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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.

The implementation of snmalloc has evolved significantly since the initial paper. The mechanism for returning memory to remote threads has remained, but most of the meta-data layout has changed. We recommend you read docs/security to find out about the current design, and if you want to dive into the code docs/ provides a good overview of the allocation and deallocation paths.

snmalloc CI snmalloc CI for Morello


There is a hardened version of snmalloc, it contains

  • Randomisation of the allocations' relative locations,
  • Most meta-data is stored separately from allocations, and is protected with guard pages,
  • All in-band meta-data is protected with a novel encoding that can detect corruption, and
  • Provides a memcpy that automatically checks the bounds relative to the underlying malloc.

A more comprehensive write up is in docs/security.

Further documentation


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