“Refuses to fragment” is a strong claim, but the title alone does not establish how the allocator works, what it was tested against, or whether it guarantees zero fragmentation. The key distinction is between internal waste inside allocated blocks and external fragmentation, where free memory exists but is split into pieces too small for a request. TLSF offers a useful embedded-systems comparison—not evidence about the unnamed allocator itself.
What “fragmentation” means for a microcontroller heap
Fragmentation describes more than one problem, and an allocator can address one without eliminating the other.
- Internal fragmentation: unused space within an allocated block, often because a request is rounded up for alignment or allocator bookkeeping.
- External fragmentation: free memory is divided among separate regions, so a request cannot fit in any one region even though the total free capacity is large enough.
Internal waste is shaped by allocation sizes, alignment, and metadata. External fragmentation depends on placement policy and the history of allocation and release requests. Results from one workload therefore do not establish behavior for every program.
What would justify a “refuses to fragment” claim?
The phrase needs a measurable definition. It could mean a bound on internal waste, prevention of external fragmentation under stated conditions, or simply favorable results in a particular test. Those are different claims.
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For a microcontroller allocator, a convincing account would identify its fragmentation metric, supported memory regions, allocation and free behavior, metadata cost, and test workload. It would also state the conditions under which any guarantee holds. Without those details, the title does not show that fragmentation is mathematically impossible.
How TLSF provides a comparison point
Two-Level Segregated Fit (TLSF) is an established allocator design used as a real-time reference. Its authors describe two-level segregated lists for arranging free blocks, an incomplete search policy, and a good-fit allocation strategy. TLSF also coalesces neighboring free blocks when memory is released, helping re-form larger free regions. The University of York’s record summarizes the authors’ approach as: “TLSF uses two levels of segregated lists to arrange free memory blocks and an incomplete search policy.” University of York publication record.
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The TLSF authors describe allocation and deallocation costs as asymptotically constant. That is a useful real-time property, but it is not a guarantee of zero fragmentation, nor proof that the allocator named in the title uses TLSF’s design.
What the published TLSF figures do—and do not—show
- Masmano and coauthors’ 2008 paper calculates around 3.1% worst-case internal fragmentation for a TLSF configuration with five second-level index bits. That figure belongs to the analyzed configuration, not to all TLSF implementations or the unnamed allocator. Paper abstract and publication details.
- The same paper reports a broader fragmentation evaluation with worst-case results below 30% and averages around 15% across the configurations it examined. This is a different metric and scope from the 3.1% internal-fragmentation calculation. Paper abstract and publication details.
- The University of York’s 2008 publication summary describes a reported TLSF response time of less than 200 processor instructions on an x86 processor. That result is platform-specific, not a microcontroller timing promise. University of York publication record.
Small-target costs and operating constraints
Allocator behavior is only part of the decision on a microcontroller. Pool size, alignment, metadata, concurrency, and out-of-memory handling can matter as much as a fragmentation metric.
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One widely used C TLSF implementation documents 4-byte alignment assumptions, per-allocation overhead, pool-management overhead, and no built-in thread safety. These are details of that implementation, not universal properties of TLSF or of other allocators. Matt Conte’s TLSF implementation.
The Rust TLSF documentation likewise leaves synchronization and realloc policy to application-level decisions, underscoring that a library’s allocation strategy does not settle every system-level requirement. Rust TLSF documentation.
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How to evaluate an allocator for your own firmware
Test the allocator against the patterns your firmware actually creates, while separating measured behavior from formal guarantees.
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- Define the failure you care about. Track internal waste separately from external fragmentation. Include the largest available free block as well as total free capacity.
- Describe the workload. Record request sizes, object lifetimes, allocation and release order, and whether allocations occur in interrupt or concurrent contexts.
- Measure resource costs. Include pool capacity, alignment, per-block and pool metadata, and worst-case allocation and free latency on the target MCU.
- Exercise failure and boundary cases. Test near-exhaustion, requests larger than the largest free region, repeated allocation/free cycles, pool boundaries, and the firmware’s out-of-memory response.
- State what the test proves. A stress test can show how an allocator behaved for that workload; it cannot alone prove a universal no-fragmentation guarantee.
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