AnyCall: Fast and Flexible System-Call Aggregation

TL;DR

AnyCall is a system that accelerates system calls by executing safety-checked user bytecode directly in kernel mode, significantly reducing transition overhead and improving performance for IO-heavy applications.

Contribution

It introduces a novel in-kernel compiler for user bytecode, enabling fast, flexible, and secure system call execution without multiple user/kernel transitions.

Findings

System call bursts up to 55 times faster with AnyCall.

Real-world applications can be sped up by 24%.

Reduces overhead caused by processor mode switches.

Abstract

Operating systems rely on system calls to allow the controlled communication of isolated processes with the kernel and other processes. Every system call includes a processor mode switch from the unprivileged user mode to the privileged kernel mode. Although processor mode switches are the essential isolation mechanism to guarantee the system's integrity, they induce direct and indirect performance costs as they invalidate parts of the processor state. In recent years, high-performance networks and storage hardware has made the user/kernel transition overhead the bottleneck for IO-heavy applications. To make matters worse, security vulnerabilities in modern processors (e.g., Meltdown) have prompted kernel mitigations that further increase the transition overhead. To decouple system calls from user/kernel transitions we propose AnyCall, which uses an in-kernel compiler to execute safety-checked user bytecode in kernel mode. This allows for very fast system calls interleaved with error checking and processing logic using only a single user/kernel transition. We have implemented AnyCall based on the Linux kernel's eBPF subsystem. Our evaluation demonstrates that system call bursts are up to 55 times faster using AnyCall and that real-world applications can be sped up by 24% even if only a minimal part of their code is run by AnyCall.