Openai·Software Engineer·Onsite - System Design / Architecture·Senior

SeniorPrefer not to say

Jun 2026

Summary

OpenAI systems interview, one meaty design question about building a memory allocator from scratch. The kind of problem that sounds manageable until you're actually in it trying to explain coalescing out loud.

Questions Asked (1)

Design and implement a memory allocator with allocate(size) and free(ptr) over a fixed-size memory region. Walk through your choice of data structures, allocation policy, block splitting, and coalescing. What are the trade-offs?

System DesignAlgorithms & Data StructuresTechnical Trade-offs

Author's notes

I started with the naive linked list of free blocks and they let me talk for a bit before asking about performance.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements (alignment, thread safety, performance goals) and then propose a design using a free list with boundary tags for O(1) coalescing. Walk through allocation (first-fit with splitting) and deallocation (coalescing with neighbors), and discuss trade-offs like fragmentation vs. speed.

Pro tip: Mention that production allocators often use size-class segregated free lists to reduce fragmentation and improve speed, and that you'd consider thread-local caches for scalability.

1. Clarify Requirements

Ask about alignment, thread safety, performance targets, and whether the allocator needs to handle arbitrary sizes or can use size classes.

2. Choose Data Structures

Propose a free list (doubly-linked) with boundary tags (headers/footers) to enable coalescing. Mention segregated free lists for efficiency.

3. Define Allocation Policy

Describe first-fit (or best-fit) search, block splitting to minimize internal fragmentation, and how to handle alignment.

4. Implement Deallocation and Coalescing

Explain how free() marks the block free and coalesces with adjacent free blocks using boundary tags, updating the free list.

5. Discuss Trade-offs

Compare first-fit vs. best-fit, splitting vs. no splitting, immediate vs. deferred coalescing, and the impact on fragmentation and performance.

Key Points to Mention

Boundary tags (headers/footers) for O(1) coalescing
Free list management (doubly-linked, segregated by size)
Allocation policies: first-fit, best-fit, worst-fit
Block splitting and coalescing to reduce fragmentation
Alignment requirements and padding
Thread safety and scalability (locks, thread-local caches)

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.