Have your CAKE and eat it too: Scaling software rate limiting across CPU cores

• Published in: Proceedings of the ... IEEE Workshop on Local and Metropolitan Area Networks pp. 1 - 6
• Main Authors: Koppeler, Jonas, Hoiland-Jorgensen, Toke, Schmid, Stefan
• Format: Conference Proceeding
• Language: English
• Published: IEEE 07.07.2025
• Subjects: Distance measurement; Hardware; Kernel; Limiting; Linux; Local area networks; Metropolitan area networks; Performance evaluation; Qdisc; Rate Limiting; Software; Synchronization; Traffic Shaping
• ISSN: 1944-0375
• DOI: 10.1109/LANMAN66415.2025.11154565

Traffic shaping is a critical function for the efficient operation of modern networks, with applications ranging from data center networks to home routers. To correctly fulfill their expected task, traffic shapers must keep up with increasing line rates. While it is possible to scale traffic shapers across multiple CPUs via hardware queues, in some cases - such as when enforcing a global rate limit - these algorithms underperform due to lock contentions. This is especially true within the Linux kernel, where scheduling policies are realized as so-called queuing disciplines (qdiscs) and enforcing a global rate limit can only be achieved on what is effectively a single CPU core.In this work, we design and implement a lockless synchronization mechanism that allows qdiscs to efficiently scale rate limiting across multiple hardware queues. To demonstrate its practicality, we integrate this mechanism into the CAKE qdisc, enabling multiple CAKE instances to operate under the MQ qdisc while maintaining a global rate limit. We perform an extensive performance evaluation and find that the implementation achieves close to perfect scaling across cores, with an accuracy deviation with less that 0.25% of the configured rate, while keeping latencies low.