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==Real-Time-Deferrable-Server-Based CPU Scheduler==
 
===Introduction===
 
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.
 
 
===Description===
 
Each VCPU of each domain is assigned a '''budget''' and a '''period'''. The VCPU with ('''budget''', '''period''') is supposed to have '''budget''' us in every '''period''' us.
 
 
Note: The VCPUs of the same domain have the same parameters right now.
 
 
===Usage===
 
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>
 
 
===Algorithm===
 
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===
 
*'''us''': microsecond
 
*'''Host''': The physical hardware running Xen and hosting guest VMs.
 
*'''VM''': Guest virtual machine.
 
*'''VCPU''': Virtual CPU (one or more per VM).
 
*'''CPU/PCPU''': Physical host CPU.
 
*'''Period''': The period when a VCPU's budget is replenished or discarded.
 
*'''Budget''': The amount of time a VCPU can execute within its period.
 
 
===Also See===
 
*[http://xenbits.xen.org/docs/unstable/man/xl.1.html#scheduler_subcommands Scheduler Subcommands]
 

Latest revision as of 18:36, 19 December 2014

The content of this page was moved to RTDS-Based-Scheduler