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#47 in Memory management

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7,152 downloads per month
Used in 3 crates

MIT/Apache

170KB
2.5K SLoC

rlsf

docs.rs

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

The result of latency measurement on STM32F401 is shown here. rlsf: 260–320 cycles. buddy-alloc: 340–440 cycles. umm_malloc: 300–700 cycles. dlmalloc: 450–750 cycles.

The result of code size measurement on WebAssembly is shown here. rlsf: 1267 bytes, rlsf + pool coalescing: 1584 bytes, wee_alloc: 1910 bytes, dlmalloc: 9613 bytes.

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

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.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

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.

#[cfg(all(target_arch = "wasm32", not(target_feature = "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

Dependencies

~1.7–3.5MB
~66K SLoC