Comparing runs and the multiverse problem

Building the rr replay debugger wasn't about academic purity. It was about survival.

Today, creator Rob O'Callahan reveals the "necessary” complexity that makes time-travel debugging possible on Firefox. We dig into the engineering nightmares behind the tool: from patching 2-byte system calls to the constant fear of silent hardware failures.

Rob explains how rr became a "protected species" inside the Linux kernel—and why messy, manual hacks often beat six months of theoretical work.