Meta·Software Engineer·Technical Phone Screen·Senior

Senior

Jun 2026

Summary

Meta Production Engineer coding screen, one question, pretty systems-flavored. The problem looked like a scripting exercise but had enough moving parts that I had to think carefully about the rolling window logic.

Questions Asked (1)

Write a script that reads vmstat output from stdin and, given a metric column name, a threshold, a max violation count, and a time window in seconds, tracks how many lines in the rolling window have the metric strictly above the threshold. If violations exceed the max, print the offending lines.

System DesignTechnical Trade-offsAlgorithms & Data Structures

Author's notes

The rolling window part is where I got tripped up.

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AI HintsAI Generated

Suggested Approach

Clarify the input format and edge cases, then design a solution using a sliding window (e.g., deque) to track violations efficiently. Implement the script with proper parsing, window management, and output logic, and discuss trade-offs like memory vs. time and handling of malformed input.

Pro tip: Demonstrate awareness of real-world vmstat output variations (e.g., extra header lines, variable column spacing) and propose a robust parsing strategy. Also, mention that the window should be based on timestamps if available, not just line count, to handle irregular intervals.

1. Clarify requirements and assumptions

Ask about input format (e.g., header presence, column separators), metric column identification, and whether the time window is based on timestamps or line count. Confirm output format and error handling expectations.

2. Design the algorithm

Choose a sliding window approach using a deque to store relevant lines within the time window. Track the count of violations and evict expired entries as new lines arrive.

3. Implement parsing and validation

Parse each line to extract the metric value, handling headers and malformed lines gracefully. Validate that the metric column exists and the value is numeric.

4. Manage the rolling window and violations

Maintain a window of lines (or timestamps) and a violation count. When a new line arrives, add it if it violates, remove expired lines, and update the count. If count exceeds max, output the offending lines.

5. Test and discuss edge cases

Walk through examples: empty input, no violations, exactly max violations, window boundaries, and malformed lines. Discuss time/space complexity and potential optimizations.

Key Points to Mention

Use of a deque (or circular buffer) for O(1) insertion and deletion in the sliding window.
Handling of timestamps vs. line count for the time window, and implications for irregular data.
Robust parsing: skipping headers, handling variable whitespace, and validating numeric values.
Definition of 'strictly above' threshold and how to handle equality.
Output format: printing the offending lines exactly as they appeared, possibly with line numbers.
Edge cases: empty input, metric column not found, non-numeric values, and window larger than available data.

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