Saving another 100TB of RAM with math (and Rust)

Cloudflare operates at a scale so big that even after working here for years, it doesn’t seem real. We have thousands of servers all over the world with petabytes of RAM and millions of CPU cores, and all of it is pushed to the max. As vast as those resources feel, they are still finite, and when you need every service to run on every node, it doesn’t leave room for wasted space.

At this scale, small improvements are greatly magnified, so even 1%-at-a-time improvements are worth celebrating. And some tweaks add up to a lot more: in this post, we’ll look at how small changes to a single algorithm reduced the memory footprint of one of our Pingora-based services significantly. That allowed us to reclaim more than 100TB of RAM globally, on top of the 100TB of memory the DNS team was able to shed last month.

Waste not

Maintaining equitable resource sharing between teams is not easy, especially in large organizations. One of the ways Cloudflare ensures the balance is kept is through the tireless efforts of the wonderful Performance team. 

This story starts with a ticket filed by Ivan who found: Excessive memory usage from pingora-ketama in Pingora Backend Router. The finding was that our internal load-balancing service, Pingora Backend Router (yes, PBR), was using significantly more memory than expected — specifically in structures associated with pingora-ketama, which is our open-source library for handling consistent hashing.

In order to talk about how we addressed this seeming overuse of memory, we need to talk about what consistent hashing even is, why we are using it in PBR, and how it became so memory hungry. Along the way, we’ll learn some Rust and even a little math.

Consistent hashing