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We’re going to take one benchmark and stop looking at it as a pair of numbers. First we’ll see why a pair misleads, then we’ll build a small time series and read it. Start with a pair that misled me. On dd-trace-go #4891, the benchmark bot’s comment now says BenchmarkOTLPProtoSize/1span is 7.6% to 8.0% faster than its baseline. Eleven days after #4891 was opened, on #4926, the bot said the same sub-benchmark was 8.1% to 8.5% slower. Both comments are correct arithmetic over two sets of runs, and the baselines differ: 05d9b8a on the first, 1e830f6 on the second. Same benchmark, opposite verdicts. 🔀 ...

Part 4 ended with a promise: the PR gate blocks only when benchstat says the delta is real and bigger than a floor we chose. A fair reader question is how. benchstat prints a table, and I checked that it exits 0 even when the table says +199.00% (two made-up result files, the pinned version, Go 1.27.1). Who, then, turns that table into a red check? We’ll build that job together. We’ll write one deliberately slow commit, let the job judge it, revert the commit, and let the job judge the revert. Two runs, two verdicts, both on a GitHub-hosted runner: ...

Every route in this series goes around the Go runtime: rewrite the source at build time, inject something at process start, or watch from the kernel, because Go gives us no agent API to plug into. This time we ask the runtime what it could hand us. The honest summary for 2026: one long-awaited feature shipped, two open proposals document gaps that have caused real production problems for years, and two experiments of mine are aspirations, not proposals. ...

Say we put a benchmark gate on every pull request: if performance regresses, the check fails. After a week, something strange shows up. A PR that does nothing performance-sensitive fails the gate. Another PR, one that rewrites a hot path, sails through with a green check. Nobody is imagining it. Both outcomes are correct given the data, and the data is just wrong. A shared CI runner is a multi-tenant machine, so our benchmark lands on a host that also runs other teams’ compile jobs, Docker builds and test suites. A real 10% regression can vanish into that noise, and a phantom one can appear from nowhere. The gate isn’t measuring our code. It’s measuring the lottery of what happens to be running next door. 🎰 ...

We’re going to follow one request, GET /orders/42, and ask what “propagation” means at each hop. Three different jobs hide behind that word, the tools in this series do different ones, and the request’s own data falls into the gaps between them. The cheapest way to see a gap is a short Go program. This is a companion to the How to Instrument Go Without Changing a Single Line of Code series, between Part 4 and Part 5. The series accompanies my GopherCon UK talk. Part 1 showed tools hacking a field into g. Here we look at what that field holds, what it can’t, and who does the rest. ...