recommended way. Or you can use a WP Super Cache filter to do the job but you can't use mod_rewrite mode caching. You have to switch to PHP or legacy caching.

WP Super Cache 1.4 introduced a cacheaction filter called wpsc_cachedata. The cached page to be displayed goes through this filter and allows modification of the page. If the page contains a placeholder tag the filter can be used to replace that tag with your dynamically generated html. The function that hooks on to the wpsc_cachedata filter should be put in a file in the WP Super Cache plugins folder unless you use the late_init feature. An example plugin is included. Edit dynamic-cache-test.php to see the example code. There are two example functions there. There's a simple function that replaces a string (or tag) you define when the cached page is served. The other example function uses an output buffer to generate the dynamic content. Due to a limitation in how PHP works the output buffer code MUST run before the wpsc_cachedata filter is hit, at least for when a page is cached. It doesn't matter when serving cached pages. See this post for a more technical and longer explanation. To execute WordPress functions you must enable the 'Late init' feature on the advanced settings page.

How do I use WordPress functions in cached dynamic pages?

How do I delay serving the cache until the "init" action fires?

Cached files are served before almost all of WordPress is loaded. While that's great for performance it's a pain when you want to extend the plugin using a core part of WordPress. Enable 'Late init' mode on the Advanced settings page and cached files will be served when "init" fires. WordPress and it's plugins will be loaded now.

Why don't WP UserOnline, Popularity Contest, WP Postratings or plugin X not work or update on my blog now?

This plugin caches entire pages but some plugins think they can run PHP code every time a page loads. To fix this, the plugin needs to use Javascript/AJAX methods or the wpsc_cachedata filter described in the previous answer to update or display dynamic information.

Why do my WP Super Cache plugin disappear when I upgrade the plugin?

WordPress deletes the plugin folder when it updates a plugin. This is the same with WP Super Cache so any modified files in wp-super-cache/plugins/ will be deleted. You can define the variable $wp_cache_plugins_dir in wp-config.php or wp-content/wp-cache-config.php and point it at a directory outside of the wp-super-cache folder. The plugin will look there for it's plugins.

What does the Cache Rebuild feature do?

When a visitor leaves a comment the cached file for that page is deleted and the next visitor recreates the cached page. A page takes time to load so what happens if it receives 100 visitors during this time? There won't be a cached page so WordPress will serve a fresh page for each user and the plugin will try to create a cached page for each of those 100 visitors causing a huge load on your server. This feature stops this happening. The cached page is not cleared when a comment is left. It is marked for rebuilding instead. The next visitor within the next 10 seconds will regenerate the cached page while the old page is served to the other 99 visitors. The page is eventually loaded by the first visitor and the cached page updated. See this post for more.

Why doesn't the plugin cache requests by search engine bots by default?

Those bots usually only visit each page once and if the page is not popular there's no point creating a cache file that will sit idle on your server. However if you're not using legacy caching you can allow these visits to be cached by removing the list of bots from "Rejected User Agents" on the Advanced settings page.

A category page is showing instead of my homepage

A tiny proportion of websites will have problems with the following configuration:

  1. Uses a static page for the front page.
  2. Uses /%category%/%postname%/ permalink structure.

Sometimes a category page is cached as the homepage of the site instead of the static page. I can't replicate the problem but a simple solution is to switch the plugin to PHP mode. For normal traffic you will see no difference in the speed of your site. You can also enable "Extra homepage checks" on the Advanced Settings page.

Why do I get warnings about caching from http://ismyblogworking.com/

"Your blog doesn't support client caching (no 304 response to If-modified-since)." "Your feed doesn't support caching (no 304 response to If-modified-since)"

Supercache doesn't support 304 header checks in mod_rewrite mode but does support it in PHP mode. This is caching done by your browser, not the server. It is a check your browser does to ask the server if an updated version of the current page is available. If not, it doesn't download the old version again. The page is still cached by your server, just not by your visitors' browsers. Try the Cacheability Engine at http://www.ircache.net/cgi-bin/cacheability.py or http://redbot.org/ for further analysis.

How should I best use the utm_source tracking tools in Google Analytics with this plugin?

That tracking adds a query string to each url linked from various sources like Twitter and feedreaders. Unfortunately it stops pages being supercached. See Joost's comment here for how to turn it into an anchor tag which can be supercached.

The plugin complains that wp-content is writable! htdocs is writable!

It's not good when the web server can write to these directories but sometimes shared hosting accounts are set up in this way to make administration easier. Use chmod 755 directory to fix the permissions or find the permissions section of your ftp client. This Google search will lead you to more information on this topic and there's also this codex page too. Unfortunately some hosts require that those directories be writable. If that's the case just ignore this warning.

How do I delete the WP_CACHE define from wp-config.php?

Load your desktop ftp client and connect to your site. Navigate to the root (or the directory below it) of your site where you'll find wp-config.php. Download that file and edit it in a text editor. Delete the line define( 'WP_CACHE', true ); and save the file. Now upload it, overwriting the wp-config.php on your server.

How do I delete the Super Cache rules from the .htaccess file?

Load your desktop ftp client and connect to your site. You may need to enable "Show hidden files" in the preferences of the ftp client. Navigate to the root of your site where you'll find the .htaccess file. Download that file and edit it in a text editor. Delete the lines between "# BEGIN WPSuperCache" and "# END WPSuperCache" and save the file. Now upload it, overwriting the .htaccess file on your server.

How do I change file permissions?

This page on the WordPress Codex explains everything you need to know about file permissions on your server and various ways of changing them.

Why do I get load spikes when new posts are made?

You may have the "clear all cached files when new posts are made" option set. Clearing those files can take time plus your visitors will now be visiting uncached pages. Are you using Google Analytics campaign tracking with utm_source in the url? Those pages aren't cached. See the question, "How should I best use the utm_source tracking tools in Google Analytics with this plugin" above for how to use them properly. Cached pages have to be refreshed when posts are made. Perhaps your server just isn't up to the job of serving the amount of traffic you get. Enable the "cache rebuild" feature as that may help.

How many pages can I cache?

The only real limit are limits defined by your server. For example, EXT2 and EXT3 allow a maximum of 31,999 sub directories so if you have a flat permalink structure (like /%POSTNAME%/) and more than 32,000 posts you may run into problems. Likewise, if you run a multisite network and have more than 31,999 sites (blogs) you won't be able to cache all of them. Realistically if you had that many active sites you wouldn't be running on one server.

How do I serve cached mobile pages to clients on small screens like phones and tablets?

You'll have to use a separate mobile plugin to render a page formatted for those visitors. The following plugins have been tested but YMMV depending on mobile client.


If things don't work when you installed the plugin here are a few things to check:

  1. Is wp-content writable by the web server?
  2. Is there a wp-content/wp-cache-config.php ? If not, copy the file wp-super-cache/wp-cache-config-sample.php to wp-content/wp-cache-config.php and make sure WPCACHEHOME points at the right place.
  3. Is there a wp-content/advanced-cache.php ? If not, then you must copy wp-super-cache/advanced-cache.php into wp-content/. You must edit the file and change the path so it points at the wp-super-cache folder.
  4. If pages are not cached at all, remove wp-content/advanced-cache.php and recreate it, following the advice above.
  5. Make sure the following line is in wp-config.php and it is ABOVE the "require_once(ABSPATH.'wp-settings.php');" line:

    define( 'WP_CACHE', true );

  6. Try the Settings->WP Super Cache page again and enable cache.
  7. Look in wp-content/cache/supercache/. Are there directories and files there?
  8. Anything in your php error_log?
  9. If your browser keeps asking you to save the file after the super cache is installed you must disable Super Cache compression. Go to the Settings->WP Super Cache page and disable it there.
  10. The plugin does not work very well when PHP's safe mode is active. This must be disabled by your administrator.
  11. If pages are randomly super cached and sometimes not, your blog can probably be viewed with and without the "www" prefix on the URL. You should choose one way and install the Enforce www preference plugin if you are using an old WordPress install. The latest versions redirect themselves (you should always be running the latest version of WordPress anyway!)
  12. Private Server users at Dreamhost should edit wp-content/wp-cache-config.php and set the cache dir to "/tmp/" if they are getting errors about increasing CPU usage. See this discussion for more.
  13. File locking errors such as "failed to acquire key 0x152b: Permission denied in..." or "Page not cached by WP Super Cache. Could not get mutex lock." are a sign that you may have to use file locking. Edit wp-content/wp-cache-config.php and uncomment "$use_flock = true" or set $sem_id to a different value. You can also disable file locking from the Admin screen as a last resort.
  14. Make sure cache/wp_cache_mutex.lock is writable by the web server if using coarse file locking.
  15. The cache folder cannot be put on an NFS or Samba or NAS share. It has to be on a local disk. File locking and deleting expired files will not work properly unless the cache folder is on the local machine.
  16. Garbage collection of old cache files won't work if WordPress can't find wp-cron.php. If your hostname resolves to it could be preventing the garbage collection from working. Check your access_logs for wp-cron.php entries. Do they return a 404 (file not found) or 200 code? If it's 404 or you don't see wp-cron.php anywhere WordPress may be looking for that script in the wrong place. You should speak to your server administator to correct this or edit /etc/hosts on Unix servers and remove the following line. Your hostname must resolve to the external IP address other servers on the network/Internet use. See http://yoast.com/wp-cron-issues/ for more. A line like " localhost localhost.localdomain" is ok. myhostname.com

  17. If old pages are being served to your visitors via the supercache, you may be missing Apache modules (or their equivalents if you don't use Apache). 3 modules are required: mod_mime, mod_headers and mod_expires. The last two are especially important for making sure browsers load new versions of existing pages on your site.
  18. The error message, "WP Super Cache is installed but broken. The path to wp-cache-phase1.php in wp-content/advanced-cache.php must be fixed!" appears at the end of every page. Open the file wp-content/advanced-cache.php in your favourite editor. Is the path to wp-cache-phase1.php correct? This file will normally be in wp-content/plugins/wp-super-cache/. If it is not correct the caching engine will not load.
  19. Caching doesn't work. The timestamp on my blog keeps changing when I reload. Check that the path in your .htaccess rules matches where the supercache directory is. You may have to hardcode it. Or use the plugin in PHP or legacy caching mode.
  20. If supercache cache files are generated but not served, check the permissions on all your wp-content/cache/supercache folders (and each of wp-content cache and supercache folders) and wp-content/cache/.htaccess. If your PHP runs as a different user to Apache and permissions are strict Apache may not be able to read the PHP generated cache files. To fix you must add the following line to your wp-config.php (Add it above the WP_CACHE define.) Then clear your cache.

    umask( 0022 );

  21. If you see garbage in your browser after enabling compression in the plugin, compression may already be enabled in your web server. In Apache you must disable mod_deflate, or in PHP zlib compression may be enabled. You can disable that in three ways. If you have root access, edit your php.ini and find the zlib.output_compression setting and make sure it's "Off" or add this line to your .htaccess:

    php_flag zlib.output_compression off If that doesn't work, add this line to your wp-config.php:

    ini_set('zlib.output_compression', 0);

  22. The "white screen of death" or a blank page when you visit your site is almost always caused by a PHP error but it may also be caused by APC. Disable that PHP extension if you have trouble and replace with eAccelerator or Xcache.
  23. After uninstalling, your permalinks may break if you remove the WordPress mod_rewrite rules too. Regenerate those rules by visiting the Settings->Permalink page and saving that form again.
  24. If your blog refuses to load make sure your wp-config.php is correct. Are you missing an opening or closing PHP tag?
  25. Your front page is ok but posts and pages give a 404? Go to Settings->permalinks and click "Save" once you've selected a custom permalink structure. You may need to manually update your .htaccess file.
  26. If certain characters do not appear correctly on your website your server may not be configured correctly. You need to tell visitors what character set is used. Go to Settings->Reading and copy the 'Encoding for pages and feeds' value. Edit the .htaccess file with all your Supercache and WordPress rewrite rules and add this at the top, replacing CHARSET with the copied value. (for example, 'UTF-8')

    AddDefaultCharset CHARSET

  27. Use Cron View to help diagnose garbage collection and preload problems. Use the plugin to make sure jobs are scheduled and for what time. Look for the wp_cache_gc and wp_cache_full_preload_hook jobs.
  28. The error message, "WP Super Cache is installed but broken. The constant WPCACHEHOME must be set in the file wp-config.php and point at the WP Super Cache plugin directory." appears at the end of every page. You can delete wp-content/advanced-cache.php and reload the plugin settings page or edit wp-config.php and look for WPCACHEHOME and make sure it points at the wp-super-cache folder. This will normally be wp-content/plugins/wp-super-cache/ but you'll likely need the full path to that file (so it's easier to let the settings page fix it). If it is not correct the caching engine will not load.
  29. If your server is running into trouble because of the number of semaphores used by the plugin it's because your users are using file locking which is not recommended (but is needed by a small number of users). You can globally disable file locking by defining the constant WPSC_DISABLE_LOCKING, or defining the constant WPSC_REMOVE_SEMAPHORE so that sem_remove() is called after every page is cached but that seems to cause problems for other processes requesting the same semaphore. Best to disable it.

Requires: 3.0 or higher
Compatible up to: 4.7.2
Last Updated: 3 weeks ago
Active Installs: 1+ million


4.3 out of 5 stars


7 of 123 support threads in the last two months have been marked resolved.

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9 people say it works.
0 people say it's broken.

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