When a cronjob really starts?

Question

My question comes from an observation that I made while seeing an every minute cronjob running like that:

12:00:15 (cron started)
12:01:20 (cron started)
12:02:02 (cron started)
.
.
.

As it seems the cron doesn't run every minute (every 60 seconds). That cron actually runs a php script that does a mysql query that selects and updates the database. It completes in less than 2 seconds.

A cronjob scheduled for every minute is supposed to run on the start of the minute (12:00:00) or it would run on any other particular second of that minute?

Has the load of the system anything to do with that? Is there any way to make sure that a cron starts at the very beginning of a minute?

Answer 1

What you observe doesn't surprise me at all, cron is not precise to the second. You can trust* it to run the job within the minute you programmed it to run, but you shouldn't really rely on more precision.

If you need your job to start at a precise moment, you should start it earlier and wait inside the script for that moment to arrive. And since 1 minute is the smallest interval supported by cron, I'd think about different options out there to start it (e.g. a Python script).

_{(*) see the comment below}