In MediaWiki 1.6, a job queue was introduced to perform long-running tasks asynchronously. The job queue is designed to hold many short tasks using batch processing.
Performance issue 
By default, each time a request runs, one job is taken from the job queue and executed. If the performance burden of this is too great, you can reduce $wgJobRunRate by putting something like this in your LocalSettings.php:
$wgJobRunRate = 0.01;
Alternatively, you can set $wgJobRunRate to 0, and then use a cron job to empty the job queue. This is done by running maintenance/runJobs.php. For instance:
0 0 * * * /usr/bin/php /var/www/wiki/maintenance/runJobs.php > /var/log/runJobs.log 2>&1
In the other case, if the job length stays very high after changing a central template, you can empty the job queue by executing from command line:
/path-to-my-wiki/maintenance$ php ./runJobs.php
MediaWiki 1.6 adds a job to the job queue for each article using a template. Each job is a command to read an article, expand any templates, and update the link table accordingly. So null edits are no longer necessary, although it may take a while for big operations to complete. This can help to ease strain on a virtual person.
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, jobs are added when more than 500 pages need to be invalidated, one job per 500 operations (see Manual:$wgUpdateRowsPerJob).
Typical values 
During a period of low load, the job queue might be zero. At Wikimedia, the job queue is, in practice, almost never zero. In off-peak hours, it might be a few hundred to a thousand. During a busy day, it might be a few million, but it can quickly fluctuate by 10% or more.
See also 
- Job queue redesign
- Job queue for developers
- Manual:Job table
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