eBPF UDP Load Balancer with Weighted Round-Robin

Software, Networking
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Introduction

I’ve been working on a new project that required high-performance UDP load balancing with dynamic weight adjustment. Traditional userspace load balancers introduce latency that’s unacceptable for our use case, so I decided to implement a kernel-level solution using eBPF (extended Berkeley Packet Filter).

The result is ebpflb_udp_wrr, an eBPF-based UDP load balancer that distributes incoming UDP traffic to local listeners using a weighted round-robin algorithm.

Why eBPF and XDP?

eBPF has revolutionized how we can extend kernel functionality without writing kernel modules or modifying the kernel source. Combined with XDP (eXpress Data Path), we can process packets at the earliest possible point in the networking stack—right when they arrive at the network interface—minimizing latency.

The key advantages for a load balancer:

  1. Performance: Packets are processed before they hit the normal network stack
  2. Safety: eBPF programs are verified by the kernel before execution
  3. Dynamic Updates: eBPF programs can be loaded and unloaded without restarting
  4. Low Latency: Processing happens in kernel space, avoiding userspace context switches

Architecture and Design

The load balancer implements a weighted round-robin scheduling algorithm. What makes it interesting is the ability to dynamically adjust weights at runtime. This allows daemon processes to modify their weight based on current system load—a feature that’s crucial for adaptive load distribution.

Technical Stack

The project is written primarily in C (81.7% of the codebase) using:

  • libbpf: The standard library for working with eBPF programs
  • XDP: For high-performance packet filtering
  • clang/LLVM: Required for compiling eBPF programs to BPF bytecode

The Atomic Operations Challenge

During development, I hit an interesting compiler bug. The original plan was to use atomic variable updates for managing weights across multiple CPU cores. However, the clang compiler’s BPF target had a bug with atomic operations.

The solution? Fall back to spin locks. While not as elegant as lock-free atomic operations, spin locks work reliably and the performance overhead is acceptable for our use case. This is a good reminder that when working with eBPF, you’re sometimes dealing with toolchain limitations that require creative workarounds.

Building and Deployment

Build Requirements

You’ll need:

  • clang compiler
  • bpftool utility
  • gcc

Building is straightforward:

make

Deployment Options

Option 1 - Direct Execution:

The simplest approach is to run the loader directly:

sudo bin/ebpflb_udp_wrr lo

This attaches the eBPF program to the loopback interface (or any interface you specify).

Option 2 - Using xdp-loader:

For more flexible management, you can attach the compiled eBPF object using xdp-loader:

xdp-loader load <interface> bin/ebpflb_udp_wrr.o

This gives you better control over program lifecycle and allows for easier integration with existing infrastructure.

Configuration

Compile-Time Constants

Several parameters can be configured in the source code:

  • Gateway expiration timeout: How long (in nanoseconds) before a gateway registration expires
  • Tracing flag: Enable/disable kernel tracing
  • Registration mode: Process registrations in kernel or userspace

Runtime Flags

The program supports several runtime flags:

# Enable kernel tracing (view output via /sys/kernel/debug/tracing/trace_pipe)
sudo bin/ebpflb_udp_wrr -t lo

# Set gateway registration expiration window (in seconds)
sudo bin/ebpflb_udp_wrr -e 30 lo

# Switch registration processing to userspace (useful for development/debugging)
sudo bin/ebpflb_udp_wrr -u lo

Use Cases

This load balancer is particularly useful for:

  1. High-Throughput UDP Services: When you need to distribute UDP traffic across multiple backend services with minimal latency
  2. Adaptive Load Distribution: Services that need to adjust their capacity based on current load
  3. Local Service Discovery: Perfect for distributing traffic to multiple local listeners
  4. Microservices Communication: When UDP is used for inter-service communication and you need intelligent routing

Current Status and Future Work

The project is currently in active development. Performance evaluation is still pending, which will provide concrete metrics on throughput, latency, and CPU overhead compared to traditional userspace load balancers.

Some areas I’m considering for future development:

  • Health Checks: Automatic detection and removal of failed backends
  • Additional Algorithms: Beyond weighted round-robin (least connections, random, etc.)
  • Metrics and Monitoring: Better visibility into load distribution
  • IPv6 Support: Currently focused on IPv4

Lessons Learned

Working with eBPF is both powerful and humbling. The toolchain is still maturing, and you’ll occasionally hit compiler bugs or unexpected limitations. The verification process means you can’t do everything you could in regular C code—and that’s by design for safety.

But when it works, the performance gains are significant. Being able to process packets in kernel space with minimal overhead opens up possibilities that simply aren’t feasible with userspace solutions.

Getting Started

If you’re interested in trying it out or contributing, check out the repository:

github.com/zazolabs/ebpflb_udp_wrr

The project is licensed under AGPL-3.0. Contributions, bug reports, and feedback are welcome.

Conclusion

eBPF and XDP provide powerful tools for building high-performance networking solutions. While there are rough edges and toolchain quirks to work around, the performance benefits make it worthwhile for use cases that demand low latency and high throughput.

This load balancer is just one example of what’s possible. As the eBPF ecosystem continues to mature, we’ll see more innovative applications that push kernel programmability to new limits.