e9ed219fd8
Presently, our flat pagemap can be configured to take...
32-bit AS 48-bit AS
USE_SMALL_CHUNKS 16 KiB 1 GiB
default 4 KiB 256 MiB
USE_LARGE_CHUNKS 256 B 16 MiB
At 1 GiB, we're already past the 512 MiB threshold imposed when
src/test/func/memory/memory.cc, when configured to TEST_LIMITED, probes the
effect of rlimit.
Instead, restrict flat pagemaps to at most 256 MiB of AS by default (override
by defining SNMALLOC_MAX_FLATPAGEMAP_SIZE), which forces the USE_SMALL_CHUNKS &
48-bit AS configuration to use the tree-based version.
While here, rename USE_FLATPAGEMAP to CHUNKMAP_USE_FLATPAGEMAP.
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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This project has adopted the Microsoft Open Source Code of Conduct. For more information see the Code of Conduct FAQ or contact opencode@microsoft.com with any additional questions or comments.