Alternative to cron

Question

I am unhappy with cron on linux.

• Load peaks: @hourly jobs create load peaks. I want a random offset (per job).
• high load: skip some jobs: If the load is too high, I want to skip some jobs.

I could solve this with shell scripts wrappers, but that is "dirty".

Up to now, I use the default vixie cron.

How do you solve this? Any alternative to vixie cron?

Answer 1

Take a look at fcron:

and, of course, in order to make it really useful, the time remaining until next execution is saved each time the system is stopped. You can also say:

run that command once between 2am and 5am which will be done if the system is running at any time in this interval.

Fcron also includes a useful system of options, which can be applied either to every lines following the declaration or to a single line. Some of the supported options permit to:

• run jobs one by one (fcrontab option serial),
• set the max system load average value under which the job should be run (fcrontab option lavg),
• set a nice value for a job (fcrontab option nice),
• run jobs at fcron's startup if they should have been run during system down time (fcrontab option bootrun),

Answer 2

What you need is simple. Leave your jobs in @Hourly, but instead of calling a direct command, use a bash wrapper with the following function in it:

#! /bin/bash

# Random wait function
RANDOMWAIT=60
random_wait() {
  sleep $(( $RANDOM % ($RANDOMWAIT * 60) + 1 ))
}

run_job() {
  your-cron-command-here
}

random_wait
run_job

This will delay the execution by a certain amount of time (from 1 to 60 minutes). Change the RANDOMWAIT to a higher value for a bigger range. ($RANDOM is a special shell variable on Linux, and is always a random integer)

That is common practice precisely for the reason you describe. I took this code sample from the cron script of the package yum-cron actually (located in /etc/cron.daily.

Answer 3

Depending on what you're doing, would a background job queuing system like Resque work?

Answer 4

There's nothing "dirty" about writing a shell wrapper to run your cron jobs under certain conditions, random intervals is not going to negate your load issues.

/etc/cron.hourly/wrapper_script

within you conditions such as what to do when under load.

Though if you can not run your cron tasks when under load I would be re-evaluating your cron tasks, such as could they be offset to run elsewhere, would they benefit from database sharding etc ...

Answer 5

Vixie Cron supports jitter, see here (1)
As far as skipping based on load goes, you will need to hack this, i am not aware of any cron-like tool which can do that.