# 1 Basics

Status: | Experimental
Component: | Hypervisor

# 2 Overview

RTDS is one of the virtual CPU (vCPU) scheduler available in the Xen hypervisor.

RTDS is a real-time scheduler, so its purpose is enabling deterministic scheduling of the virtual machine’s vCPUs. It has been originally developed in the context of the RT-Xen project.

# 3 User details

RTDS is not in use by default. In order to use it as the Xen scheduler the following parameter should be passed to the hypervisor at boot:

    `sched=rtds`

Once the system is live, for creating a cpupool with RTDS as its scheduler, either compile a cpupool configuration file, as described in `docs/man/xlcpupool.cfg.pod.5` (and as exemplified in `tools/examples/cpupool`), or use just `xl` directly:

    xl cpupool-create name=\"pool-rt\" sched=\"rtds\" cpus=[4,5,6,8]

For checking or changing a VM’s scheduling parameters from xl, do as follows: * `xl sched-rtds -d vm-rt -v all` * `xl sched-rtds -d vm-rt -v all -p 10000 -b 2500`

It is possible, for a multiple vCPUs VM, to change the parameters of each vCPU individually: * `xl sched-rtds -d vm-rt -v 0 -p 20000 -b 10000 -e 1 -v 1 -p 45000 -b 12000 -e 0`

# 4 Technical details

Implementation entirely lives in the hypervisor. Xen has a pluggable, hook based, architecture for schedulers. Thanks to this, RTDS code is all contained in `xen/common/sched_rtds.c`.

In libxl, the availability of the RTDS scheduler is advertised by the presence of the LIBXL_HAVE_SCHED_RTDS symbol. The ability of specifying different scheduling parameters for each vcpu has been introduced later, and is available if the following symbols are defined: * LIBXL_HAVE_VCPU_SCHED_PARAMS, * LIBXL_HAVE_SCHED_RTDS_VCPU_PARAMS, * LIBXL_HAVE_SCHED_RTDS_VCPU_EXTRA.

# 5 Limitations

RTDS is a special purpose scheduling. This is by design, and not at all a limitation, but it is certainly something to keep in mind when thinking about using it. The purpose of the scheduler is enabling deterministic and statically analyzable behavior (as per the real-time academic literature), according to the scheduling parameters assigned to each vCPU.

Using RTDS a the Xen scheduler, and/or for general purpose workloads is definitely possible, but the vCPU scheduling parameters (of both Domain0 and of the various VMs) would probably require tweaking, with respect to their default values.

# 6 Testing

Any change done in RTDS must be tested by doing the following:

- create a cpupool with RTDS as its scheduler,
- create a few virtual machines a move them in and out of the pool,
- create a few virtual machines, directly inside the pool, and verify that they boot and can run some basic workload (e.g., login into them and run simple commands),
- shutdown/reboot the virtual machines,

The fact that the system boots fine when passing `sched=rtds` to Xen should also be verified.

Finally, to check that the scheduler is working properly (although only at a macroscopic level), the following should be done:

- create a VM with 1 vCPU and put it in the RTDS cpupool,
- set the scheduling parameters such as it has a 50% reservation, with `xl sched-rtds -d vm -v all -p 100000 -b 50000`,
- run a CPU-burning process inside the VM (e.g., `yes`),
- check with `xentop` (in Domain0) that the VM is getting no more than 50% pCPU time.

# 7 Areas for improvement

- performance assessment, especially focusing on what level of real-time behavior the scheduler enables.

# 8 Known issues

- OSSTest reports occasional failures on ARM.

# 9 References

- “RT-Xen: Real-Time Virtualization” XPDS14 Presentation
- “Scheduling in Xen” XPDS15 Presentation
- RT-Xen Project
- RTDS-Based-Scheduler
- “The RTDS Scheduler” on the Xen-Project wiki
- “Xen Project Schedulers” on the Xen-Project wiki

# 10 History

Date | Revision | Version | Notes
--- | --- | --- | ---
2016-10-14 2017-08-31 | 1 2 | Xen 4.8 Xen 4.10 | Document written Revise for work conserving feature