FAQ

Q: My web site is slow or unresponsive
A: Ultimate Cron launches jobs in parallel through an HTTP request (unless using Drush). If a lot jobs are scheduled to run, ensure that the "Simultaneous connections" setting is lower than the MaxClient of your web server.

Q: Jobs are stuck in "Starting" state.
A: If the Drupal site is not reachable at $base_url from the web server, Background Process can fail in launching requests, especially if migrating db's from one environment to another. One solution can be to click the "Determine default service host" button on the Background Process settings page. Another solution could be to manually configure the service hosts in the settings file. See Background Process for more.
A: The web server accepts the connection but never actually executes it. This a somewhat theoretical problem that hasn't been confirmed yet, but seem to have happened to some people, especially when using SSL.
A: This will happen with the combination Drupal 6, clean urls and nginx. This has been fixed in the dev version

Q: Jobs are running but never finishes.
A: It can happen from time to time that a job dies without Ultimate Cron being able to catch this. There are two known reasons for this.
Reason 1: Memory exhaust so much, that shutdown handlers cannot be executed.
Solution 1: Pre-allocate memory for shutdown handlers through settings.php. (see Ultimate Cron project page)

Reason 2: The web server process dies or was killed.
Solution 2: This is uncatchable by nature, but the Background Process Apache Server status can somewhat remedy this effect by cleaning up jobs that are in fact not running. This requires Apache and the mod_status module. Please see the Background Process documentation for this.

Q: What is the difference between Ultimate Cron and Elysia Cron?
A: wanted, see here for now https://drupal.org/node/1518488