Yenya's World

Tue, 03 May 2011

Rethinking Cron

cron(8) is one of the oldest tools in UNIX. Despite of that, I think cron is not something to be proud of. In my opinion, it falls to the unfixable designs category. The recent attempts to fix it (factoring out atd(8), a dirty hack that is anacron(8), etc.) show some of the problems of cron. My recent experience confirms it:

Cronserver load average

This is the load average graph from our server, which runs periodical jobs of IS MU. Around 2 pm, I have rewritten the main crontab joining several similar tasks to one line, and adding several seconds delay between their startup. The groups of tasks are now started by a simple Perl script which handles redirecting STDOUT and STDERR, and handling the return code. The Perl script is started using exec in the crontab line, saving one more process.

This way, I have managed to get the number of jobs which are simultaneously started in the peak minutes of an hour from 155 to 13. The system does exactly the same amount of work as before, but most of the work is evenly distributed across the whole timeframe, not started in parallel the first second of a minute. This is one of the big weaknesses of cron. I think the future cron will need to support the following use cases:

What periodical and semi-periodical tasks scheduler do you use? Will systemd be the answer to these problems?

Section: /computers (RSS feed) | Permanent link | 2 writebacks

2 replies for this story:

Vašek Stodůlka wrote:

I miss the feature not to start when previous instance of the same line is still running. I use "lockrun" for this, but it is not a package in most distributions, so it has to be some better way...

xbezdick wrote: systemd

Lately when I was complaining about systemd somewhere I mentioned that it's already big enough to take over from cron and do even it's job...

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