--- title: "Find the bottleneck" subtitle: "Inside a computer · Specs and speed · Lesson 2" --- # Find the bottleneck **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** An old laptop takes a minute and a half to load a game level. You can upgrade one part. Which? :::teacher-note **In one sentence:** 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). **Students should be able to say:** 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. **Width:** the unmarked parts are the floor every student does (predict, run, investigate and make). Typical adds the modify practice; wider adds the real-world contexts. They add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Predict :::keep-together **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?** ☐ A processor twice as fast ☐ A faster internet connection ☐ Twice the RAM ☐ Swap the hard disk for an SSD My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** The SSD: the hard disk takes 80 of the 90 seconds. A processor twice as fast saves only 5 seconds. ::: ## Run Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Worked example: where the 90 seconds go :::keep-together ### Loading a game level on the old laptop 1. $\text{processor} = 40 \div (2 \times 2) = 10\ \text{s}$ — Work ÷ (cores it can use × GHz). 2. $\text{storage} = 8000\ \text{MB} \div 100\ \text{MB/s} = 80\ \text{s}$ — A hard disk reads about 100 MB a second. 3. $\text{total} = 10 + 80 = 90\ \text{s}$ — RAM is big enough (no swapping) and nothing is downloaded. 4. $\text{bottleneck: storage}$ — 80 s is the biggest part. 5. $\text{SSD: } 8000 \div 2000 = 4\ \text{s, total } 14\ \text{s}$ — Upgrade the bottleneck: 90 s becomes 14 s. Now try a faster processor instead. **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?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 85 Processor 40 ÷ (2 × 4) = 5 s, storage still 80 s: 85 s. Upgrading a part that isn't the bottleneck barely helps. ::: :::keep-together ### Find the mistake **Sam picks an upgrade. Click the line where it first goes wrong.** ``` 1 The old laptop takes 90 seconds to load a game level. 2 The storage takes 80 of those seconds: it is the bottleneck. 3 A 2 TB hard disk is bigger than the 1 TB one, so it will load faster. 4 So Sam buys the 2 TB hard disk. ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** 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. ::: :::keep-together ### Fix the cause **The tabs job swaps to the hard disk because the RAM is too small. Which upgrade fixes the cause?** ☐ A bigger hard disk ☐ More RAM, so nothing needs swapping ☐ A faster network ☐ Closing the laptop lid ::: :::teacher-note **Answer:** 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.) ::: :::typical ## Vary: one change each **More tabs.** **Download.** **AI.** **Not the bottleneck.** Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: ## Name the bottleneck :::keep-together ### Video call **A video call goes blocky and freezes, but the laptop is new and fast. What is the most likely bottleneck?** ☐ The screen ☐ The network ☐ The storage ☐ The processor ::: :::teacher-note **Answer:** The network Live video needs data to keep arriving: the network is the bottleneck. ::: :::keep-together ### Starting up **An old computer with a hard disk takes two minutes to start up. Which upgrade helps most?** ☐ An SSD ☐ A bigger hard disk ☐ A graphics card ☐ A faster network ::: :::teacher-note **Answer:** An SSD Starting up reads gigabytes of programs from storage: an SSD reads them about 20 times faster. ::: :::keep-together ### AI **Why do companies train AI models on graphics cards rather than processors?** ☐ Processors can't do maths ☐ Graphics cards have more storage ☐ AI is millions of small, simple sums, and a graphics card does thousands at once ☐ AI needs a screen ::: :::teacher-note **Answer:** 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. ::: :::keep-together ### Review: specs **Review: which spec empties when the power goes off?** ☐ RAM ☐ Processor speed ☐ Storage ☐ Network ::: :::teacher-note **Answer:** RAM RAM is the working space; storage is what it keeps. ::: ## Make: fix the slow export Then: what is the bottleneck on your own device when it feels slow? Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ## Exit ticket :::keep-together **A laptop with 8 GB of RAM gets slow with many tabs open. Name the bottleneck and the best upgrade, and say why a faster processor won't fix it.** {{lines-3}} ::: :::teacher-note **Answer:** RAM is the bottleneck: it runs out and swaps to storage. More RAM stops the swapping. A faster processor only speeds the part that wasn't slow. :::