Find the bottleneck

Students run jobs on an old laptop, see how long each part takes, find each job's bottleneck and upgrade only that part (AC9TDI8K01 node 2).

Inside a computer · Specs and speed · about 40 minutes · Open the student lesson

Classroom resources

Load these into Waypoints Prepare: choose Load URL and paste the copied address, or download the file and import it. In Prepare, the worksheet's Teacher edition shows the answers; the student copy hides them.

Slides Download Present from the browser, or export to PowerPoint or PDF.
Worksheet Download Print the student copy; the Teacher edition has the answers.

Students will be able to

I can explain how a spec affects how fast a job runs.
I can find the bottleneck for a job.
I can say which upgrade will help, and why others won't.

Timing

Predict and Try 7 min · Worked example 10 min · Vary 12 min · Practise and Make 11 min

Before class

Try the boards yourself.

The one idea: the slowest part decides. This is the hinge for lesson 3: without it, choosing hardware becomes picking the biggest numbers.

The model is simplified on purpose and says so: times add up part by part, the work amounts are made up but in proportion, a hard disk reads about 100 MB/s and an SSD about 2000 MB/s, and when RAM is too small the computer swaps to storage. Real computers overlap these steps and use caches, so real timings are messier; the idea that the slowest part dominates holds.

Hands-on: open Task Manager (Windows) or Activity Monitor (macOS) while a big download or video export runs, and look at which graph is at the top.

Running the lesson

Predict: students who chose the processor will meet the 85 s answer in the worked example.
Try: ask what is different about the three red bars.
Worked example: before the SSD line, ask which part an upgrade should target.
Vary: after task 4, ask why 32 cores didn't help.
Make: ask who used 16 cores and whether that was better than 8 (the export can only use 8).

The hard part (on purpose)

Students reach for "a faster processor" for everything. Let them try it on the download (Vary task 4) and see almost nothing change, rather than telling them.

When students get stuck

What you will see	A question to ask	The fix
A student buys a bigger hard disk to speed it up (Sam's mistake).	Does a bigger pantry make the chef walk faster?	Size is how much; SSD versus HDD is how fast.
A student upgrades everything.	Which bar got shorter, and which ones didn't change?	Only the bottleneck's upgrade made a real difference.

Answers to the checks

one upgrade: An old laptop (2 cores, 2.0 GHz, 8 GB RAM, 1 TB hard disk) takes 90 seconds to load a game level. Which one upgrade helps most?
The SSD: the hard disk takes 80 of the 90 seconds. A processor twice as fast saves only 5 seconds.
Try: run three jobs: Run all three jobs on the old laptop. Which part is the slowest for each one?

Tabs: RAM (it swaps to the hard disk). Game: storage. Download: network. Same laptop, three different bottlenecks.
Loading a game level on the old laptop: Same old laptop, but with a processor twice as fast (4 GHz) and still the hard disk. How many seconds does the game level take now?
85
Processor 40 ÷ (2 × 4) = 5 s, storage still 80 s: 85 s. Upgrading a part that isn't the bottleneck barely helps.
Find the mistake: Sam picks an upgrade. Click the line where it first goes wrong.
line 3
Line 3: size is how much it holds, not how fast it reads. A 2 TB hard disk still reads at hard disk speed; an SSD is about 20 times faster.
Fix the cause: The tabs job swaps to the hard disk because the RAM is too small. Which upgrade fixes the cause?
More RAM, so nothing needs swapping
With enough RAM, nothing is swapped: the RAM time goes to zero. (An SSD makes swapping faster but doesn't stop it.)
More tabs: Switching tabs takes 86 s. Change one part to get it under 5 seconds, then run it.

More RAM: nothing swaps, so the RAM bar disappears. (An SSD only gets it to about 7 s: it makes swapping faster but doesn't stop it.)
Download: Now the download. Change one part to get it under 200 seconds.

A faster network. 5 GB is 40,000 megabits: at 50 Mb/s that is 800 s, at 300 Mb/s about 133 s.
AI: Train the small AI model. Get it under 60 seconds.

A graphics card does thousands of small sums at once. Even 32 cores at 4 GHz would take over 2 minutes.
Not the bottleneck: Non-example: give the old laptop 32 cores at 4.0 GHz and run the download again. Is it much faster?

Hardly: the processor was never the bottleneck for a download. The network was.
Video call: A video call goes blocky and freezes, but the laptop is new and fast. What is the most likely bottleneck?
The network
Live video needs data to keep arriving: the network is the bottleneck.
Starting up: An old computer with a hard disk takes two minutes to start up. Which upgrade helps most?
An SSD
Starting up reads gigabytes of programs from storage: an SSD reads them about 20 times faster.
AI: Why do companies train AI models on graphics cards rather than processors?
AI is millions of small, simple sums, and a graphics card does thousands at once
Many simple sums at once is exactly what a graphics card is built for.
Review: specs: Review: which spec empties when the power goes off?
RAM
RAM is the working space; storage is what it keeps.
fix the slow export: This laptop takes 103 s to export the video. Get it under 60 seconds by changing only one spec.

8 cores: the export uses all of them, so the processor time halves to 50 s. Faster GHz alone only gets to 78 s; more RAM or a bigger SSD saves nothing.