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PunchyHamster • last Tuesday at 4:29 PM • 2 replies • view on HN

That's simple but consider "run something 4x per day but randomize a delay by hour so all of the 200 servers doing that task won't run it all at once"

In cron, you basically have to either use your configuration management to generate those times, or have a random delay script running before the command

In systemd timers, it's just

    OnCalendar=0/6:00:00
    RandomizedOffsetSec=60m
and the offset generated will be stable for the job on a given machine (i.e. always same on this machine but different on others) so you will get nice uniform distribution of load.

If you add

    Persist=true
the job will also be run once if there was one or more scheduled runs when the machine was down

Replies

PenguinCoder • last Tuesday at 5:32 PM

`* 1 * * * sleep $(( $(od -N1 -tuC -An /dev/urandom) \% 45 ))m ; <your command here>`

simoncion • last Tuesday at 4:48 PM

> In cron, you basically have to either use your configuration management to generate those times, or have a random delay script running before the command

Nope. From crontab(5)

  The  RANDOM_DELAY variable allows delaying job startups by random amount
  of minutes with upper limit specified by the variable. The random  scal‐
  ing  factor  is  determined during the cron daemon startup so it remains
  constant for the whole run time of the daemon.
That's from my cronie install, but it looks like this has been a feature of some crons for at least a decade. (Notice that the post date of [0] is in 2016.) Given that cronie is based on vixie-cron, and I think I was was using vixie-cron in 2002, I bet it's been a thing for at least twenty years.

[0] <https://stackoverflow.com/a/34815984>

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