dac1ba6a6c
Introduce Metaslab::from_link(SlabLink*) to encapsulate the "container of" dance. Note that Metaslab was not a standard layout type prior to this change (since both SlabLink and Metaslab defined non-static data members), and so the reinterpret_cast<>s replaced here with ::from_link() were UB, but everyone lays out classes as one expects so it was fine in practice. Most of the uses of ::from_link() are already guarded by checks that the link pointer is not nullptr, but in src/mem/corealloc.h:/debug_is_empty_impl we shift to testing the link pointer explicitly before converting to the metaslab. Despite that Metaslab is now standard layout, we still don't fall back to the inter-convertibility of a standard layout class and its first[*] data member since we're going to want to put a common initial sequence across Metaslab and ChunkRecord and the SlabLink isn't likely to be in it.
snmalloc
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
Contributing
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