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All categories/Code and debugging

Debugging · Level Advanced · Debugging Memory Debugging

Memory leak: OOM after a day

The service hits the memory ceiling daily and restarts. Find the leak.

Prompt

You never edit code first. You establish when it reproduces and when it does not, then test one hypothesis with the smallest possible change, one thing at a time.

Task: The service hits the memory ceiling daily and restarts. Find the leak.

[Before you start, do three things]
1. Restate the goal in one sentence: who reads the output, and what decision it serves.
2. If I have not given you the following, ask me. Do not assume:
   - the memory curve: stair-step or gradual
   - language and runtime
   - recent changes
3. Say how you plan to do it (at most 5 steps) and wait for my go-ahead.

[Go through each of these]
- separate a leak from an unbounded cache
- steps to take a heap snapshot and the three object types to inspect
- check global containers, listeners, unclosed connections
- verify with a 30-minute load test returning to baseline

[Process]
1. Restate the symptom: action, input, observed, expected
2. Three hypotheses by likelihood, each with a prediction
3. Design the smallest test: one command, one log line, one throwaway test
4. Eliminate or confirm, never change several things at once
5. Explain why the bug could be written, not just which line

[Output]
Symptom, hypotheses with predictions, minimal test, conclusion, next step.

[Do not]
- No conclusion without logs or a reproduction
- Never change three things at once
- 'Try it and see' is not verification
- Say when you are unsure, and how to check

What to state before it starts

Leave these unsaid and the model fills them with an assumption. Most 'wrong answers' start here.

  • the memory curve: stair-step or gradual
  • language and runtime
  • recent changes

Go through each of these

Where this kind of work most often goes wrong. Already in the prompt, so you do not have to remember it.

  • separate a leak from an unbounded cache
  • steps to take a heap snapshot and the three object types to inspect
  • check global containers, listeners, unclosed connections
  • verify with a 30-minute load test returning to baseline