Scaling Guest OS Critical Sections with eCS

Multi-core virtual machines (VMs) are now a norm in data center environments. However, one of the well-known problems that VMs suffer from is the vCPU scheduling problem that causes poor scalability behaviors. More specifically, the symptoms of this problem appear as preemption problems in both under- and over-committed scenarios. Although prior research efforts attempted to alleviate these symptoms separately, they fail to address the common root cause of these problems: the missing semantic gap that occurs when a guest OS is preempted while executing its own critical section, thereby leading to degradation of application scalability.

In this work, we strive to address all preemption problems together by bridging the semantic gap between guest OSes and the hypervisor: the hypervisor now knows whether guest OSes are running in critical sections and a guest OS has hypervisor's scheduling context. We annotate all critical sections by using the lightweight para-virtualized APIs, so we called enlightened critical sections (eCS), that provide scheduling hints to both the hypervisor and VMs. The hypervisor uses the hint to reschedule a vCPU to fundamentally overcome the double scheduling problem for these annotated critical sections and VMs use the hypervisor provided hints to further mitigate the blocked-waiter wake-up problem. Our evaluation results show that eCS guarantees the forward progress of a guest OS by 1) decreasing preemption counts by 85--100% while 2) improving the throughput of applications up to 2.5X in an over-committed scenario and 1.6X in an under-committed scenario for various real-world workloads on an 80-core machine.