Forking workers per CPU core in Node.js - Step-by-Step Execution

Concept Flow - Forking workers per CPU core

Start Master Process

↓

Get CPU Core Count

↓

For each CPU core

↓

Fork a Worker Process

↓

Workers run tasks in parallel

↓

Master listens for worker exit

↓

If worker exits, fork a new one

↓

End

The master process counts CPU cores, forks one worker per core, and restarts workers if they exit.

Execution Sample

Node.js

import cluster from 'node:cluster';
import os from 'node:os';

if (cluster.isPrimary) {
  const cpuCount = os.cpus().length;
  for (let i = 0; i < cpuCount; i++) {
    cluster.fork();
  }
}

This code forks one worker process per CPU core from the master process.

Execution Table

Step	Action	Value/Result	Notes
1	Check if process is master	cluster.isPrimary = true	Master process starts
2	Get CPU core count	cpuCount = 4	Assuming 4 cores on machine
3	Start loop i=0	i=0	First iteration
4	Fork worker	Worker 1 created	Worker process 1 starts
5	Increment i	i=1	Next iteration
6	Fork worker	Worker 2 created	Worker process 2 starts
7	Increment i	i=2	Next iteration
8	Fork worker	Worker 3 created	Worker process 3 starts
9	Increment i	i=3	Next iteration
10	Fork worker	Worker 4 created	Worker process 4 starts
11	Increment i	i=4	Loop ends, i == cpuCount
12	Master listens for worker exit	If worker exits, fork new worker	Ensures continuous availability
13	Workers run tasks	Parallel processing	Workers handle workload independently
14	Exit	All workers running	Master process waits

💡 Loop ends when i equals cpuCount; all workers forked.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
i	undefined	0	1	2	3	4
cpuCount	undefined	4	4	4	4	4
cluster.isPrimary	undefined	true	true	true	true	true

Key Moments - 3 Insights

Why do we check if cluster.isPrimary before forking?

Because only the master process should fork workers. Workers should not fork more processes. See execution_table step 1.

What happens if we don't fork one worker per CPU core?

We might not fully use all CPU cores, leading to less efficient processing. The loop in steps 3-11 ensures one worker per core.

Why does the master listen for worker exit and fork new ones?

To keep the system running smoothly by replacing any worker that crashes or exits unexpectedly, as shown in step 12.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'i' when the third worker is forked?

A 2

B 3

C 1

D 4

💡 Hint

Check steps 7 and 8 where i increments and the third worker is created.

At which step does the loop end because i equals cpuCount?

A Step 10

B Step 11

C Step 12

D Step 9

💡 Hint

Look for the step where i becomes 4 and the loop stops.

If the machine has 8 CPU cores, how many times will the loop run?

A 4

B 6

C 8

D 1

💡 Hint

Refer to variable_tracker for cpuCount and loop iterations.

Concept Snapshot

Forking workers per CPU core in Node.js:
- Use cluster.isPrimary to check master process
- Get CPU count with os.cpus().length
- Loop from 0 to cpuCount-1
- Fork a worker each iteration
- Master listens for worker exit to restart
- This uses all CPU cores for parallel work

Full Transcript

In Node.js, to use all CPU cores, the master process first checks if it is the primary process using cluster.isPrimary. It then gets the number of CPU cores with os.cpus().length. A loop runs from zero up to one less than the CPU count, and in each loop iteration, the master forks a new worker process. Each worker runs independently to handle tasks in parallel. The master also listens for any worker exiting and forks a new one to keep the system running smoothly. This approach maximizes CPU usage by having one worker per core.