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.
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:
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 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 -v 1 -p 45000 -b 12000
Implementation entirely lives in the hypervisor. Xen has a pluggable, hook based, architecture for schedulers. Thanks to this, RTDS code is all contained in
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: *
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.
Any change done in RTDS must be tested by doing the following:
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:
xl sched-rtds -d vm -v all -p 100000 -b 50000,
xentop(in Domain0) that the VM is getting no more than 50% pCPU time.
|2016-10-14||1||Xen 4.8||Document written|