General Discussions

XEN – SEDF and Credit Schedulers

This post gives the brief of two schedulers present in Xen.

Credit Scheduler


  • Credit scheduler has three states:
    • OVER – gone over the credit allocation
    • UNDER – credits remaining
    • BOOST – to achieve low I/O response latency
  • When sum of the credits for all of the domains go negative all domains are given new credits.
  • Domains in UNDER are always run before in the OVER state.
  • A domain over its credit allocation is only executed if there are no domains in the UNDER state that are ready to run.
  • Considers only whether it is in OVER / UNDER state, the absolute number of credits that a domain has remaining is irrelevant.
  • Domains in same state are run in first-in, first-out manner.


SEDF Scheduler


  • Each domain is allocated processing resources according to 3 parameters: domains Period and slice; and a Boolean flag indicating whether domain is eligible to receive extra CPU time.
  • For each domain there is also a deadline, the time at which the domains current period ends and the amount of processing time that the domain is due before the deadline passes.
  • Domains are ordered in Queue according to deadlines.
  • Domain with Earliest deadline is executed first.
  • Deadlines move forward in time as domains consume processing time.

Note: Official website and documentation can be referred for more.


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