Most of the content comes from this article.

Scenario

Have you ever experienced out of memory error on a Linux server? Or, have you been bothered by the freeze that conda install caused? Well, the problem probably due to insufficient memory. But, how? Linux should have a good memory management. 😫 So, let’s have a look at our memory.

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free -h
           total   used    free    shared  buff/cache   available
Mem:       3.3Gi   632Mi   2.1Gi   2.0Mi       665Mi       2.5Gi
Swap:      0B      0B      0B

Why there is no swap?😵‍💫 I don’t know the reason, but maybe it is not configured by default. So in this post, I’m going to introduce how to configure swap file for a Linux server.

1. Configure Swap Space

1.1 Checking Configuration

Before we start, we can have a brief look at our existing configurations. For example, we can see if the system has any configured swap by typing this.

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sudo swapon --show

No output means that you don’t yet have any swap space. And you can use free command to confirm that, like we used just now.

Then, to add a swap file, we may need to check our disk usage to make sure we have enough space.

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df -h
Filesystem      Size  Used Avail Use% Mounted on
tmpfs           341M 1012K  340M   1% /run
/dev/vda2        59G   36G   21G  64% /
tmpfs           1.7G   24K  1.7G   1% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs           340M     0  340M   0% /run/user/1000

The device with / in the Mounted on column is our disk in this case, and we still have plenty of space (21G) available.

1.2 Creating a Swap File

Now that we know the absence of our swap file, and how much disk space we have, we can start to create one now.

The best way of creating a swap file is with the fallocate program. This command instantly creates a file of the specified size.

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sudo fallocate -l 6G /swapfile

How much should be the swap size? For more information, you can refer to this article.

To ensure that we have created the swap file, we can verify it simply by ls command.

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ls -lh /swapfile
-rw-r--r-- 1 root root 6.0G Aug 29 09:36 /swapfile

If you care about accessibility, you can make it only accessible by root.

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sudo chmod 600 /swapfile
ls -lh /swapfile
-rw------- 1 root root 6.0G Aug 29 09:36 /swapfile

A little tip here, you can use -h parameter to show space size in automatic units rather than huge numbers in Byte.

After that, we have to mark it as swap by mkswap command.

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sudo mkswap /swapfile
Setting up swapspace version 1, size = 6 GiB (6442446848 bytes)
no label, UUID=6175c84f-76b6-4252-a318-b50ae039e2a2

Now, we can enable the swap file. And we can see the properties by --show parameter, instead of nothing at the very beginning.

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sudo swapon /swapfile
sudo swapon --show
NAME      TYPE SIZE USED PRIO
/swapfile file   6G   0B   -2

1.3 Marking the Swap File Permanent

Although we enabled swap file, the changes are only for the current session, and will be lost if we reboot. So we can make it permanent by add it to /etc/fstab file.

It is recommended to back up /etc/fstab in case anything goes wrong.

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sudo cp /etc/fstab /etc/fstab.bak

We can add our new swap file information at the end of /etc/fstab file simply by this.

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echo '/swapfile none swap sw 0 0' | sudo tee -a /etc/fstab

1.4 Tuning Swap Settings

There are a few options that you can configure that will have an impact on your system’s performance when dealing with swap.

1.4.1 Swappiness

The swappiness parameter configures how often your system swaps data out of RAM to the swap space. This is a value between 0 and 100 that represents a percentage. With values close to zero, the kernel will not swap data to the disk unless absolutely necessary. On the contrary, values that are closer to 100 will try to put more data into swap in an effort to keep more RAM space free.

We can see our current swappiness value by this command. You may see “60” as a default value.

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cat /proc/sys/vm/swappiness
60

For server, this value might better be close to zero. We can change this value by this command.

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sudo sysctl vm.swappiness=10
vm.swappiness = 10

Still, this will persist only during the current session. You can add it to /etc/sysctl.conf to make it persistent.

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sudo vim /etc/sysctl.conf

Then, add this line at the bottom, or change the value is entry already exists.

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vm.swappiness=10

1.4.2 Cache Pressure

Another related value that you might want to modify is the vfs_cache_pressure. This setting configures how much the system will choose to cache inode and dentry information over other data. And by default it is 100, which removes cache too quickly.

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cat /proc/sys/vm/vfs_cache_pressure
100

We can set this to a more conservative setting like 50, and the method is similar to how we configure swapiness.

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sudo sysctl vm.vfs_cache_pressure=50
vm.vfs_cache_pressure=50

Also, we can make it permanent by adding an extra entry in /etc/sysctl.conf.

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vm.vfs_cache_pressure=50

Epilogue

Now that we have configured swap space for our Linux server, we won’t worry about memory problem any more. 😁

For example, in the case below, used swap space is more than free memory we have, which would likely to cause memory error before. But now, it won’t! 🥳

image-20230829115505920