rlsf

This crate implements the TLSF (Two-Level Segregated Fit) dynamic memory allocation algorithm¹. Requires Rust 1.61.0 or later.

• Allocation and deallocation operations are guaranteed to complete in constant time. TLSF is suitable for real-time applications.

• Fast and small. You can have both. It was found to be smaller and faster² than most no_std-compatible allocator crates.

• Accepts any kinds of memory pools. The low-level type Tlsf just divides any memory pools you provide (e.g., a static array) to serve allocation requests. The high-level type GlobalTlsf automatically acquires memory pages using standard methods on supported systems.

• This crate supports #![no_std]. It can be used in bare-metal and RTOS-based applications.

_{¹ M. Masmano, I. Ripoll, A. Crespo and J. Real, "TLSF: a new dynamic memory allocator for real-time systems," Proceedings. 16th Euromicro Conference on Real-Time Systems, 2004. ECRTS 2004., Catania, Italy, 2004, pp. 79-88, doi: 10.1109/EMRTS.2004.1311009.}

_{² Compiled for and measured on a STM32F401 microcontroller using FarCri.rs.}

Measured Performance

Drawbacks

• It does not support concurrent access. A whole pool must be locked for allocation and deallocation. If you use a FIFO lock to protect the pool, the worst-case execution time will be O(num_contending_threads). You should consider using a thread-caching memory allocator (e.g., TCMalloc, jemalloc) if achieving a maximal throughput in a highly concurrent environment is desired.

• Segregated freelists with constant-time lookup cause internal fragmentation proportional to free block sizes. The SLLEN paramter allows for adjusting the trade-off between fewer freelists and lower fragmentation.

• No special handling for small allocations (one algorithm for all sizes). This may lead to inefficiencies in allocation-heavy applications compared to modern scalable memory allocators, such as glibc and jemalloc.

Examples

Tlsf: Core API

```rust use rlsf::Tlsf; use std::{mem::MaybeUninit, alloc::Layout};

let mut pool = [MaybeUninit::uninit(); 65536];

// On 32-bit systems, the maximum block size is 16 << FLLEN = 65536 bytes. // The worst-case internal fragmentation is (16 << FLLEN) / SLLEN - 2 = 4094 bytes. // 'pool represents the memory pool's lifetime (pool in this case). let mut tlsf: Tlsf<', u16, u16, 12, 16> = Tlsf::new(); // ^^ ^^ ^^ // | | | // 'pool | SLLEN // FLLEN tlsf.insertfree_block(&mut pool);

unsafe { let mut ptr1 = tlsf.allocate(Layout::new::()).unwrap().cast::(); let mut ptr2 = tlsf.allocate(Layout::new::()).unwrap().cast::(); ptr1.as_mut() = 42; *ptr2.as_mut() = 56; assert_eq!(ptr1.asref(), 42); asserteq!(*ptr2.as_ref(), 56); tlsf.deallocate(ptr1.cast(), Layout::new::().align()); tlsf.deallocate(ptr2.cast(), Layout::new::().align()); } ```

GlobalTlsf: Global Allocator

GlobalTlsf automatically acquires memory pages through platform-specific mechanisms. It doesn't support returning memory pages to the system even if the system supports it.

```rust

[cfg(all(targetarch = "wasm32", not(targetfeature = "atomics")))]

[global_allocator]

static A: rlsf::SmallGlobalTlsf = rlsf::SmallGlobalTlsf::new();

let mut m = std::collections::HashMap::new(); m.insert(1, 2); m.insert(5, 3); drop(m); ```

Details

Changes from the Original Algorithm

• The end of each memory pool is capped by a sentinel block (a permanently occupied block) instead of a normal block with a last-block-in-pool flag. This simplifies the code a bit and improves its worst-case performance and code size.

Cargo Features

• unstable: Enables experimental features that are exempt from the API stability guarantees.

License

MIT/Apache-2.0