In operating systems, what is the buddy system of dynamic memory allocation and how does it manage free blocks to reduce fragmentation?

Introduction / Context:
Dynamic memory allocation in operating systems and low level allocators must balance fast allocation with low fragmentation. The buddy system is a classic algorithm used in some kernels and runtime libraries to manage variable sized blocks in main memory. This question checks whether you understand the basic idea of how the buddy system splits and merges blocks, rather than confusing it with paging, file system redundancy or a simple first fit scheme.

Given Data / Assumptions:

• We are allocating and freeing variable sized regions of main memory.

• The allocator should be able to split large free regions into smaller ones.

• The allocator should also be able to merge compatible free regions back into larger blocks when possible.

• The term buddy system refers to the classic power of two based scheme described in operating system textbooks.

Concept / Approach:
In the buddy system, the total memory region is treated as a block whose size is a power of two. When a request arrives, the allocator finds the smallest block that is still large enough. If that block is larger than needed, it is repeatedly split into two equal halves, called buddies, until the best size is obtained. When a block is freed, the allocator checks whether its buddy is also free. If both are free and correctly aligned, they are coalesced back into a larger block. This process continues up the tree, reducing external fragmentation while keeping the free list structure relatively simple.

Step-by-Step Solution:
Step 1: Recall that in the buddy system every free block size is a power of two, such as 2^k bytes. Step 2: On allocation, the system searches for a block of size 2^k that can satisfy the request and splits larger blocks into two equal halves until the required size is reached. Step 3: On deallocation, the system finds the buddy of the freed block, which is the block of the same size that differs from it only in the k-th bit of the address. Step 4: If the buddy is also free, the allocator merges them into a larger block of size 2^(k+1) and may repeat this merge up the hierarchy. Step 5: Compare this behavior with each option and select the description that mentions power of two sizes, splitting and merging buddy blocks to manage free space.

Verification / Alternative check:
A quick way to verify is to check whether the option talks about block pairs and combining them after deallocation. The buddy system relies on a simple mathematical relationship between addresses so that the allocator can quickly locate a buddy using bit operations. It does not duplicate data for redundancy, which would be more typical of RAID, and it does not limit itself to fixed page sizes only. Any answer that confuses it with strict paging or simple first fit allocation is therefore suspect.

Why Other Options Are Wrong:
Option B is wrong because it describes a pure paging scheme that never combines blocks, which is unrelated to the power of two splitting and merging strategy of the buddy system. Option C is wrong because it suggests storing duplicates on different disks, which belongs to storage level redundancy rather than main memory allocation. Option D is wrong because first fit allocation does not require or describe any buddy relationship or power of two size constraint, so it does not capture the essence of the buddy algorithm.

Common Pitfalls:
Students often confuse the buddy system with paging simply because both use power of two sizes. However, paging operates at the level of virtual memory pages and frames, whereas the buddy system is a dynamic allocator for variable sized requests inside a region. Another pitfall is thinking that the buddy system removes all fragmentation, when in reality it reduces external fragmentation but may still suffer from internal fragmentation, because a request may not exactly match a power of two. Understanding that buddy merges occur only when both halves are free is also important to avoid overestimating how often coalescing happens in practice.

Final Answer:
Therefore, the buddy system splits memory into blocks whose sizes are powers of two and combines adjacent buddy blocks on deallocation to manage free space efficiently.