Despite a decade of research, most high-performance userspace network stacks remain impractical for public cloud tenants developing their applications atop Virtual Machines (VMs). We identify two root causes: (1) reliance on specialized NIC features (e.g., flow steering, deep buffers) absent in commodity cloud vNICs, and (2) rigid execution models ill-suited to diverse application needs. We present Machnet, a highperformance and flexible userspace network stack designed for public cloud VMs. Machnet uses only a minimal set of vNIC features that any major cloud provider supports. It also relies on a microkernel architecture to enable flexible application execution. We evaluate Machnet across three major public clouds and on production-grade applications, including a key-value store, an HTTP server, and a statemachine replication system. We release Machnet at https: //github.com/microsoft/machnet.

eBPF has emerged as a popular platform for building high-performance I/O programs. However, eBPF's programming model limits the use of commonly used performance optimizations, leaving programs vulnerable to cache-miss-induced performance overheads. Specifically, the programming model restricts the choice of data structures used by a program, thus limiting the programmers' ability to improve data locality. Furthermore, it also makes it hard for programmers to overlap computation with data movement.

BPF support in Linux has made kernel extensions easier. Recent efforts have shown that using BPF to offload portions of server applications, e.g., memcached and service proxies, can improve application performance and efficiency. However, thus far, the community has not looked at the question of what parts of an application should be offloaded? This paper first shows that blindly offloading application functionality to the kernel is neither beneficial nor desirable, and care must be taken when deciding what to offload. Furthermore, when deciding what to offload, developers must consider not just the application, but also the workload being handled, and the kernel being targetted, Therefore, we advocate automating this decision process in a compiler, that can analyze application code, and produce two executables, a kernel offload and a userspace program, that jointly implement the application's functionality. This paper discusses the challenges that must be addressed to build such a compiler, and why they can be feasibly addressed.

This paper makes a case for writing unrestricted eBPF network functions which then get automatically decomposed between kernel and user-space.