How does cron internally schedule jobs?

问题

How do "modern" cron daemons internally schedule their jobs? Some cronds used to schedule a run every so often via at. So after a crontab is written out, does crond:

1. Parse the crontab for all future events and the sleep for the intervals?
2. Poll an aggregated crontab database every minute to determine if the current time matches the schedule pattern?
3. Other?

Thanks,

回答1:

A few crickets heard in this question. Good 'ol RTFC with some discrete event simulation papers and Wikipedia:

http://en.wikipedia.org/wiki/Cron#Multi-user_capability

The algorithm used by this cron is as follows:

1. On start-up, look for a file named .crontab in the home directories of all account holders.
2. For each crontab file found, determine the next time in the future that each command is to be run.
3. Place those commands on the Franta-Maly event list with their corresponding time and their "five field" time specifier.
4. Enter main loop:
   1. Examine the task entry at the head of the queue, compute how far in the future it is to be run.
   2. Sleep for that period of time.
   3. On awakening and after verifying the correct time, execute the task at the head of the queue (in background) with the privileges of the user who created it.
   4. Determine the next time in the future to run this command and place it back on the event list at that time

来源：https://stackoverflow.com/questions/3982957/how-does-cron-internally-schedule-jobs