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==Real-Time-Deferrable-Server(RTDS)-Based CPU Scheduler==
The content of this page was moved to [[RTDS-Based-Scheduler]]
===Introduction===
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.

'''[WARN]''' The scheduler has been included as an experimental and in-development feature.

===Description===
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.

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>us and period to <period>us of the specified <domain>

===Algorithm===
The design of this rtds scheduler is as follows:

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.

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.

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.

===TODO===
The following features will be implemented after Xen 4.5
# Burn budget in finer granularity instead of 1ms; [medium]
# Use separate timer per VCPU for each VCPU's budget replenishment,
instead of scanning the full runqueue every now and then [medium]
# Handle time stolen from domU by hypervisor. When it runs on a machine
with many sockets and lots of cores, the spin-lock for global RunQ used in rtds
scheduler could eat up time from domU, which could make domU have less budget
than it requires. [not sure about difficulty right now]
# Toolstack supports assigning/display each VCPU's parameters of each
domain.

===Glossary of Terms===
*'''us''': microsecond
*'''Host''': The physical hardware running Xen and hosting guest VMs.
*'''VM/domU''': 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]
*[https://sites.google.com/site/realtimexen/ RT-Xen website]
*[https://blog.xenproject.org/2013/11/27/rt-xen-real-time-virtualization-in-xen/ Blog descripting RT-Xen]

Latest revision as of 18:36, 19 December 2014

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

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