Go Optimization Guide

Every perf guide recommends to minimize allocations to reduce GC times, but if you look at pprof of a Go app, GC mark phase is what takes time, not GC sweep. GC mark always starts with known live roots (goroutine stacks, globals, etc) and traverse references from there colouring every pointer. To minimize GC time it is best to avoid _long living_ allocations. Short lived allocations, those which GC mark phase will never reach, has almost neglible effect on GC times.

Allocations of any kind have an effect on triggering GC earlier, but in real apps it is almost hopeless to avoid GC, except for very carefully written programs with no dependenciesm, and if GC happens, then reducing GC mark times gives bigger bang for the buck.

Side note: see https://tip.golang.org/doc/gc-guide for more on how the Go GC works and what triggers it.

GC frequency is directly driven by allocation rate (in terms of bytes) and live heap size. Some examples:

  - If you halve the allocation rate, you halve the GC frequency.
  - If you double the live heap size, you halve the GC frequency (barring changes away from the default `GOGC=100`).

> ...but if you look at pprof of a Go app, GC mark phase is what takes time, not GC sweep.

It is true that sweeping is a lot cheaper than marking, which makes your next statement:

> Short lived allocations, those which GC mark phase will never reach, has almost neglible effect on GC times.

...technically correct. Usually, this is the best kind of correct, but it omits two important considerations:

  - If you generate a ton of short-lived allocations instead of keeping them around, the GC will trigger more frequently.
  - If you reduce the live heap size (by not keeping anything around), the GC will trigger more frequently.

So now you have cheaper GC cycles, but many more of them. On top of that, you have vastly increased allocation costs.

It is not a priori clear to me this is a win. In my experience, it isn't.