c33f355736
Summary of changes: - Add a new PAL that doesn't allocate memory, which can be used with a memory provider that is pre-initialised with a range of memory. - Add a `NoAllocation` PAL property so that the methods on a PAL that doesn't support dynamically reserving address space will never be called and therefore don't need to be implemented. - Slightly refactor the memory provider class so that it has a narrower interface with LargeAlloc and is easier to proxy. - Allow the address space manager and the memory provider to be initialised with a range of memory. This may eventually also remove the need for (or, at least, simplify) the Open Enclave PAL. This commit also ends up with a few other cleanups: - The `malloc_useable_size` CMake test that checks whether the parameter is const qualified was failing on FreeBSD where this function is declared in `malloc_np.h` but where including `malloc.h` raises an error. This should now be more robust. - The BSD aligned PAL inherited from the BSD PAL, which does not expose aligned allocation. This meant that it exposed both the aligned and non-aligned allocation interfaces and so happily accepted incorrect `constexpr` if blocks that expected one or the other but accidentally required both to exist. The unaligned function is now deleted so the same failures that appear in CI should appear locally for anyone using this PAL.
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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