9d97a38806
These are almost entirely backend concerns, so move their definitions over there. Use C++ friend classes to ensure that MetaCommon structures are opaque to frontend code (at least, at compile time, and neglecting the rest of C++). (These structures contain high-authority pointers and so should be as closely guarded as we can make them.) The bits that leak out are - the encoding of RemoteAllocator* and sizeclass_t into the uinptr_t within a MetaEntry. This, however, is almost entirely a frontend concern, so detach the method definitions from the class and leave those in mem/metaslab.h for the moment. - the size of metadata structures pointed to by the MetaEntry meta field. Rather than use sizeof(Metaslab) (and assert that sizeof(ChunkRecord) is smaller), instead, define PAGEMAP_METADATA_STRUCT_SIZE once and assert that all records fit. Additionally, add an assertion that Metaslab is exactly this size, not for semantic reasons, but because we expect it to be true. The bits that leak in are - the need to zero memory corresponding to a chunk. Rather than having an escape hatch that reveals the MetaCommon.chunk, move the zeroing call into a small wrapper method within the MetaCommon class itself. - the need to get the address of a chunk. We want to assert that we've got the right chunk on occasion (well, at least once so far) and so add a class method to expose the address_t view of the chunk pointer without exposing the pointer itself.
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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