Custom C++ Memory Pools: Fixed Blocks, TLS, and Benchmarks [#48-3]

Introduction: “When new/delete show up in the profiler”

Why memory pools?

Many same-sized allocations and frees amplify heap fragmentation and allocator cost. This article implements fixed-block pools, thread-local pools, object pools, frame allocators, and stack allocators, and compares against ::operator new with benchmarks.

See also: Network handler allocators, PMR allocators.

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Scenarios

Problem	Pool angle
malloc hot in profiler	Fixed blocks + reuse
Long-run OOM despite free RAM	Reduce global heap fragmentation
Many threads, poor scaling	Thread-local pools avoid heap lock contention
Cache misses on pointer chasing	Contiguous pool storage

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Design: fixed blocks

• Allocate a chunk (e.g. with ::operator new).
• Split into fixed-size blocks; first bytes of free blocks store next pointers (intrusive free list).
• allocate: pop head; deallocate: push head (LIFO).
• Expand with new chunks when the free list is empty.
• Alignment: round block and chunk sizes to alignof(std::max_align_t).
• Fallback: for requests larger than block size, delegate to global new.

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Thread-local pool

thread_local FixedBlockPool gives lock-free alloc/free on that thread—good for per-thread workers.

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Game-oriented patterns

Pattern	Lifetime	Typical use
Object pool	Per object	Bullets, particles
Frame allocator	Per frame	Temp per-frame data
Stack allocator	LIFO scope	Scratch buffers

Placement new on pooled storage + explicit destructor on release.

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Benchmarking

Compare N allocate+free cycles: batch vs interleaved; scale thread count with TLS pools. Numbers are machine-specific—always re-measure on your target.

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Common mistakes

1. Pool destroyed before objects → UAF; pool must outlive users.
2. Wrong pool on deallocate → UB.
3. Object larger than block → overflow; use fallback or larger class.
4. Double free → corrupt free list.
5. Misaligned blocks → UB on some types—pad/align.
6. Global pool + multi-thread without TLS/mutex → data race.
7. Frame allocator reset then use old pointers → UAF.

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Production patterns

• Size-class pools (64/128/256…) for varied small sizes.
• Monitored pool counters (peak usage, allocations).
• RAII wrappers with pool deleters.
• Asio handlers allocated from pools when sizes are bounded—see network guide.

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Related posts

• Memory pool guide
• PMR
• Custom allocator article

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FAQ

Q. Block size?
A. At least max(sizeof(T), sizeof(void*)); profile real allocation sizes.

Q. Next read?
A. Segfault debugging #49-1

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Summary

Fixed pools cut allocator overhead and fragmentation for uniform hot allocations; TLS avoids locks; validate with benchmarks and watch lifetime and ownership at pool boundaries.

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