I started with a free list, which felt safe, but they pushed back almost immediately asking how I'd find a block of the right size efficiently.
Start by clarifying requirements (e.g., alignment, thread safety, fragmentation) and then propose a segregated free list with size classes, using a balanced BST (e.g., red-black tree) per size class to track free blocks. Explain how allocate finds the smallest suitable block in O(log n) and free inserts the block back into the appropriate tree, also in O(log n).
Pro tip: Mention that real allocators like jemalloc and tcmalloc use size-class segregation and per-thread caches to reduce lock contention, and that O(log n) is often achieved with intrusive data structures to avoid extra memory overhead.
Ask about alignment, thread safety, fragmentation tolerance, and performance goals to tailor the design. This shows you consider real-world trade-offs.
Propose segregated free lists by size class, each implemented as a balanced BST (e.g., red-black tree) keyed by block size. This allows O(log n) search, insert, and delete.
Round up size to the next size class, find the smallest free block >= requested size in the corresponding tree (or next larger class), remove it, and split if necessary. Splitting may require inserting the remainder into a smaller size class.
Locate the block's metadata (e.g., via a header before the pointer), determine its size class, and insert it into the appropriate free tree. Optionally coalesce with adjacent free blocks, which may involve removing neighbors from their trees and inserting the merged block.
Explain that each operation is O(log n) due to tree operations, and discuss trade-offs like memory overhead of trees, fragmentation, and potential improvements (e.g., per-thread caches, lock-free structures).
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
This came naturally after the main design question.
Start by defining fragmentation and its two types (internal and external), then discuss strategies to mitigate each, such as segregated free lists, size classes, and compaction. Explain coalescing as a technique to merge adjacent free blocks, and describe how boundary tags and free list management enable efficient coalescing.
Pro tip: Mention that coalescing can be done immediately or deferred, and that immediate coalescing reduces external fragmentation but may increase allocation latency; deferred coalescing can improve throughput at the cost of more fragmentation. Also, note that modern allocators like jemalloc and tcmalloc use size-class-based segregation to minimize fragmentation.
Briefly explain internal and external fragmentation, and why they matter for allocator performance and memory utilization.
Describe techniques like segregated free lists, size classes, slab allocation, and compaction to reduce fragmentation.
Detail how adjacent free blocks are merged using boundary tags and free list pointers, and the role of immediate vs. deferred coalescing.
Compare immediate vs. deferred coalescing in terms of latency, throughput, and fragmentation, and mention real-world allocator examples.
Summarize how you would choose an approach based on workload characteristics and performance goals.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.