performance on semikolan

When Big-O Lies

Mon, 20 Jul 2026 00:00:00 +0000

There’s a particular kind of confidence that comes from writing O(n log n) in a design doc. It feels like a proof. And it is one — about a limit you will, in practice, never reach.¹ The trouble starts when we treat the asymptote as advice for the machine in front of us.

Last week I replaced a “slow” quadratic scan with a “fast” sort-then-sweep. The theory was unimpeachable. The result was slower — for every realistic input we actually saw.

The Quiet Cost of a Fast Loop

Tue, 30 Jun 2026 00:00:00 +0000

The first time I saw a sorted array process faster than an identical unsorted one, I assumed I’d measured wrong. Same data, same instructions, same count. The only difference was order — and order, I’d been taught, doesn’t change complexity.

It doesn’t. It changes the constant, and the constant is a physical thing living in the branch predictor.

The branch that couldn’t guess #

A modern CPU runs ahead of itself. When it hits an if, it doesn’t wait to learn the answer — it bets, and speculatively executes the branch it thinks you’ll take. Guess right and the work is already done. Guess wrong and it throws away a dozen in-flight instructions and starts over.

The Profiler Is Always Right

Fri, 03 Apr 2026 00:00:00 +0000

I keep a mental list of times I was certain I knew where the time was going, ran a profiler to confirm it, and got humbled. The list is long. The profiler’s record against me is perfect.

The bottleneck was never where I looked #

I’d stare at the gnarly nested loop, the obvious O(n²), and optimize it for an afternoon. The profiler would then point, flatly, at a json.Marshal call in the logging path that ran on every request. The scary algorithm ran twice a day. The boring serialization ran a million times an hour.