Editing Help:Job queue (section)

===HTML cache invalidation===
A wider class of operations can result in invalidation of the HTML cache for a large number of pages:

* Changing an image (all the thumbnails have to be re-rendered, and their sizes recalculated)
* Deleting a page (all the links to it from other pages need to change from blue to red)
* Creating or undeleting a page (like above, but from red to blue)
* Changing a template (all the pages that transclude the template need updating)

Except for template changes, these operations do not invalidate the links tables, but they do invalidate the HTML cache of all pages linking to that page, or using that image. Invalidating the cache of a page is a short operation; it only requires updating a single database field and sending a multicast packet to clear the caches. But if there are more than about 1000 to do, it takes a long time. By default, one job is added per 300 operations.

Note, however, that even if purging the cache of a page is a short operation, reparsing a complex page that is not in the cache may be expensive, specially if a highly used template is edited and causes lots of pages to be purged in a short period of time and your wiki has lots of concurrent visitors loading a wide spread of pages.