Manual:$wgRunJobsAsync/de-formal

When the execution of jobs during normal page requests is enabled (by setting to a number greater than 0; it defaults to 1), then this variable controls whether to execute them asynchronously or not.

Details
When running jobs asynchronously, an internal HTTP connection for handling the execution of jobs will be opened, and MediaWiki will return the contents of the page immediately to the client without waiting for the job to complete. Otherwise, the job will be executed in the same process as the returning of the page and the client will have to wait until the job is completed. When the job does not run asynchronously, if a fatal error occurs during job execution, it will propagate to the client, aborting the loading of the page.

Pages do not appear in their categories
Many users have problems with pages, which do not appear in their categories although they are categorized correctly in the wikitext. In many cases this is a problem with the job queue. The issue people usually have is that pages do not immediately appear inside their categories. Reasons may be different. It is possible that the job queue is not working at all or that the queue is not executing enough jobs per page view due to the way the wiki is being used. In the latter case, more jobs get queued than are executed, stalling execution.

In these cases, one should try the following:

If there still are any jobs (no matter if they are queued, claimed or delayed), then TRUNCATE the database table jobs in the database. This will remove all the unrun jobs from the DB. This might possibly also remove jobs, which were scheduled to be run in the future. These jobs logically will then not be run as they are getting deleted just as all other jobs. After that point, category pages will show their member pages correctly again (at least for the current categorization state of the pages). Then monitor the contents of the table or use the  maintenance script to check, if jobs are getting executed now.
 * Try running the jobs directly with.
 * Afterwards, use the maintenance script to verify that there are 0 jobs in the queue.
 * Afterwards, run maintenance script to get the link tables updated.
 *  Set $wgRunJobsAsync to false and $wgJobRunRate to 2 or higher. This often helps (but performance gets worse). 
 * Afterwards, do a few edits, adding or removing categories from pages.