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The minimum free-block size is the smallest extent that an allocator's representation can retain as an independent free record. It depends on where the allocator stores a region's size, links and other metadata, as well as memory alignment. A region may contain unused bytes yet be too short to enter the free list. OSTEP demonstrates why allocator metadata must be counted: free records can be embedded inside the free regions themselves.^[ref]

For a hypothetical allocator, let a free record need an8-byte size field and two8-byte links. Its raw metadata requires24 bytes. If all block lengths are multiples of16 bytes, its minimum independently representable size isThe exact fields and sizes are illustrative. An allocator with separate metadata, a singly linked record or another block granularity can have another minimum.

Suppose the same allocator uses an8-byte allocated-block header. An8-byte payload plus that header would occupy16 bytes before enforcing the minimum. Reserving32 bytes ensures that this block can later become an independent free record. Extra allocation space of this kind contributes to internal fragmentation. The allocator might instead use a distinct representation for tiny blocks, so the formula is a design constraint for the stated representation rather than a universal allocation rule.

A proposed split also needs to respect the minimum. Taking80 bytes from a96-byte extent leaves16 bytes, an unsplittable free-block remainder under this layout. Giving the caller the full96-byte extent avoids making an invalid free record but consumes extra storage. Taking32 bytes would leave64, which can be retained. Memory block coalescing can later join adjacent returned extents, but the allocator must have a valid way to represent each extent until that happens.

An unsplittable free-block remainder is the positive suffix left by a proposed allocation that is too small to retain as a valid independent free list record. Its size is below the allocator's minimum free-block size. The region still contains unused bytes, but the chosen representation cannot manage it as a separate free extent. The issue arises because free-region metadata can occupy the regions themselves.^[ref]

Let a chosen block have size $B$, let the proposed allocated extent after header/alignment rounding be $N\le B$, and let the minimum free size be $M$. With remainder $r=B-N$, an allocator that absorbs an undersized suffix usesThis is one possible policy, not a mandatory allocator interface. The absorbed suffix adds $r$ bytes of internal fragmentation beyond the bookkeeping and rounding already included in $N$.

For an original example, take $B=96$ bytes, an8-byte allocated header,16-byte block granularity and $M=32$ bytes. A56-byte payload gives $N=16\lceil (56+8)/16\rceil =64$, leaving a representable32-byte free block. A57-byte payload instead gives $N=80$, leaving an unusable16-byte suffix. Absorbing it reserves all96 bytes. Increasing the request by just one byte has therefore increased this allocation's occupied extent by32 bytes.

If $M$ and all extents are multiples of a granularity $q$, a positive absorbed suffix is at most $M-q$ bytes. Without that granularity condition, the general integer-byte bound is $M-1$. Choosing another free block or another representation can change the tradeoff. Later memory block coalescing can recover adjacent space when allocations are returned, but it does not make an invalid intermediate free record representable.