Manual:$wgInvalidateCacheOnLocalSettingsChange

Details
Setting this to true will invalidate all cached pages whenever  is changed.

This is useful so that if you install a new extension, any old pages that were cached before the extension was installed are re-rendered.

On high-traffic wikis, this should be set to false, to avoid the need to check the file modification time, and to avoid the performance impact of unnecessary cache invalidations.

In older versions of MediaWiki, this was done by default in LocalSettings.php using code like  