Explain inuse_space vs alloc_space in a heap profile. Which do you use to find a memory leak vs GC pressure?

The heap profile records allocation sites, sampled on average once per runtime.MemProfileRate bytes (default 512 KiB). It offers four sample types:

• inuse_space / inuse_objects: memory allocated and not yet freed as of the most recent GC. Use this for leaks and high RSS.
• alloc_space / alloc_objects: everything allocated since program start, freed or not. Use this for GC pressure / CPU spent in mallocgc.

go tool pprof -sample_index=inuse_space http://host:6060/debug/pprof/heap
go tool pprof -sample_index=alloc_objects http://host:6060/debug/pprof/allocs

# leak hunting: diff two snapshots taken minutes apart
go tool pprof -base heap1.pb.gz heap2.pb.gz

Gotchas: (1) the data reflects the last completed GC, so it lags reality; (2) it shows where memory was allocated, not who is retaining it — a leak often shows up as a growing slice/map in a long-lived struct, or as goroutines holding references; (3) the heap profile does not include goroutine stacks, OS memory not yet returned, or cgo allocations, so RSS can exceed it. Check runtime/metrics or GODEBUG=gctrace=1 for the full picture.