Communications of the ACM

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Bare-Metal Performance For Virtual Machines with Exitless Interrupts

By Nadav Amit, Abel Gordon, Nadav Har'El, Muli Ben-Yehuda, Alex Landau, Assaf Schuster, Dan Tsafrir
Communications of the ACM, January 2016, Vol. 59 No. 1, Pages 108-116
10.1145/2845648
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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 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 ExitLess Interrupts (ELI), a software-only approach for handling interrupts within guest virtual machines directly and securely. By removing the host from the interrupt handling path, ELI improves the throughput and latency of unmodified, untrusted guests by 1.3×–1.6×, allowing them to reach 97–100% of bare-metal performance even for the most demanding I/O-intensive workloads.

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1. Introduction

I/O activity is a dominant factor in the performance of virtualized environments,^{17, 25} motivating direct device assignment where the host assigns physical I/O devices directly to guest virtual machines. Examples of such devices include disk controllers, network cards, and GPUs. Direct device assignment provides superior performance than alternative I/O virtualization approaches, because it almost entirely removes the host from the guest's I/O path. Without direct device assignment, I/O-intensive workloads might suffer unacceptable performance degradation.^{17, 19, 25} Still, on x86 CPUs (the most popular platform for virtualization), direct assignment alone does not allow I/O-intensive workloads to approach bare-metal (nonvirtual) performance^{6, 9, 16, 25}; by our measurements, such workloads achieve only 60–65% of bare-metal performance. We find that nearly the entire performance difference is induced by interrupts of assigned devices.

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