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Lesson plan 02: Hardware and operating-system costs
Trace a request through CPU caches, memory, the operating system, and persistent storage.
Placement: Part 1: Foundations: understanding work, time, and resources.
Prerequisites: Lesson 01: Workloads and correctness contracts, plus the earlier dependency chain.
Learning outcomes
- Trace a request through CPU caches, memory, the operating system, and persistent storage.
- Calculate copying and storage costs using consistent byte units and distinguish elapsed time from CPU occupancy.
- Identify which failure boundary a memory write, filesystem write, and durable flush cross.
- Separate a general mechanism, a documented implementation choice, and an illustrative numerical assumption.
Teaching sequence
- Establish the motivating situation: A database with ample RAM stalls because random pointer chasing, remote memory, and copying dominate execution.
- Introduce the vocabulary and the correctness obligation before an optimization. Define CPU caches, RAM, NUMA.
- Develop the mechanism: Trace one read from a CPU instruction through address translation and caches; contrast a buffered write with a durable one.
- Follow the complete worked example: Copying a 4 MiB result three times moves 12 MiB before transport overhead. Explain the assumptions, intermediate units, and limiting interpretation.
- Extend the scenario to changed load, skew, or failure, explaining SSDs, page cache, system calls, copying, persistence boundaries.
- Connect the mechanism to a documented implementation and an alternative with a different workload fit. Close by stating the remaining cost and the next lesson dependency.
Required chapter material
Use a complete execution trace or worked calculation, supported by an annotated static diagram or table. Include language-neutral pseudocode or labeled SQL when it clarifies the mechanism. State preconditions, ownership, visibility, and failure boundaries for algorithms. Describe operational costs as well as the normal path. All questions raised in the narrative must be resolved in the text.
Visual explanation
A memory-and-storage hierarchy labels relative cost categories and a write timeline separates application return, kernel acceptance, and durable persistence. The accompanying prose explains the arrows, state changes, or table entries so the lesson remains understandable in a plain Markdown reader.
Editorial checks
Confirm that the numerical example agrees with its units and assumptions. Check that a faster path preserves the stated contract, or explicitly identify the changed guarantee. Keep hypothetical figures separate from product measurements. Explain both an appropriate use and a limitation of the case design. Check terminology against the glossary and links against the course index.