I am aware of dlmallc/mspaces and GNU Obstacks. Both were in a way, original inspirations for spaces. Though I hadn't looked at mspaces source in years, I remember its inline boundary tags enabling zero overhad per allocation and there were no alignment constraints on the allocator itself (and is hardened across countless archs, not just x64 :) Spaces uses 64kb aligned slabs and a metadata find is a bitop. so potentially, a buffer overflow can corrupt the heap metadata in mspaces while spaces eats a cache-line on free.

mspaces was one mutex per heap for entire task (no tlc or lockfree paths). Spaces has per thread-heaps, local caches (no atomic ops on same thrad alloc/free), and a lock-free Treiber stack (ABA tagging) for cross-thread frees. mspaces doesnt track large allocs (>= 256 or 512kb) that hit mmap, so unless one knows to explicitly call mspace_track_large_chunks(...), destroy_mspace silently leaks them all (I think obstacks is good this way but is not a general fit imo). In Spaces, a chunk_destroy walks and frees all the page types unconditionally.

Another small thing may matter is error callbacks: Spaces triggers a cb allowing the application to shed load/degrade gracefully. Effectively, the heap walking (inspection?) in msapces is a compile-time switch that holds the lock whole time and doesnt track mmap (direct) allocs, and shares the thresholds like mmap_threashold, etc. globally, whereas Spaces lets you tune everything per-heap. So I'd say Spaces is a better candidate for use cases mspaces bolts on: concurrent access, hard budgets, complete heap walking and per-heap tuning.