Design Memory Allocator

Design Memory Allocator

What is this problem about?

The Design Memory Allocator coding problem simulates how an operating system manages memory blocks. You are given an array of size $n$ representing memory units. You need to implement two operations: allocate, which finds the first available contiguous block of a certain size and assigns it a unique mID, and free, which releases all blocks associated with a specific mID.

Why is this asked in interviews?

This problem is frequently asked by companies like Microsoft and Qualcomm because it reflects real-world system programming challenges. It tests your ability to manage contiguous segments within an array and your efficiency in searching for free space. It evaluations your proficiency with array simulation interview patterns and your ability to optimize search and deletion using secondary data structures like Hash Maps to track which blocks belong to which IDs.

Algorithmic pattern used

The primary pattern is Simulation using an array. To find a block for allocate, you perform a linear scan to find $k$ consecutive zeros. To optimize the free operation, you can maintain a Map<mID, List<Integer>> that stores the indices occupied by each ID, allowing you to clear them without scanning the entire array.

Example explanation

Memory size = 10.

1. allocate(size=3, mID=1): Search for the first 3 zeros. Indices 0, 1, 2 are free. Occupy them. Memory: [1, 1, 1, 0, 0, 0, 0, 0, 0, 0].
2. allocate(size=2, mID=2): Search for 2 zeros. Indices 3, 4 are free. Memory: [1, 1, 1, 2, 2, 0, 0, 0, 0, 0].
3. free(mID=1): Find all '1's and set them to '0'. Memory: [0, 0, 0, 2, 2, 0, 0, 0, 0, 0]. Returns 3 (the number of units freed).

Common mistakes candidates make

• Fragmentation: Not correctly identifying that blocks must be contiguous.
• Slow Deletion: Iterating through the entire array for every free command instead of using a Map to track indices.
• Search Efficiency: Not resetting the "consecutive zeros" counter when hitting an occupied block during the search.

Interview preparation tip

Always consider the trade-off between time and space. While a Map speeds up the free operation, it uses extra memory. Mentioning this trade-off to your interviewer shows that you understand the constraints of system-level programming.