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RTDS Scheduler

Real-Time-Deferrable-Server-Based CPU Scheduler


The rtds scheduler is a real-time CPU scheduler built to provide guaranteed CPU capacity to real-time workload on SMP hosts. It is introduced in Xen 4.5 as an experimental scheduler.


Each VCPU of each domain is assigned a budget and a period. The VCPU with (budget, period) is supposed to have budget ms in every period ms.

Note: The VCPUs of the same domain have the same parameters right now.


The "xl sched-rtds" command can be used to tune the per VM guest scheduler parameters.

  • xl sched-rtds -d <domain> :List the parameter of the specified <domain>
  • xl sched-rtds -d <domain> -p <period> -b <budget> : Set each VCPU's budget to <budget> and period to <period> of the specified <domain>


The design of this rtds scheduler is as follows:

This scheduler follows the Preemptive Global Earliest Deadline First (EDF) theory in real-time field. At any scheduling point, the VCPU with earlier deadline has higher priority. The scheduler always picks the highest priority VCPU to run on a feasible PCPU. A PCPU is feasible if the VCPU can run on this PCPU and the PCPU is idle or has a lower-priority VCPU running on it.

Each VCPU has a dedicated period and budget. The deadline of a VCPU is at the end of each period; A VCPU has its budget replenished at the beginning of each period; While scheduled, a VCPU burns its budget. The VCPU needs to finish its budget before its deadline in each period; The VCPU discards its unused budget at the end of each period. If a VCPU runs out of budget in a period, it has to wait until next period.

Each VCPU is implemented as a deferable server. When a VCPU has a task running on it, its budget is continuously burned; When a VCPU has no task but with budget left, its budget is preserved.

Queue scheme: A global runqueue and a global depletedq for each CPU pool. The runqueue holds all runnable VCPUs with budget and sorted by deadline; The depletedq holds all VCPUs without budget and unsorted.

Glossary of Terms

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