Blog 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.

Reading the Standard Library for Fun

Thu, 11 Jun 2026 00:00:00 +0000

The standard library is the best-reviewed code most of us will ever have free access to, and almost nobody reads it. I’ve started treating it like a museum you’re allowed to touch — pick one function, follow it all the way down, and notice every decision that isn’t the obvious one.

sort.Slice is not quicksort #

If you’d asked me to implement sort.Slice, I’d have written quicksort and moved on. Go’s implementation is a pattern-defeating quicksort — pdqsort — and the difference is a catalogue of the ways plain quicksort betrays you in production.

Cache Invalidation, Mostly Naming

Tue, 12 May 2026 00:00:00 +0000

The joke is that there are two hard problems in computer science: cache invalidation and naming things. I’ve come to think they’re the same problem wearing two hats.

What a key really is #

A cache key is a name for a claim: “the answer to this question is that value.” Invalidation is just noticing the claim stopped being true. The reason invalidation is hard is almost never the eviction mechanics — it’s that the key doesn’t actually name the claim.

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.

A Year of Reading Postmortems

Tue, 02 Dec 2025 00:00:00 +0000

I spent last year reading one public postmortem a week — outages from companies large enough to be forced into candor. I expected exotic failures. What I found was the same three or four stories wearing different logos.

The retry that became the outage #

The single most common shape: a small failure triggers automatic retries, the retries multiply the load, and the load turns a blip into a self-sustaining fire. The system’s own recovery mechanism is the accelerant.

The Case for Boring Databases

Sun, 14 Sep 2025 00:00:00 +0000

My default database is Postgres, and my second choice is also Postgres. This isn’t nostalgia. It’s that “boring” is a technical property, and it’s one I want more of as a system gets important.

What boring buys you #

A boring database is one whose failure modes are already written down — in blog posts, in Stack Overflow answers, in the scar tissue of every engineer you’ll ever hire. When it’s 3am and something is wrong, you are not the first person to see this error. That is worth more than almost any feature.