ESCA: Effective System Call Aggregation for Event-Driven Servers

The switches between a non-privileged application and the OS kernel running in the CPU's supervisor mode have been inducing performance costs despite manufacturer efforts to provide special instructions for such transition. Software that heavily interacts with the underlying OS (e.g., I/O intensive and event-driven applications) suffers from system call overhead. To deteriorate this situation, security vulnerabilities in modern processors have prompted kernel mitigations that further increase the transition overhead. Particularly system-call-heavy applications have been reported to be slowed down by up to 30% with kernel page-Table isolation (KPTI), the widely deployed mitigation for the Meltdown vulnerability. To decouple system calls from mode transitions, we revisit an old idea known as system-call batching or multi-calls: The bundling of system calls into a combined call, which only incurs the mode-Transition costs of a single one. And then, we have implemented ESCA scheme to adapt system-call batching to Linux-based servers in the light of Meltdown and Spectre, effectively eliminating the slowdown of KPTI-Affected applications. Our evaluation shows that the throughputs of real-world applications, benefiting from ESCA, can be improved with only 2 lines of code changed respectively: Nginx by up to 12%, lighttpd by up to 23%, and Redis by 4%. Meanwhile, using aggregated transitions, our approach allows faster system calls interleaved with full compatibility but without requiring Linux kernel patches.