Deriving Systems

Your programs print numbers at you all day. A duration in a log line, a benchmark summary, a p99 on a dashboard you scroll past. How many of them have you ever checked?

Not measured again. Checked: worked out what the number should have been, then looked to see whether the one on your screen was anywhere near it. Hardly anyone does, and it takes about ten seconds.

Here is what those ten seconds buy. Write a megabyte to a file, time the call, and it comes back in twenty-five microseconds. But a megabyte cannot cross a memory bus that quickly. At ten gigabytes a second it takes a hundred microseconds to move, and that is the floor for the fastest thing that could possibly have happened. Twenty-five is under the floor. So the work did not happen. Something you have done a thousand times just reported success, and it was not telling you the truth.

One division. No profiler, no flame graph, no afternoon gone.

This book is twelve of them. Nothing is looked up: each chapter opens with a measurement that cannot be true, spends a couple of pages on long division, and ends with one sentence worth remembering.

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There is a preface if you would rather know what you are holding first, and an introduction if you want the method stated plainly before you watch it used. Neither is required, and the first chapter assumes you skipped both.