It is a simplest bandwidth-preserving server that improves response time of aperiodic jobs as compared to polling server.

Consumption rule:

  • The budget is consumed at the rate of one per unit time whenever the server executes.
  • Unused budget is retained throughout the period, to be used whenever there are aperiodic jobs to execute.
  • If no aperiodic job is available to execute at start of period then keep it instead of discarding with the hope of new arrival of new aperiodic job.

Replenishment rule:

  • The budget is set to e S at multiples of the period e. time instants k⋅P S, for k = 0, 1, 2, …

For example: consider a system with two independent periodic tasks T1 (F = 2, P = 3.5, e = 1.5) and T2 (P = 6.5, e = 0.5) with aperiodic jobs A (r=2.75, e = 1.75) be executed in deferrable server TDS = (Ps = 3, es =1). The RM schedule of periodic task and execution of aperiodic task on deferrable server is:

Limitations of Deferrable Servers:

They may delay lower-priority tasks for more time than a periodic task with the same period and execution time.