NVIDIA·Software Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say

Jul 2026

Summary

NVIDIA systems interview, one meaty low-level question about implementing a memory allocator inside a fixed-size array with no external data structures allowed. The kind of problem that sounds manageable until you're actually sitting there trying to figure out where to stash your metadata.

Questions Asked (1)

You have an array of 32 elements, each holding 64 bits of data. Implement allocate(size) and free(handle) to manage memory blocks within this array. You cannot use any external data structures; all bookkeeping must live inside the array itself.

System DesignAlgorithms & Data StructuresTechnical Trade-offs

Author's notes

My first instinct was to reach for a separate free list and I literally started describing one before remembering the constraint.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Start by clarifying the problem constraints: the array is the only storage, so metadata must be embedded within the 64-bit elements. Propose a design where each element stores either a block header (size, free/used flag, links) or user data, and implement allocate/free using in-array linked lists and boundary tags. Discuss trade-offs between internal fragmentation, metadata overhead, and allocation speed, and suggest optimizations like segregated free lists or bitmaps.

Pro tip: Emphasize that the design must handle the array as a contiguous memory pool; use boundary tags to enable coalescing of adjacent free blocks, which is crucial for preventing fragmentation. Also, mention that you would validate the design with edge cases like allocating the entire array or freeing non-allocated handles.

1. Clarify requirements and constraints

Confirm that the array is the sole memory and that no external data structures are allowed. Discuss expected allocation patterns, handle representation, and whether alignment or block size limits exist.

2. Design metadata layout

Decide how to store bookkeeping within the 64-bit elements: e.g., each block starts with a header containing size, free/used flag, and pointers to next/prev free blocks. Consider using boundary tags at both ends for coalescing.

3. Implement allocate(size)

Traverse the free list to find a suitable block (first-fit, best-fit, etc.). If the block is larger than needed, split it and update metadata. Return a handle (e.g., index of the data portion) to the user.

4. Implement free(handle)

Locate the block header from the handle, mark it free, and insert it into the free list. Coalesce with adjacent free blocks using boundary tags to reduce fragmentation.

5. Analyze trade-offs and optimizations

Discuss time/space trade-offs: metadata overhead vs. allocation speed, fragmentation, and potential improvements like segregated free lists, bitmaps, or buddy system within the array.

Key Points to Mention

Use of boundary tags (headers/footers) to enable coalescing of adjacent free blocks.
Handle representation: index into the array, possibly encoding block size or offset.
Free list management: singly or doubly linked list embedded in free blocks.
Allocation strategies: first-fit, best-fit, or next-fit and their impact on fragmentation.
Splitting and coalescing logic to maintain large contiguous free space.
Trade-offs: metadata overhead reduces usable memory; internal fragmentation from alignment or minimum block size.

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