Zero-copy in Go: sendfile, splice, and the cost of io.Copy

Mon, 22 Jan 2024 10:00:00 +0100

A small file-serving service of mine slowed to a crawl one afternoon after a “harmless” middleware change. CPU on the server box doubled, throughput roughly halved. The diff was a single line: instead of handing a *os.File to io.Copy, somebody had wrapped it in a tiny logging reader to count bytes.

That one wrap quietly turned off sendfile(2).

This post is about that fast path: what Go does for you for free, how to see it actually fire, and the surprisingly easy ways to lose it.

Io.Copy on Segflow

Zero-copy in Go: sendfile, splice, and the cost of io.Copy