sendfile and tcp_nopush in Nginx - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the sendfile directive do in nginx?

The sendfile directive enables nginx to use the operating system's sendfile() system call to transfer files directly from disk to the network socket, improving performance by avoiding copying data between user space and kernel space.

beginner

What is the purpose of the tcp_nopush directive in nginx?

The tcp_nopush directive controls how nginx sends TCP packets. When enabled, it delays sending partial packets to send larger packets at once, reducing overhead and improving network efficiency.

intermediate

How do sendfile and tcp_nopush work together in nginx?

When sendfile is enabled, tcp_nopush helps by delaying packet sending until a full packet is ready. This combination improves file transfer speed and reduces CPU usage.

beginner

How do you enable sendfile and tcp_nopush in nginx configuration?

In the nginx configuration file, add these lines inside the http block:

sendfile on;
tcp_nopush on;

beginner

What is a real-life analogy for sendfile and tcp_nopush working together?

Imagine sending a big package by mail. sendfile is like using a direct conveyor belt to move the package without extra handling. tcp_nopush is like waiting to fill the box completely before sealing it, so fewer boxes are sent and shipping is efficient.

What does enabling sendfile in nginx do?

A Caches files in memory

B Compresses files before sending

C Encrypts network traffic

D Transfers files directly from disk to network socket

Sendfile uses the OS system call to send files directly, improving performance.

What is the effect of enabling tcp_nopush in nginx?

A Forces immediate sending of all packets

B Delays sending partial TCP packets to send larger packets

C Disables TCP connections

D Splits packets into smaller pieces

Tcp_nopush delays sending to optimize packet size and reduce overhead.

Which nginx block typically contains sendfile on; and tcp_nopush on; directives?

A http

B server

C location

D events

These directives are usually set in the http block for global effect.

What is a benefit of using sendfile with tcp_nopush?

A Better logging of requests

B Increased file encryption

C Improved file transfer speed and reduced CPU usage

D Automatic file compression

They work together to optimize file sending and reduce CPU load.

If sendfile is off, what happens?

A Files are copied through user space before sending

B Files are sent directly from disk

C Network connections are closed

D Files are compressed automatically

Without sendfile, data moves through user space, which is slower.

Explain how sendfile and tcp_nopush improve nginx performance when serving static files.

Think about how data moves from disk to network and how packets are sent.

Describe how to enable sendfile and tcp_nopush in nginx and where to place these directives.

Consider nginx configuration structure and applying global settings.

Practice

1. What is the main purpose of the sendfile directive in nginx?

easy

A. To compress files before sending them

B. To speed up file sending by avoiding extra copying between kernel and user space

C. To encrypt files during transfer

D. To limit the number of simultaneous connections

Solution

1. Step 1: Understand what sendfile does

   The sendfile directive allows nginx to send files directly from disk to the network socket without copying data to user space, which speeds up file sending.

2. Step 2: Compare options with this behavior

   Options B, C, and D describe unrelated features like compression, encryption, or connection limits, which are not the purpose of sendfile.

3. Final Answer:

   To speed up file sending by avoiding extra copying between kernel and user space -> Option B

4. Quick Check:

   sendfile = avoid extra copying [OK]

Hint: Remember: sendfile skips copying to user space [OK]

Common Mistakes:

• Thinking sendfile compresses data
• Confusing sendfile with encryption
• Assuming sendfile limits connections

2. Which of the following is the correct syntax to enable tcp_nopush in nginx configuration?

easy

A. tcp_nopush enable;

B. tcp_nopush = on;

C. tcp_nopush true;

D. tcp_nopush on;

Solution

1. Step 1: Recall nginx directive syntax

   In nginx, directives are set with the format directive value;. The correct value for enabling tcp_nopush is on.

2. Step 2: Check each option's syntax

   tcp_nopush on; uses tcp_nopush on; which is correct. Options A, C, and D use invalid keywords or syntax not supported by nginx.

3. Final Answer:

   tcp_nopush on; -> Option D

4. Quick Check:

   Enable directives with on; in nginx [OK]

Hint: Use 'on;' to enable nginx directives like tcp_nopush [OK]

Common Mistakes:

• Using 'enable' instead of 'on'
• Adding '=' sign in directive
• Using 'true' instead of 'on'

3. Given this nginx snippet:

sendfile on;
tcp_nopush on;

What is the expected effect on static file delivery?

medium

A. Files are sent faster by sending larger TCP packets

B. Files are sent slower due to extra copying

C. Files are compressed before sending

D. Files are sent with encryption

Solution

1. Step 1: Understand combined effect of sendfile and tcp_nopush

   sendfile on; avoids extra copying, and tcp_nopush on; allows grouping data into bigger TCP packets, improving speed.

2. Step 2: Analyze options

   Files are sent faster by sending larger TCP packets correctly states files are sent faster by sending larger TCP packets. Options B, C, and D describe incorrect effects.

3. Final Answer:

   Files are sent faster by sending larger TCP packets -> Option A

4. Quick Check:

   sendfile + tcp_nopush = faster large packets [OK]

Hint: sendfile + tcp_nopush = faster static file delivery [OK]

Common Mistakes:

• Thinking tcp_nopush compresses data
• Assuming sendfile slows down transfer
• Confusing tcp_nopush with encryption

4. You enabled sendfile on; but forgot to enable tcp_nopush. What issue might you observe?

medium

A. nginx configuration syntax error

B. Files fail to send completely

C. Small TCP packets causing inefficient network usage

D. Files get corrupted during transfer

Solution

1. Step 1: Understand the role of tcp_nopush with sendfile

   tcp_nopush helps group data into larger TCP packets. Without it, sendfile sends smaller packets inefficiently.

2. Step 2: Analyze consequences of missing tcp_nopush

   Small TCP packets causing inefficient network usage correctly identifies small TCP packets causing inefficient network usage. Options A, B, and D describe errors or corruption which do not happen just by missing tcp_nopush.

3. Final Answer:

   Small TCP packets causing inefficient network usage -> Option C

4. Quick Check:

   Missing tcp_nopush = small packets [OK]

Hint: Without tcp_nopush, packets stay small and inefficient [OK]

Common Mistakes:

• Expecting syntax errors without tcp_nopush
• Thinking files get corrupted
• Assuming files fail to send

5. You want to optimize nginx for serving large static files efficiently. Which configuration snippet correctly uses sendfile and tcp_nopush together?

hard

A. sendfile on;
tcp_nopush on;

B. sendfile on;
tcp_nopush off;

C. sendfile off;
tcp_nopush off;

D. sendfile off;
tcp_nopush on;

Solution

1. Step 1: Identify best practice for static file optimization

   To optimize static file delivery, sendfile should be enabled to avoid extra copying, and tcp_nopush enabled to send larger TCP packets.

2. Step 2: Evaluate each configuration snippet

   sendfile on; tcp_nopush on; enables both sendfile and tcp_nopush, which is the correct combination. Options A, B, and C disable one or both, reducing efficiency.

3. Final Answer:

   sendfile on; tcp_nopush on; -> Option A

4. Quick Check:

   Enable both for best static file speed [OK]

Hint: Enable both sendfile and tcp_nopush for best speed [OK]

Common Mistakes:

• Disabling sendfile but enabling tcp_nopush
• Enabling sendfile but disabling tcp_nopush
• Disabling both directives