Xen Project Schedulers

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Contents

Overview

The Xen Project Hypervisor supports several different schedulers with different properties.

Different schedulers can be assigned to

  • an entire host
  • a pool of physical CPU’s on a host (VMs need to be assigned to a pool or pinned to a CPU)

Scheduler parameters can be modified per

  • an entire host
  • a CPU pool
  • A Virtual Machine


Schedulers in Xen 4.5 and beyond

Legend:

  • likely in 4.6
  • possible in 4.6


Scheduler Use-cases Xen 4.5 Plans for 4.6+
Credit General Purpose Supported
Default
Supported
Credit 2 General Purpose

Optimized for lower latency, high VM density

Experimental Supported
Default
RTDS Soft & Firm Real-time
Multicore
Embedded, Automotive, Graphics & Gaming in the Cloud, Low Latency Workloads
Experimental Hardening
Optimization
Better XL support
<1μs granularity
Supported
ARINC 653 Hard Real-time
Single core
Avionics, Drones, Medical
Supported
Compile time
No change

Also See

History of Xen Schedulers

This content was originally compiled by Jacob Mathai.

1. Borrowed Virtual Time (Xen 2.0/3.0)

sched=bvt
Global Parameters
ctx_allow - The context switch allowance is similar to the ''quantum'' in traditional schedulers. 
It is the minimum time that a scheduled domain will be allowed to run before being preempted.

Per-domain parameters
mcuadv - the MCU (Minimum Charging Unit) advance determines the proportional share of the CPU
that a domain receives. It is set inversely proportionally to a domain's sharing weight.
warp - the amount of `virtual time' the domain is allowed to warp backwards
warpl - the warp limit is the maximum time a domain can run warped for
warpu - the unwarp requirement is the minimum time a domain must run unwarped for before it can warp again

2. Atropos (Xen 2.0)

sched=atropos
Atropos is a soft real time scheduler. It provides guarantees about absolute shares of the CPU, 
with a facility for sharing slack CPU time on a best-effort basis. It can provide timeliness 
guarantees for latency-sensitive domains.

Every domain has an associated period and slice. The domain should receive `slice' nanoseconds 
every `period' nanoseconds. This allows the administrator to configure both the absolute share 
of the CPU a domain receives and the frequency with which it is scheduled.

Note: don't over-commit the CPU when using Atropos (i.e. don't reserve more CPU than is 
available -- the utilization should be kept to slightly less than 100% in order to ensure predictable 
behavior).

Per-domain parameters :
period - The regular time interval during which a domain is guaranteed to receive its allocation of CPU time.
slice - The length of time per period that a domain is guaranteed to run for (in the absence of voluntary yielding of the CPU).
latency - The latency hint is used to control how soon after waking up a domain it should be scheduled.
xtratime - This is a boolean flag that specifies whether a domain should be allowed a share of the system slack time.

3. Round Robin (Xen 2.0)

sched=rrobin
The round robin scheduler is included as a simple demonstration of Xen's internal scheduler 
API. It is not intended for production use.

Global Parameters
rr_slice - The maximum time each domain runs before the next scheduling decision is made.

4. sEDF scheduler (Xen 3.0)

sched=sedf
(from docs/misc/sedf_scheduler_mini-HOWTO.txt)
This scheduler provides weighted CPU sharing in an intuitive way and uses realtime-algorithms 
to ensure time guarantees.

Per-domain parameters
use "xm sched-sedf <dom-id> <period> <slice> <latency-hint> <extra> <weight>"
-period/slice are the normal EDF scheduling parameters in nanosecs
-latency-hint is the scaled period in case the domain is doing heavy I/O
(unused by the currently compiled version)
-extra is a flag (0/1), which controls whether the domain can run in extra-time
-weight is mutually exclusive with period/slice and specifies another way of setting a domains cpu slice
See wikipedia for a short intro to EDF:
http://en.wikipedia.org/wiki/Earliest_deadline_first_scheduling

5. ARINC 653 (Xen 4.0)

sched=arinc653
The arinc653 scheduler follows the ARINC 653 specification for scheduling, giving each partition (domain) a
fixed, dedicated time slot for execution.

Note: Current implementation does not support multicore, so 'maxcpus=1' must be set at boot.

System Calls and Scheduling

Some Scheduling System Calls
/schedule.c
SCHEDOP_yield
SCHEDOP_block
SCHEDOP_shutdown
 *nice( )
getpriority( )
setpriority( )
sched_getscheduler( )
sched_setscheduler( )
sched_getparam( )
sched_setparam( )
sched_yield( )
sched_get_ priority_min( )
sched_get_ priority_max( )
sched_rr_get_interval( )

A related wiki topic on Real Time Applications & Preemption .

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