Brussels / 30 & 31 January 2016


Scheduling in The Age of Virtualization

Genral Thoughts and a Case Study

Once upon a time, an operating system scheduler was TheOne_ entity in charge of deciding what "activity" would run on a physical processor. Furthermore, such "activity" would pretty much all the time be an application (or system) program. Well, guess what: these days, things are more complicated! Applications and physical processors have moved away from each other, as one or more layers have been put in between them. In fact, in a typical virtualization/cloud scenario, we have a hypervisor, providing the abstraction of virtual processors, and it is onto them that applications run. And here we are: running an application inside a virtual machine requires a virtual processor to be scheduled on a physical processor --at the host level-- and the application itself to be scheduler on the virtual processor --at the guest level. So, two schedulers (at least!).

But how do these two schedulers interact right now? How should they interact ideally? Should they be closely related, completely independent, or something in the middle? Are they better be based on the same algorithm? Or do different layers have different enough needs to require highly specific solutions?

This talk will go through the above (and more) questions, trying to outline pros and cons of a few approaches. That will happen both in a "theoretical" and practical sense. The latter meaning that we will be looking at some real scheduling traces (at both host and guest level), and at numbers from different benchmarks. The main focus, especially of the 'case study' part, will be on the Xen's scheduler(s) (i.e., on the interactions between Xen's and Linux's schedulers) but we will have a look at a few KVM data points as well.

One Scheduler to rule them all. One Scheduler to find them, One Scheduler to bring them all and...

Recently, while investigating, and trying to improve, performance of the Xen hypervisor, a number of non previously obvious interactions between host and guest level scheduling have been identified. This means, in case of Xen, interactions between the Xen's scheduler (at the host/hypervisor layer) and the Linux scheduler (at the guest layer).

Traces have been collected and benchmarks have been run, in order to figure out the exact nature of those interactions, and even more in order to amend or, at least, mitigate, the ones that were causing badly performing behaviours. Similar situations when other hypervisors (namely, KVM) were in use, have been inspected too, out of interest and for reference.

The results of such investigation, both in terms of generic thoughts and reasoning, and in terms of traces and numbers is what the talk will cover.


Photo of Dario Faggioli Dario Faggioli