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KERN_BUF_SIZE.MD

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KERN_BUF_SIZE.MD

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How to Tune Kernel Network Buffer Sizes (rmem_max & wmem_max)

This tutorial explains how to view and increase the maximum network socket send (TX) and receive (RX) buffer sizes in Linux.

  • net.core.wmem_max: The maximum send (write) buffer size, in bytes, that a single socket can allocate. This is for TX (Transmission).

  • net.core.rmem_max: The maximum receive (read) buffer size, in bytes, that a single socket can allocate. This is for RX (Reception).

Tuning these values can sometimes improve network performance, especially on high-speed (e.g., 10Gb+) or high-latency (long-distance) networks. It allows the kernel to buffer more data for a connection, which can help saturate the link.

Warning: Read This First!

Changing kernel parameters can affect system stability and performance. For most standard desktop or server workloads (e.g., web browsing, local file sharing, simple web hosting), the default values are sufficient.

Only change these values if you are trying to solve a specific, measurable network performance problem (like packet loss or an inability to reach full bandwidth on a high-speed, high-latency link) and you understand the risk. Increasing buffers too much can consume significant kernel memory and potentially increase latency (a problem known as "bufferbloat").

1. Check Your Current Values

Before making changes, check your system's current default values. The values are in bytes.

# Check current max receive buffer size
sysctl net.core.rmem_max

# Check current max send buffer size
sysctl net.core.wmem_max

You will likely see an output like net.core.rmem_max = 212992, which is 208KB.

2. How to Change Values Temporarily (for Testing)

This method is for testing only. These changes will be lost after you reboot.

We will use sysctl -w to "write" a new value. You must run these commands as root or with sudo.

Let's say you want to test a new value of 16MB. (16MB = 16 * 1024 * 1024 = 16777216 bytes)

Example: Set max receive buffer to 16MB
sudo sysctl -w net.core.rmem_max=16777216

Example: Set max send buffer to 16MB
sudo sysctl -w net.core.wmem_max=16777216

You can immediately verify the change by running the check commands from Step 1 again. Now is the time to run your network performance tests (e.g., with iperf3) to see if this change actually helped.

3. How to Make Changes Permanent (Survive Reboot)

Once you have tested your values and are confident you want to keep them, you must add them to a sysctl configuration file.

The best practice is to create a new, custom configuration file in the /etc/sysctl.d/ directory.

a. Create a new configuration file

Use a text editor like nano or vim to create a file. We'll call it 90-network-buffers.conf (the 90- prefix ensures it's loaded late in the boot sequence, overriding defaults).

sudo nano /etc/sysctl.d/90-network-buffers.conf

b. Add your settings to this file

Add the following lines to the file. This is a configuration file, so do not use sudo or sysctl -w inside the file. Use the key = value format.

Custom network buffer settings:

Set max RX buffer to 16MB
net.core.rmem_max = 16777216

Set max TX buffer to 16MB
net.core.wmem_max = 16777216

Save the file and exit the text editor.

c. Apply the changes immediately

The system will read this file automatically on the next reboot. To apply the settings now without rebooting, run the following command:

Load all sysctl configuration files, including your new one
sudo sysctl --system

This command will parse all files in /etc/sysctl.d/ and /etc/sysctl.conf and apply the settings. Your changes are now active and will persist after a reboot. You can verify again using sysctl net.core.rmem_max.

How to Revert Your Changes

If your changes cause problems, you can easily revert them.

  1. To revert temporary changes: Simply reboot your machine.

  2. To revert permanent changes:

    • Delete the file you created: sudo rm /etc/sysctl.d/90-network-buffers.conf

    • ...or, edit the file (sudo nano /etc/sysctl.d/90-network-buffers.conf) and put a # at the beginning of each line to comment it out.

    • After deleting or commenting, run sudo sysctl --system again to re-load the default values.