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The content of this page was moved to [[RTDS-Based-Scheduler]] |
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===Introduction=== |
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The Real-Time Deferrable Server (rtds) scheduler is a real-time CPU scheduler built to provide guaranteed CPU capacity to guest VMs on SMP hosts. It is introduced with name '''rtds''' in Xen 4.5 as an experimental scheduler. |
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['''WARN'''] The scheduler has been included as an experimental and in-development feature. |
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===Description=== |
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Each VCPU of each domain is assigned a budget and a period. The VCPU with <budget>, <period> is supposed to run for <budget>us (not necessarily continuously) in every <period>us. |
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Note: The VCPUs of the same domain have the same parameters right now. |
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===Usage=== |
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The '''xl sched-rtds''' command can be used to tune the per VM guest scheduler parameters. |
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* '''xl sched-rtds -d <domain>''' :List the parameter of the specified <domain> |
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* '''xl sched-rtds -d <domain> -p <period> -b <budget>''' : Set each VCPU's budget to <budget>us and period to <period>us of the specified <domain> |
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===Algorithm=== |
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The design of this rtds scheduler is as follows: |
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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. |
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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. |
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This scheduler follows the Preemptive Global Earliest Deadline First (EDF) theory in real-time field to schedule these VCPUs. 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 to a VCPU if the PCPU is idle or has a lower-priority VCPU running on it. |
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Queue scheme: |
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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. |
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===Glossary of Terms=== |
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*'''us''': microsecond |
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*'''Host''': The physical hardware running Xen and hosting guest VMs. |
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*'''VM''': Guest virtual machine. |
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*'''VCPU''': Virtual CPU (one or more per VM). |
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*'''CPU/PCPU''': Physical host CPU. |
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*'''Period''': The period when a VCPU's budget is replenished or discarded. |
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*'''Budget''': The amount of time a VCPU can execute within its period. |
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===Also See=== |
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*[http://xenbits.xen.org/docs/unstable/man/xl.1.html#scheduler_subcommands Scheduler Subcommands] |
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*[https://sites.google.com/site/realtimexen/ RT-Xen website] |
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*[https://blog.xenproject.org/2013/11/27/rt-xen-real-time-virtualization-in-xen/ Blog descripting RT-Xen] |
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Latest revision as of 18:36, 19 December 2014
The content of this page was moved to RTDS-Based-Scheduler
