Chapter 15Memory Management MechanicsPaid chapter

The Allocator — Size Classes, the Tiny Allocator, and Scan vs Noscan

Understand how mallocgc places objects: size-class rounding, the tiny allocator, and why pointer-free allocations are cheaper for the GC — connecting allocation (Part I) to GC cost (Chapter 16).

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What this chapter teaches

Common belief

Allocating N bytes uses N bytes, and all allocations cost the GC the same.

What actually happens

mallocgc rounds every request up to a fixed size class, batches tiny pointer-free objects, and marks pointer-free (noscan) memory far more cheaply than pointer-containing memory.

Mechanisms covered

The tiny allocator packs multiple small (<16 B) pointer-free allocations into one block, reducing both allocation count and overhead.

Spans are marked scan or noscan. The GC must scan pointer-containing objects for references; pointer-free objects (e.g., []byte, []int) are noscan and skipped during marking — so they impose far less GC mark cost.

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