Virtualization is a prominent technology used in data centers around the world. While many kinds of workloads can run at near-native performance even when virtualized, I/O intensive workloads still suffer from high overhead precluding the use of virtualization in many applications. In this paper we tackle the problem of improving the performance of paravirtual I/O. We propose an exitless paravirtual I/O model, under which guests and the hypervisor, running on distinct cores, exchange exitless notifications instead of costly exit-based notifications. Our initial proof of concept improved throughput by 45% and latency by 25µsec compared to a traditional network paravirtual I/O model. We show that a single hypervisor I/O core can become saturated when serving multiple I/O intensive guests, and further research is required to improve scalability in this scenario.
Direct device assignment enhances the performance of guest virtual machines by allowing them to communicate with I/O devices without host involvement. But even with device assignment, guests are still unable to approach bare-metal performance, because the host intercepts all interrupts, including those interrupts generated by assigned devices to signal to guests the completion of their I/O requests. The host involvement induces multiple unwarranted guest/host context switches, which significantly hamper the performance of I/O intensive workloads. To solve this problem, we present ELI (ExitLess Interrupts), a software-only approach for handling interrupts within guest virtual machines directly and securely. By removing the host from the interrupt handling path, ELI manages to improve the throughput and latency of unmodified, untrusted guests by 1.3x-1.6x, allowing them to reach 97%-100% of bare-metal performance even for the most demanding I/O-intensive workloads.
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