The BugBash Podcast How rr Became a Protected Species: A Story of Necessary Hacks
Feb 25, 2026
Rob O'Callaghan, creator of the rr record-and-replay debugger and co-founder of Pernosco, is a systems engineer who made time-travel debugging practical. He recounts building rr with pragmatic hacks, patching syscalls and courting the kernel to keep rr safe. Talks include why browsers needed record-and-replay, Pernosco's data-flow visualizations, and the tradeoff of practical hacking over academic purity.
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How RR Got Started With Interns And A Sabbatical Engineer
- RR began at Mozilla via small incremental proofs of concept driven by interns and a one-year sabbatical engineer.
- Rob described three intern POCs that validated counters, syscall interception, then Chris Jones turned those into a Firefox-capable tool.
Counting Branches And Intercepting Syscalls Made RR Practical
- RR uses retired conditional branches as a practical proxy for instruction counting because available performance counters weren't sufficient.
- They validated accuracy on large workloads and then solved syscall overhead by intercepting syscalls in user space to avoid per-call context switches.
Record And Replay Targeted Flaky Tests But Found A Different Niche
- RR aimed to reduce flaky CI test pain by enabling reproducible recordings so low-rate failures could be debugged before reaching customers.
- Rob found RR became more popular as a human-facing time-travel debugger than as an automated CI fix.