e1ba7cd592
Replace the generic check_size() calls with per-{small,medium,large} code paths,
which follow the progression...
* _unchecked: no validation has been performed; check the chunkmap and move
to...
* _checked_chunkmap: the chunkmap indicates that this is the correct slab-type,
and so we have reason to believe that our supposed Allocslab pointers are of
the correct types. Use those pointers to read the sizeclass and move to...
* _checked_sizeclass: additionally, the Allocslab metadata confirms that our
sizeclass matches the expected value. Check that we are at the start of an
object and move to...
* _start: we now believe we have a pointer to the start of a live object. On
a sufficiently capable architecture, we will eventually atomically check that
our pointer is not (scheduled to be) revoked (and/or not reversioned). If
this test passes, our prior pointer arithmetic and loads are justified
(assuming correctness of the quarantine/revocation/reuse) machinery.
The size-less dealloc(void*) path by necessity reads the chunkmap and sizeclass
data itself and so there's no reason to re-validate; as such, it jumps directly
to the _okcmsc point. Similarly, we assume that remote queues are full of
already validated objects, and so the remote handling paths continue to jump
further along even than the _start methods.
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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