ghOSt: Fast & Flexible User-Space Delegation of Linux Scheduling

We present ghOSt, our infrastructure for delegating kernel scheduling decisions to userspace code. ghOSt is designed to support the rapidly evolving needs of our data center workloads and platforms. Improving scheduling decisions can drastically improve the throughput, tail latency, scalability, and security of important workloads. However, kernel schedulers are difficult to implement, test, and deploy efficiently across a large fleet. Recent research suggests bespoke scheduling policies, within custom data plane operating systems, can provide compelling performance results in a data center setting. However, these gains have proved difficult to realize as it is impractical to deploy a custom OS image(s) at an application granularity, particularly in a multi-tenant environment, limiting the practical applications of these new techniques. ghOSt provides general-purpose delegation of scheduling policies to userspace processes in a Linux environment. ghOSt provides state encapsulation, communication, and action mechanisms that allow complex expression of scheduling policies within a userspace agent, while assisting in synchronization. Programmers use any language to develop and optimize policies, which are modified without a host reboot. ghOSt supports a wide range of scheduling models, from per-CPU to centralized, run-to-completion to preemptive, and incurs low overheads for scheduling actions. We demonstrate ghOSt's performance on both academic and real-world workloads, including Google Snap and Google Search. We show that by using ghOSt instead of the kernel scheduler, we can quickly achieve comparable throughput and latency while enabling policy optimization, non-disruptive upgrades, and fault isolation for our data center workloads. We open-source our implementation to enable future research and development based on ghOSt.