Software & AppsOperating SystemLinux

Fixing “Cannot allocate memory” error on Ubuntu Server

Ubuntu 11

If you’re an Ubuntu Server user, you may have encountered the “Cannot allocate memory” error. This error typically means that your server has run out of memory. There could be several reasons for this, such as a process consuming all the memory or a lack of swap memory. This article will guide you through the steps to resolve this issue and prevent it from reoccurring.

Understanding the Error

The “Cannot allocate memory” error occurs when there isn’t enough memory to perform a specific task. This could be due to a single process using up all the available memory, or several smaller processes collectively consuming all the memory.

How to Fix the Error

Step 1: Reboot Your Server

The quickest way to free up memory is by rebooting your server. You can do this through your client control panel. For instance, if you’re using DigitalOcean, select “Reboot” for your droplet. This action will restart your server and free up any locked memory.

Step 2: Check Memory Usage

After rebooting, SSH into your server and run the top or htop command. These commands will display a list of running processes along with their memory usage. Identify the process that is using up all the memory. If it is a faulty program or process, you can try killing or removing it.

Here’s an example of how to use the top command:


The top command will display a live, scrolling view of the current state of your system, including memory usage.

Step 3: Adjust Configuration

If the problematic program or process is necessary, you can try changing its configuration to prevent it from consuming excessive memory. Consult the program’s documentation or seek online resources for guidance on how to do this.

Preventing the Error

To prevent this issue from happening again in the future, consider the following suggestions:

Add Swap Memory

Allocate additional swap memory to your server. This can help allocate more memory when you are running out. You can refer to the DigitalOcean tutorial on how to add swap memory for Ubuntu 16.04.

Configure Programs Correctly

When installing new programs, ensure that you configure them correctly to avoid unintended memory usage.

Regularly Check Memory Usage

After installing new packages or making changes, use htop or top to monitor your memory usage. If you notice that you are using almost all of it, remove unnecessary programs or processes to free up memory.

Review Auto-started Processes

Check for any processes that are auto-started on your server. If you don’t recognize or want them to be auto-started, remove them. However, always research the process before killing or deleting it to avoid disrupting system functions.

Last Resort: Manually Triggering the OOM Killer

If you are unable to reboot your server or run commands due to the memory error, you can manually trigger the OOM (Out of Memory) killer. This can be done by executing the following commands:

echo 1 > /proc/sys/kernel/sysrq
echo f > /proc/sysrq-trigger
echo 0 > /proc/sys/kernel/sysrq

The first command enables the magic SysRq key, a key combination understood by the Linux kernel which allows the user to perform various low-level commands. The second command triggers the OOM killer, which will kill processes to free up memory. The last command disables the magic SysRq key again.

Please note that this should be used as a last resort and may not always resolve the issue.


Fixing the “Cannot allocate memory” error on Ubuntu Server involves identifying the problematic process and adjusting its configuration or killing it. To prevent this error in the future, consider adding swap memory, configuring programs correctly, regularly checking memory usage, and reviewing auto-started processes. If all else fails, triggering the OOM killer can be a last resort. Always remember to exercise caution when making changes to your server and research any processes or configurations before making modifications.

What is the difference between the “top” and “htop” commands?

The "top" command displays a live, scrolling view of the current state of your system, including memory usage. On the other hand, "htop" provides a more user-friendly and interactive interface with additional features such as sorting processes by various criteria and the ability to kill processes with a single keystroke.

How can I add swap memory to my Ubuntu server?

To add swap memory to your Ubuntu server, you can follow the instructions in the DigitalOcean tutorial on how to add swap space for Ubuntu 16.04. It provides a step-by-step guide on creating and enabling swap space on your server.

Can I manually trigger the OOM killer to free up memory?

Yes, you can manually trigger the OOM (Out of Memory) killer to free up memory on your server. By executing the commands echo 1 > /proc/sys/kernel/sysrq, echo f > /proc/sysrq-trigger, and echo 0 > /proc/sys/kernel/sysrq in the terminal, you enable the magic SysRq key, trigger the OOM killer, and disable the magic SysRq key again. However, this should be used as a last resort and may not always resolve the issue.

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