snmalloc: High-Performance Message-Passing Allocator

snmalloc is a high-performance allocator. snmalloc can be used directly in a project as a header-only C++ library, it can be LD_PRELOADed on Elf platforms (e.g. Linux, BSD), and there is a crate to use it from Rust.

Its key design features are:

  • Memory that is freed by the same thread that allocated it does not require any synchronising operations.
  • Freeing memory in a different thread to initially allocated it, does not take any locks and instead uses a novel message passing scheme to return the memory to the original allocator, where it is recycled. This enables 1000s of remote deallocations to be performed with only a single atomic operation enabling great scaling with core count.
  • The allocator uses large ranges of pages to reduce the amount of meta-data required.
  • The fast paths are highly optimised with just two branches on the fast path for malloc (On Linux compiled with Clang).
  • The platform dependencies are abstracted away to enable porting to other platforms.

snmalloc's design is particular well suited to the following two difficult scenarios that can be problematic for other allocators:

  • Allocations on one thread are freed by a different thread
  • Deallocations occur in large batches

Both of these can cause massive reductions in performance of other allocators, but do not for snmalloc.

Comprehensive details about snmalloc's design can be found in the accompanying paper, and differences between the paper and the current implementation are described here. Since writing the paper, the performance of snmalloc has improved considerably.

Further documentation

  • Instructions for building snmalloc
  • Instructions for porting snmalloc


This project welcomes contributions and suggestions. Most contributions require you to agree to a Contributor License Agreement (CLA) declaring that you have the right to, and actually do, grant us the rights to use your contribution. For details, visit

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This project has adopted the Microsoft Open Source Code of Conduct. For more information see the Code of Conduct FAQ or contact with any additional questions or comments.

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Author: Microsoft

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License: MIT

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snmalloc: High-Performance Message-Passing Allocator
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