Buddy Memory Allocator | mysimulator.uk

Request Memory

Block size to allocate 20 KB

Selected Block

Click an allocated block to select it

Pool State

Free

0 KB

Allocated

0 KB

Internal fragmentation

0 KB

Blocks in pool

How it works

The buddy system manages a memory pool of size 2^K as a binary tree of power-of-two blocks. To satisfy a request of R bytes it picks the smallest order o with 2^o ≥ R, then repeatedly halves the smallest available larger block until a block of exactly that order exists:

split(block, order):
  while block.order > order:
    left, right = halve(block)     // two "buddies"
    freeList[block.order-1] += right
    block = left
  return mark_allocated(block)

Freeing reverses the process. Two buddies — blocks of the same order whose start addresses differ by exactly one bit, buddy = start XOR 2^order — coalesce back into their parent the moment both are free, which is what keeps the pool from fragmenting into unusable slivers:

free(block):
  mark_free(block)
  b = buddy_of(block)
  while b exists and b.free and b.order == block.order:
    block = merge(block, b)
    b = buddy_of(block)

Allocate — rounds your request up to the next power of two (in 4 KB units) and carves it out of the largest available free block, splitting as many times as needed.
Free selected — click any allocated (colored) block in the view, then release it; watch it merge with its buddy, and its buddy's buddy, whenever both sides are free.
Internal fragmentation — the gap between what you asked for and the power-of-two block you actually got (e.g. a 20 KB request inside a 32 KB block wastes 12 KB) — the structural cost of the buddy system's O(log N) split/merge speed.

Real-world relevance: this is the exact allocator behind the Linux kernel's page allocator (mm/page_alloc.c) and early Unix/BSD kmem allocators — fast, simple, and still in every machine you own.

Free block Selected Allocated

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