mimalloc

mimalloc (pronounced "me-malloc") is a general purpose allocator with excellent performance characteristics. Initially developed by Daan Leijen for the runtime systems of the Koka and Lean languages.

Latest release tag: v2.1.7 (2024-05-21).
Latest v1 tag: v1.8.7 (2024-05-21).

mimalloc is a drop-in replacement for malloc and can be used in other programs without code changes, for example, on dynamically linked ELF-based systems (Linux, BSD, etc.) you can use it as:

> LD_PRELOAD=/usr/lib/libmimalloc.so  myprogram

It also includes a robust way to override the default allocator in Windows. Notable aspects of the design include:

small and consistent: the library is about 8k LOC using simple and consistent data structures. This makes it very suitable to integrate and adapt in other projects. For runtime systems it provides hooks for a monotonic heartbeat and deferred freeing (for bounded worst-case times with reference counting). Partly due to its simplicity, mimalloc has been ported to many systems (Windows, macOS, Linux, WASM, various BSD's, Haiku, MUSL, etc) and has excellent support for dynamic overriding. At the same time, it is an industrial strength allocator that runs (very) large scale distributed services on thousands of machines with excellent worst case latencies.
free list sharding: instead of one big free list (per size class) we have many smaller lists per "mimalloc page" which reduces fragmentation and increases locality -- things that are allocated close in time get allocated close in memory. (A mimalloc page contains blocks of one size class and is usually 64KiB on a 64-bit system).
free list multi-sharding: the big idea! Not only do we shard the free list per mimalloc page, but for each page we have multiple free lists. In particular, there is one list for thread-local free operations, and another one for concurrent free operations. Free-ing from another thread can now be a single CAS without needing sophisticated coordination between threads. Since there will be thousands of separate free lists, contention is naturally distributed over the heap, and the chance of contending on a single location will be low -- this is quite similar to randomized algorithms like skip lists where adding a random oracle removes the need for a more complex algorithm.
eager page purging: when a "page" becomes empty (with increased chance due to free list sharding) the memory is marked to the OS as unused (reset or decommitted) reducing (real) memory pressure and fragmentation, especially in long running programs.
secure: mimalloc can be built in secure mode, adding guard pages, randomized allocation, encrypted free lists, etc. to protect against various heap vulnerabilities. The performance penalty is usually around 10% on average over our benchmarks.
first-class heaps: efficiently create and use multiple heaps to allocate across different regions. A heap can be destroyed at once instead of deallocating each object separately.
bounded: it does not suffer from blowup [1], has bounded worst-case allocation times (wcat) (upto OS primitives), bounded space overhead (~0.2% meta-data, with low internal fragmentation), and has no internal points of contention using only atomic operations.
fast: In our benchmarks (see below), mimalloc outperforms other leading allocators (jemalloc, tcmalloc, Hoard, etc), and often uses less memory. A nice property is that it does consistently well over a wide range of benchmarks. There is also good huge OS page support for larger server programs.

The documentation gives a full overview of the API. You can read more on the design of mimalloc in the technical report which also has detailed benchmark results.

Enjoy!

Branches

master: latest stable release (based on dev-slice).
dev: development branch for mimalloc v1. Use this branch for submitting PR's.
dev-slice: development branch for mimalloc v2. This branch is downstream of dev (and is essentially equal to dev except for src/segment.c)

Releases

Note: the v2.x version has a different algorithm for managing internal mimalloc pages (as slices) that tends to use reduce memory usage and fragmentation compared to mimalloc v1.x (especially for large workloads). Should otherwise have similar performance (see below); please report if you observe any significant performance regression.

2024-05-21, v1.8.7, v2.1.7: Fix build issues on less common platforms. Started upstreaming patches from the CPython integration. Upstream vcpkg patches.
2024-05-13, v1.8.6, v2.1.6: Fix build errors on various (older) platforms. Refactored aligned allocation.
2024-04-22, v1.8.4, v2.1.4: Fixes various bugs and build issues. Add MI_LIBC_MUSL cmake flag for musl builds. Free-ing code is refactored into a separate module (free.c). Mimalloc page info is simplified with the block size directly available (and new block_size_shift to improve aligned block free-ing). New approach to collection of abandoned segments: When a thread terminates the segments it owns are abandoned (containing still live objects) and these can be reclaimed by other threads. We no longer use a list of abandoned segments but this is now done using bitmaps in arena's which is more concurrent (and more aggressive). Abandoned memory can now also be reclaimed if a thread frees an object in an abandoned page (which can be disabled using mi_option_abandoned_reclaim_on_free). The option mi_option_max_segment_reclaim gives a maximum percentage of abandoned segments that can be reclaimed per try (=10%).
2023-04-24, v1.8.2, v2.1.2: Fixes build issues on freeBSD, musl, and C17 (UE 5.1.1). Reduce code size/complexity by removing regions and segment-cache's and only use arenas with improved memory purging -- this may improve memory usage as well for larger services. Renamed options for consistency. Improved Valgrind and ASAN checking.
2023-04-03, v1.8.1, v2.1.1: Fixes build issues on some platforms.
2023-03-29, v1.8.0, v2.1.0: Improved support dynamic overriding on Windows 11. Improved tracing precision with asan and Valgrind, and added Windows event tracing ETW (contributed by Xinglong He). Created an OS abstraction layer to make it easier to port and separate platform dependent code (in src/prim). Fixed C++ STL compilation on older Microsoft C++ compilers, and various small bug fixes.
2022-12-23, v1.7.9, v2.0.9: Supports building with asan and improved Valgrind support. Support arbitrary large alignments (in particular for std::pmr pools). Added C++ STL allocators attached to a specific heap (thanks @vmarkovtsev). Heap walks now visit all object (including huge objects). Support Windows nano server containers (by Johannes Schindelin,@dscho). Various small bug fixes.
2022-11-03, v1.7.7, v2.0.7: Initial support for Valgrind for leak testing and heap block overflow detection. Initial support for attaching heaps to a speficic memory area (only in v2). Fix realloc behavior for zero size blocks, remove restriction to integral multiple of the alignment in alloc_align, improved aligned allocation performance, reduced contention with many threads on few processors (thank you @dposluns!), vs2022 support, support pkg-config, .
2022-04-14, v1.7.6, v2.0.6: fix fallback path for aligned OS allocation on Windows, improve Windows aligned allocation even when compiling with older SDK's, fix dynamic overriding on macOS Monterey, fix MSVC C++ dynamic overriding, fix warnings under Clang 14, improve performance if many OS threads are created and destroyed, fix statistics for large object allocations, using MIMALLOC_VERBOSE=1 has no maximum on the number of error messages, various small fixes.
2022-02-14, v1.7.5, v2.0.5 (alpha): fix malloc override on Windows 11, fix compilation with musl, potentially reduced committed memory, add bin/minject for Windows, improved wasm support, faster aligned allocation, various small fixes.
Older release notes

Special thanks to:

David Carlier (@devnexen) for his many contributions, and making mimalloc work better on many less common operating systems, like Haiku, Dragonfly, etc.
Mary Feofanova (@mary3000), Evgeniy Moiseenko, and Manuel Pöter (@mpoeter) for making mimalloc TSAN checkable, and finding memory model bugs using the genMC model checker.
Weipeng Liu (@pongba), Zhuowei Li, Junhua Wang, and Jakub Szymanski, for their early support of mimalloc and deployment at large scale services, leading to many improvements in the mimalloc algorithms for large workloads.
Jason Gibson (@jasongibson) for exhaustive testing on large scale workloads and server environments, and finding complex bugs in (early versions of) mimalloc.
Manuel Pöter (@mpoeter) and Sam Gross(@colesbury) for finding an ABA concurrency issue in abandoned segment reclamation. Sam also created the no GIL Python fork which uses mimalloc internally.

Usage

mimalloc is used in various large scale low-latency services and programs, for example:

Building

Windows

Open ide/vs2022/mimalloc.sln in Visual Studio 2022 and build. The mimalloc project builds a static library (in out/msvc-x64), while the mimalloc-override project builds a DLL for overriding malloc in the entire program.

macOS, Linux, BSD, etc.

We use cmake<sup>1</sup> as the build system:

> mkdir -p out/release
> cd out/release
> cmake ../..
> make

This builds the library as a shared (dynamic) library (.so or .dylib), a static library (.a), and as a single object file (.o).

> sudo make install (install the library and header files in /usr/local/lib and /usr/local/include)

You can build the debug version which does many internal checks and maintains detailed statistics as:

> mkdir -p out/debug
> cd out/debug
> cmake -DCMAKE_BUILD_TYPE=Debug ../..
> make

This will name the shared library as libmimalloc-debug.so.

Finally, you can build a secure version that uses guard pages, encrypted free lists, etc., as:

> mkdir -p out/secure
> cd out/secure
> cmake -DMI_SECURE=ON ../..
> make

This will name the shared library as libmimalloc-secure.so. Use ccmake<sup>2</sup> instead of cmake to see and customize all the available build options.

Notes:

Install CMake: sudo apt-get install cmake
Install CCMake: sudo apt-get install cmake-curses-gui

Single source

You can also directly build the single src/static.c file as part of your project without needing cmake at all. Make sure to also add the mimalloc include directory to the include path.

Using the library

The preferred usage is including <mimalloc.h>, linking with the shared- or static library, and using the mi_malloc API exclusively for allocation. For example,

> gcc -o myprogram -lmimalloc myfile.c

mimalloc uses only safe OS calls (mmap and VirtualAlloc) and can co-exist with other allocators linked to the same program. If you use cmake, you can simply use:

find_package(mimalloc 1.4 REQUIRED)

in your CMakeLists.txt to find a locally installed mimalloc. Then use either:

target_link_libraries(myapp PUBLIC mimalloc)

to link with the shared (dynamic) library, or:

target_link_libraries(myapp PUBLIC mimalloc-static)

to link with the static library. See test\CMakeLists.txt for an example.

For best performance in C++ programs, it is also recommended to override the global new and delete operators. For convenience, mimalloc provides mimalloc-new-delete.h which does this for you -- just include it in a single(!) source file in your project. In C++, mimalloc also provides the mi_stl_allocator struct which implements the std::allocator interface.

You can pass environment variables to print verbose messages (MIMALLOC_VERBOSE=1) and statistics (MIMALLOC_SHOW_STATS=1) (in the debug version):

> env MIMALLOC_SHOW_STATS=1 ./cfrac 175451865205073170563711388363

175451865205073170563711388363 = 374456281610909315237213 * 468551

heap stats:     peak      total      freed       unit
normal   2:    16.4 kb    17.5 mb    17.5 mb      16 b   ok
normal   3:    16.3 kb    15.2 mb    15.2 mb      24 b   ok
normal   4:      64 b      4.6 kb     4.6 kb      32 b   ok
normal   5:      80 b    118.4 kb   118.4 kb      40 b   ok
normal   6:      48 b       48 b       48 b       48 b   ok
normal  17:     960 b      960 b      960 b      320 b   ok

heap stats:     peak      total      freed       unit
    normal:    33.9 kb    32.8 mb    32.8 mb       1 b   ok
      huge:       0 b        0 b        0 b        1 b   ok
     total:    33.9 kb    32.8 mb    32.8 mb       1 b   ok
malloc requested:         32.8 mb

 committed:    58.2 kb    58.2 kb    58.2 kb       1 b   ok
  reserved:     2.0 mb     2.0 mb     2.0 mb       1 b   ok
     reset:       0 b        0 b        0 b        1 b   ok
  segments:       1          1          1
-abandoned:       0
     pages:       6          6          6
-abandoned:       0
     mmaps:       3
 mmap fast:       0
 mmap slow:       1