Xen Project Schedulers
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
- ✓ likely in 4.6
- ✓ possible in 4.6
|Scheduler||Use-cases||Xen 4.5||Plans for 4.6+|
|Credit 2||General Purpose|
Optimized for lower latency, high VM density
|RTDS||Soft & Firm Real-time|
Embedded, Automotive, Graphics & Gaming in the Cloud, Low Latency Workloads
✓ Better XL support
✓ <1μs granularity
|ARINC 653||Hard Real-time |
Avionics, Drones, Medical
|✓ No change|
- sched heading in Xen unstable boot options (for other versions of Xen, see Category:ManPage)
- XL scheduler commands under SCHEDULER SUBCOMMANDS in XL unstable man pages (for other versions of Xen, see Category:ManPage)
- Credit Scheduler
- Credit2 Scheduler Development
- ARINC653 Scheduler
- Category:Resource Management
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 4.1)
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 .