abcmalloc 🐊

a deterministic, low-latency memory allocator

abcmalloc is a header-only C++23 general-purpose allocator built for realtime and high-throughput systems, while maintaining strong security guarantees. It pairs a TLSF (Two-Level Segregated Fit) front end for small objects alongside a buddy block allocator for larger regions; enabling bounded, constant-time small-object allocation alongside efficient splitting/coalescing of large blocks

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Warning

abcmalloc is part of the actively-developed micron core library; the ABI may change without notice. It also requires that memory requested from the kernel is immediately accessible and fully addressable; configure the kernel with vm.overcommit_memory = 2

Features

• hybrid TLSF + buddy + mmap architecture: constant-time small allocs, coalescing large blocks, direct mapping for huge regions
• flat latency distribution: p10…p99.9 cluster within a few nanoseconds, with a near-zero (≈0.00%) branch-misprediction rate and ~3.8 IPC on the hot path
• near-linear multithreaded scaling: per-thread arenas, no lock on the owning-thread fast path, lock-free MPSC cross-thread frees
• a per-class free cache (LIFO) and eagerly-warmed hot tiers for fast repeated allocation
• guard pages, per-tier tombstoning, double-free detection, with opt-in provenance enforcement, redzone sanitization and zero-on-alloc/free
• temporal-allocation (launder) and tombstone-free (retire) primitives for pointer-stable / hardened data structures
• header-only, freestanding-capable, depends only on the micron core library
• thread-local or global allocator modes; libc drop-in (malloc/free/...) and an STL-style allocator wrapper

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Design

abcmalloc routes every request to a size tier (thresholds from config_amd64.hpp):

tier	size range	strategy
precise	1 – 256 B	TLSF
small	257 – 512 B	TLSF
medium	513 B – 4 KiB	TLSF
large	4 K – 32 KiB	buddy
huge	32 K – 256 KiB	buddy
1mb	256 K – 1 MiB	buddy
gb	1 MiB – 512+ GiB	buddy

Latency & realtime suitability

abcmalloc is built so the distribution, not just the mean, is predictable.

• Flat percentiles. On the hot path the per-op latency is tightly bounded: e.g. for 1–32 B round-trips, p10 ≈ 6 ns, p50 ≈ 7 ns, p90 ≈ 8 ns, p99 ≈ 8–12 ns, p99.9 ≈ 9–18 ns. The only outliers are unavoidable first-touch page faults (shared by every allocator).
• Near-zero branch misprediction. Measured branch-miss rate is ≈ 0.00% across pathways (vs ~1–2% for glibc/mimalloc/jemalloc) at ~3.8 instructions/cycle
• Bounded by construction. TLSF gives O(1) small-object placement; the buddy allocator bounds large-block work; tier routing is a handful of comparisons.

Benchmarks

(fill this out later properly)

Safety guarantees

Default posture (no flags required):

• Guard pages (PROT_NONE) between allocation regions catch overflows and out-of-bounds traversal.
• Per-tier tombstoning on large/huge tiers — freed blocks are not handed back until their page is unmapped, trapping use-after-free where it matters most.
• Double-free detection — repeated/foreign frees are rejected rather than corrupting the heap.
• salloc / calloc return zero-initialised memory; calloc / aligned_alloc are overflow-checked.
• Cross-thread frees are routed safely via the lock-free MPSC queue (no shared-arena races).

Opt-in hardening (compile-time flags):

• Provenance enforcement (__default_enforce_provenance) — verify every freed pointer was allocated by this allocator.
• Redzone sanitization (__default_sanitize), zero-on-alloc / zero-on-free, fill-on-free patterns.
• Tombstoning on every tier, read-only freeze of live regions (freeze), temporal-only allocation (launder).

Testing & validation

abcmalloc is exercised by a dedicated rigor + soak regiment under tests/rigor/ (snowball framework):

• abcmalloc.cpp: correctness (tier routing, alignment, provenance, redzones, tombstones, freezes)
• abcmalloc_sizes.cpp: exhaustive size-class coverage
• abcmalloc_realloc.cpp (+ abc_overlap_probe.cpp): realloc semantics and the in-place overlap regression
• abcmalloc_stress.cpp / abcmalloc_adversarial.cpp: exotic and nested patterns, bit-flip regression detection
• abcmalloc_mt.cpp / abcmalloc_concurrent.cpp: cross-thread frees via the MPSC route
• abcmalloc_arena_recycle.cpp: concurrent arena reclamation (>= 64 lifetime threads)
• abcmalloc_soak.cpp / abcmalloc_soak_serial_bulk.cpp / abcmalloc_soak_mt.cpp: long-running soaks

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API

All entry points live in namespace abc. Types are micron core types (byte, usize, micron::__chunk<byte>).

namespace abc {

// core (malloc-style)
byte *alloc(usize size);                    // malloc; nullptr on size 0
byte *salloc(usize size);                   // hardened alloc, zero-initialised
void  dealloc(byte *ptr);                   // free (size looked up)
void  dealloc(byte *ptr, usize len);        // free with explicit, hard-checked size
void *realloc(void *ptr, usize size);       // grow/shrink, may move
template <typename T> void dealloc(T *ptr);
template <typename T> void dealloc(T *ptr, usize len);

// chunk API (returns {ptr, actual_capacity})
micron::__chunk<byte> balloc(usize size);
micron::__chunk<byte> fetch(usize size);
template <typename T> T *fetch();           // one trivially-constructible T

// temporal & safety extensions
byte *launder(usize size);                  // temporal alloc
void  retire(byte *ptr);                    // tombstone free (use-after-free trap)
void  freeze(byte *ptr);                    // make a live region read-only
void  relinquish(byte *ptr);               // unmap the whole sheet ptr lives on
template <typename T> void retire(T *ptr);
template <typename T> void freeze(T *ptr);
template <typename T> void relinquish(T *ptr);

// aligned
void *aligned_alloc(usize alignment, usize size);   // alignment must be a power of two
void  aligned_free(void *ptr);                       // REQUIRED when alignment > 32 B

// introspection
template <typename T> usize query_size(T *ptr);      // actual allocated size
bool  is_present(byte *ptr);                          // allocated & live?
bool  within(byte *ptr);                              // owned by this allocator?
usize musage();                                       // total bytes in use
template <u64 Sz> usize musage();                     // bytes in one size class
void  which();                                        // per-tier usage report (debug)

// external-memory provenance
byte *mark_at(byte *ptr, usize size);                // track externally-mapped memory
byte *unmark_at(byte *ptr, usize size);

} // namespace abc

// libc drop-in (active unless ABCMALLOC_DISABLE is defined)
void *malloc(usize size);
void *calloc(usize num, usize size);
void *realloc(void *ptr, usize size);
void free(void *ptr);
void *aligned_alloc(usize alignment, usize size);

Configuration

Behavior is set through compile-time constants in config_amd64.hpp (workstation) / config_embed.hpp (constrained). Both presets are usable out of the box. The most important defaults:

__default_multithread_safe   = true;   // per-arena concurrency safety (off in freestanding)
__default_per_class_free_cache = true; // LIFO free cache on hot tiers for fast reuse
__default_eager_hot_tiers    = true;   // pre-warm precise/small/medium
__default_insert_guard_pages = true;   // PROT_NONE guard pages between regions
__default_tombstone (large/huge only)  // cold-tier use-after-free trapping
__default_saturated_mode     = true;   // adapt page provisioning to request bursts
__default_launder            = false;  // global address laundering (immutable structures)
__default_enforce_provenance = false;  // verify every free belongs to this allocator
__default_zero_on_alloc      = false;  // clear memory on allocation
__default_zero_on_free       = false;  // clear memory on free
__default_sanitize           = false;  // redzone/uninit-read detection patterns
__default_oom_enable         = false;  // OOM pressure monitoring (costs performance)

See config_amd64.hpp for the complete, documented flag set (tier sheet caps, cache depths, OOM thresholds, fail policy, etc.).

Building & integration

Header-only — include src/memory/allocation/abcmalloc/malloc.hpp and use abc::alloc / abc::dealloc. It depends only on the micron core library.

• LD_PRELOAD is not wired into the build; it can be added by compiling the allocator as a shared object that exports the libc allocation symbols.
• Language bindings (C / Rust / Zig) do not yet exist; but they will.

Limitations

• first allocation on a thread pays a one-time arena-initialization cost
• slightly underperforms on workloads dominated by tiny round-trip allocations
• more than __max_arenas (64) genuinely-concurrent allocator threads fall back to a shared arena; keep concurrent threads ≤ 64
• depends on the micron core library as its sole dependency

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License

Licensed under the MIT License.