Crate rlsf

This crate implements the TLSF (Two-Level Segregated Fit) dynamic memory allocation algorithm¹. Requires Rust 1.51.0 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 three randomly chosen no_std-compatible allocator crates.

• The memory pool is provided by an application. Examples of potential memory pool sources include: a static array for global memory allocation, a memory block allocated by another memory allocator for arena allocation.

• 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

Limitations

• 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.

• Free blocks cannot be returned to the underlying memory system efficiently.

Examples

Tlsf: Core API

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 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::INIT;
//                 ^^            ^^  ^^
//                  |             |  |
//                'pool           |  SLLEN
//                               FLLEN
tlsf.insert_free_block(&mut pool);

unsafe {
    let mut ptr1 = tlsf.allocate(Layout::new::<u64>()).unwrap().cast::<u64>();
    let mut ptr2 = tlsf.allocate(Layout::new::<u64>()).unwrap().cast::<u64>();
    *ptr1.as_mut() = 42;
    *ptr2.as_mut() = 56;
    assert_eq!(*ptr1.as_ref(), 42);
    assert_eq!(*ptr2.as_ref(), 56);
    tlsf.deallocate(ptr1.cast(), Layout::new::<u64>().align());
    tlsf.deallocate(ptr2.cast(), Layout::new::<u64>().align());
}

GlobalTlsf: Global Allocator

#[cfg(all(target_arch = "wasm32", not(target_feature = "atomics")))]
#[global_allocator]
static A: rlsf::SmallGlobalTlsf = rlsf::SmallGlobalTlsf::INIT;

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.

Modules

int	Provides BinInteger, a trait for types like u8 and u32.

Structs

FlexTlsf	A wrapper of Tlsf that automatically acquires fresh memory pools from FlexSource.
GlobalAllocAsFlexSource	Wraps core::alloc::GlobalAlloc to implement the FlexSource trait.
GlobalTlsf	WebAssembly and non-atomics, or Unix Tlsf as a global allocator.
SmallGlobalTlsfOptions	WebAssembly and non-atomics, or Unix GlobalTlsfOptions with all options set to optimize for code size.
Tlsf	The TLSF header (top-level) data structure.

Constants

GRANULARITY

The allocation granularity.

Traits

FlexSource	The trait for dynamic storage allocators that can back FlexTlsf.
GlobalTlsfOptions	WebAssembly and non-atomics, or Unix The options for GlobalTlsf.
Init	Provides a constant default value.

Type Definitions

SmallGlobalTlsf

WebAssembly and non-atomics, or Unix An instantiation of GlobalTlsf optimized for code size.