Crate simple_chunk_allocator

Simple Chunk Allocator

A simple no_std allocator written in Rust that manages memory in fixed-size chunks/blocks. Useful for basic no_std binaries where you want to manage a heap of a few megabytes without complex features such as paging/page table management. Instead, this allocator gets a fixed/static memory region and allocates memory from there. This memory region can be contained inside the executable file that uses this allocator. See examples down below.

⚠ Other allocators with different properties (for example better memory utilization but less performance) do exist. The README of the repository contains a section that discusses how this allocator relates to other existing allocators on <crates.io>. ⚠

TL;DR

• ✅ no_std allocator with test coverage
• ✅ uses static memory as backing storage (no paging/page table manipulations)
• ✅ allocation strategy is a combination of next-fit and best-fit
• ✅ reasonable fast with low code complexity
• ✅ const compatibility (no runtime init() required)
• ✅ efficient in scenarios where heap is a few dozens megabytes in size
• ✅ user-friendly API

The inner and low-level ChunkAllocator can be used as #[global_allocator] with the synchronized wrapper type GlobalChunkAllocator. Both can be used with the allocator_api feature. The latter enables the usage in several types of the Rust standard library, such as Vec::new_in or BTreeMap::new_in. This is primarily interesting for testing but may also enable other interesting use-cases.

The focus is on const compatibility. The allocator and the backing memory can get initialized during compile time and need no runtime init() call or similar. This means that if the compiler accepts it then the allocation will also work during runtime. However, you can also create allocator objects during runtime.

The inner and low-level ChunkAllocator is a chunk allocator or also called fixed-size block allocator. It uses a mixture of the strategies next-fit and a best-fit. It tries to use the smallest gap for an allocation request to prevent fragmentation but this is no guarantee. Each allocation is a trade-off between a low allocation time and preventing fragmentation. The default chunk size is 256 bytes but this can be changed as compile time const generic. Having a fixed-size block allocator enables an easy bookkeeping algorithm through a bitmap but has as consequence that small allocations, such as 64 byte will take at least one chunk/block of the chosen block size.

This project originates from my Diplom thesis project. Since I originally had lots of struggles to create this (my first ever allocator), I outsourced it for better testability and to share my knowledge and findings with others in the hope that someone can learn from it in any way.

Minimal Code Example

#![feature(const_mut_refs)]
#![feature(allocator_api)]

use simple_chunk_allocator::{heap, heap_bitmap, GlobalChunkAllocator, PageAligned};

// The macros help to get a correctly sized arrays types.
// I page-align them for better caching and to improve the availability of
// page-aligned addresses.

/// Backing storage for heap (1Mib). (read+write) static memory in final executable.
///
/// heap!: first argument is chunk amount, second argument is size of each chunk.
///        If no arguments are provided it falls back to defaults.
///        Example: `heap!(chunks=16, chunksize=256)`.
static mut HEAP: PageAligned<[u8; 1048576]> = heap!();
/// Backing storage for heap bookkeeping bitmap. (read+write) static memory in final executable.
///
/// heap_bitmap!: first argument is amount of chunks.
///               If no argument is provided it falls back to a default.
///               Example: `heap_bitmap!(chunks=16)`.
static mut HEAP_BITMAP: PageAligned<[u8; 512]> = heap_bitmap!();

// please make sure that the backing memory is at least CHUNK_SIZE aligned; better page-aligned
#[global_allocator]
static ALLOCATOR: GlobalChunkAllocator =
    unsafe { GlobalChunkAllocator::new(HEAP.deref_mut_const(), HEAP_BITMAP.deref_mut_const()) };

fn main() {
    // at this point, the allocator already got used a bit by the Rust runtime that executes
    // before main() gets called. This is not the case if a `no_std` binary gets produced.
    let old_usage = ALLOCATOR.usage();
    let mut vec = Vec::new();
    vec.push(1);
    vec.push(2);
    vec.push(3);
    assert!(ALLOCATOR.usage() > old_usage);

    // use "allocator_api"-feature. You can use this if "ALLOCATOR" is not registered as
    // the global allocator. Otherwise, it is already the default.
    let _boxed = Box::new_in([1, 2, 3], ALLOCATOR.allocator_api_glue());
}

Macros

heap

Helper macro that initializes a page-aligned static memory area with a correct size to get used as heap in crate::GlobalChunkAllocator.

heap_bitmap

Helper macro that initializes a page-aligned static memory area with a correct size to get used as heap bookkeeping bitmap in crate::GlobalChunkAllocator.

Structs

AllocatorApiGlue

Helper struct generated by GlobalChunkAllocator that can be used in structs that accept a custom instance of a specific allocator implementation.

ChunkAllocator

Low-level chunk allocator that operates on the provided backing memory. Allocates memory with a variant of the strategies next-fit and best-fit.

GlobalChunkAllocator

Synchronized high-level wrapper around ChunkAllocator that implements the Rust traits GlobalAlloc which enables the usage as global allocator. The method GlobalChunkAllocator::allocator_api_glue returns an object of type AllocatorApiGlue which can be used with the allocator_api feature.

PageAligned

Wrapper around a T that gets page-aligned. All important methods allow the usage in const contexts.

Enums

ChunkAllocatorError

Possible errors of ChunkAllocator.

Constants

DEFAULT_CHUNK_AMOUNT

The default number of chunks. If the default chunk size of DEFAULT_CHUNK_SIZE gets used then this equals to 1MiB of memory.

DEFAULT_CHUNK_SIZE

Default chunk size used by ChunkAllocator. 256 Bytes is a trade-off between fast allocations and efficient memory usage. However, small allocations will take up at least this amount if bytes.