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Golang / Golang Internals and Memory Management Interview Questions

How does Go's memory allocator work? Explain mcache, mcentral, and mheap.

Go uses a hierarchical, size-class-based allocator inspired by TCMalloc (Thread-Caching Malloc). The three levels minimise lock contention and fragmentation.

Go Allocator Layers
LayerScopeLockingPurpose
mcachePer-P (per logical CPU)Lock-freePer-CPU cache of spans for each size class — fast path
mcentralPer size class, globalMutex per size classCentral pool of spans; supplies mcache when empty
mheapGlobalMutex (large objects)OS memory; supplies mcentral with new spans; manages large (>32 KB) allocations directly
// Allocation path for a small object (<=32 KB):
// 1. Round up to nearest size class (e.g., 24 bytes → 32-byte class)
// 2. Check P's mcache for that size class — if span available, use it (no lock)
// 3. If mcache empty, fetch a span from mcentral (mutex on that size class)
// 4. If mcentral empty, request memory from mheap (global lock)
// 5. If mheap exhausted, request OS memory via mmap/VirtualAlloc

// Large object allocation (>32 KB):
// Directly from mheap — allocated as a multi-page span

// Tiny allocation (<=16 bytes, no pointers, not zero-sized):
// Multiple tiny objects packed into a single 16-byte slot
// e.g., allocating many bool or small int values is very cheap

// Size classes — Go has ~68 size classes:
// 8, 16, 24, 32, 48, 64, 80, 96, 112, 128, ... 32768 bytes

// Inspecting allocation stats
var m runtime.MemStats
runtime.ReadMemStats(&m)
fmt.Printf("Alloc      = %d KB\n", m.Alloc/1024)
fmt.Printf("TotalAlloc = %d KB\n", m.TotalAlloc/1024)
fmt.Printf("Sys        = %d KB\n", m.Sys/1024)
fmt.Printf("Mallocs    = %d\n", m.Mallocs)
fmt.Printf("Frees      = %d\n", m.Frees)

The size-class design eliminates fragmentation for small objects: each span serves exactly one size class, so all objects in a span are the same size and fit perfectly. This avoids the fragmentation of a general-purpose allocator.

Why does Go's allocator use a per-P (per-processor) mcache?
What path does Go take when allocating an object larger than 32 KB?

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