RT-Xen: Real-Time Virtualization in Xen

The researchers at Washington University in St. Louis and University of Pennsylvania are pleased to announce, here on this blog, the release of a new and greatly improved version of the RT-Xen project. Recent years have seen increasing demand for supporting real-time systems in virtualized environments (for example, the Xen-ARM projects and several other real-time enhancements to Xen), as virtualization enables greater flexibility and reduces cost, weight and energy by breaking the correspondence between logical systems and physical systems. As an example of this, check out the video below from the 2013 Xen Project Developer Summit

The video describes how Xen could be used in an in-vehicle infotainement system.
In order to combine real-time and virtualization, a formally defined real-time scheduler at the hypervisor level is needed to provide timing guarantees to the guest virtual machines. RT-Xen bridges the gap between real-time scheduling theory and the virtualization technology by providing a suite of multi-core real-time schedulers to deliver real-time performance to domains running on the Xen hypervisor.

Background: Scheduling in Xen

In Xen, each domain’s core is abstracted as a Virtual CPU (VCPU), and the hypervisor scheduler is responsible for scheduling VCPUs. For example, the default credit scheduler would assign a weight per domain, which decides the proportional share of CPU cycles that a domain would get. The credit scheduler works great for general purpose computing, but is not suitable for real-time applications due to the following reasons:

1. There is no reservation with credit scheduler. For example, when two VCPUs runs on a 2 GHz physical core, each would get 1 GHz. However, if another VCPU also boots on the same PCPU, the resource share shrinks to 0.66 GHz. The system manager have to carefully configure the number of VMs/VCPUs to ensure that each domain get an appropriate amount of CPU resource;
2. There is little timing predictability or real-time performance provided to the VM. If a VM is running some real-time workload (video decoding, voice processing, and feedback control loops) which are periodically triggered and have a timing requirement — for example, the VM must be scheduled every 10 ms to process the data — there is no way the VM can express this information to the underlying VMM scheduler. The existing SEDF scheduler can help with this, but it has poor support for multi-core.

RT-Xen: Combining real-time and virtualization

RT-Xen aims to solve this problem by providing a suite of real-time schedulers. The users can specify (budget, period, CPU mask) for each VCPU individually. The budget represents the maximum CPU resource a VCPU will get during a period; the period represents the timing quantum of the CPU resources provided to the VCPU; the CPU mask defines a subset of physical cores a VCPU is allowed to run. For each VCPU, the budget is reset at each starting point of the period (all in milliseconds), consumed when the VCPU is executing, and deferred when the VCPU has budget but no work to do.
Within each scheduler, the users can switch between different priority schemes: earliest deadline first (EDF), where VCPU with earlier deadline has higher priority; or rate monotonic (RM), where VCPU with shorter period has higher priority. As a results, not only the VCPU gets a resource reservation (budget/period), but also an explicit timing information for the CPU resources (period). The real-time schedulers in RT-Xen delivers the desired real-time performance to the VMs based on the resource reservations.
To be more specific, the two multi-core schedulers in RT-Xen are:

• RT-global: which uses a global run queue to hold all VCPUs (in runnable state). It is CPU mask aware, and provides better resource utilization, as VCPU can migrate freely between physical cores (within CPU mask)
• RT-partition: which uses a run queue per physical CPU. In this way, each physical CPU only looks at its own run queue to make scheduling decisions, which incurs less overhead and potentially better cache performance. However, load-balancing between physical cores is not provided in the current release.

Source Code and References

The developers of RT-Xen are looking closely at how to integrate both schedulers into the Xen mainstream. In the meantime, please check out publications at [EMSOFT’14], [EMSOFT’11], [RTAS’12] and source code.