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Systemd Timer Schedule Calculator

Work out systemd timer schedule instantly with clear inputs, formula shown and shareable results.

About Systemd Timer Schedule Calculator

systemd timers improve on cron with three controls: AccuracySec lets the kernel batch wakeups to save power, RandomizedDelaySec spreads a fleet's runs to avoid a thundering herd against a shared service, and Persistent catches up missed runs after downtime. Without randomised delay, 400 machines on the same timer all hit the package mirror in the same second.

Formula

Timer spread

runs per day = 1440 / interval; effective spread = randomised delay if set, otherwise accuracy; peak concurrency ~ instances x accuracy / spread

Worked example

With the default values below, Systemd Timer Schedule Calculator returns Runs per day per machine of 48.0. Change any input above and every figure here is recalculated in your browser.

Inputs used

OnUnitActiveSec intervalminimum 0.1 min
30 min
RandomizedDelaySecminimum 0 min
5 min
AccuracySecminimum 0.001 min
1 min
Machines running this timerminimum 1
400

Results

Runs per day per machine
48.0
Peak simultaneous runs across the fleet
80
Window over which runs are spread
5.0 min
Herd behaviour
Randomised delay spreads runs over 5 minutes

Step by step

  1. 1Values used
  2. 2Timer spread
  3. 3Runs per day per machine= 48.0
  4. 4Peak simultaneous runs across the fleet= 80
  5. 5Window over which runs are spread= 5.0 min
  6. 6Herd behaviour= Randomised delay spreads runs over 5 minutes

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

What advantage do timers have over cron?

Dependency ordering on other units, journal integration, resource limits through the service unit, and no overlapping runs because a service already active will not restart.

What does Persistent do?

It records the last run and executes immediately after boot if a scheduled run was missed, which cron's anacron behaviour only partly provides.

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