One of the easiest way of guarding against out-of-memory errors in applications is to add some swap space to your server.
Warning: Although swap is generally recommended for systems utilizing traditional spinning hard drives, using swap with SSDs can cause issues with hardware degradation over time.
The information written to disk will be significantly slower than information kept in RAM, but the operating system will prefer to keep running application data in memory and use swap for the older data. Overall, having swap space as a fallback for when your system's RAM is depleted can be a good safety net against out-of-memory exceptions on systems with non-SSD storage available.
Before we begin, we can check if the system already has some swap space available. It is possible to have multiple swap files or swap partitions, but generally one should be enough.
We can see if the system has any configured swap by typing: sudo swapon --show
If you don't get back any output, this means your system does not have swap space available currently.
You can verify that there is no active swap using the free utility:
free -h
>Output
> total used free shared buff/cache available
>Mem: 985M 84M 222M 680K 678M 721M
>Swap: 0B 0B 0B
As you can see in the Swap row of the output, no swap is active on the system.
Before we create our swap file, we'll check our current disk usage to make sure we have enough space:
df -h
>Output
>Filesystem Size Used Avail Use% Mounted on
>udev 481M 0 481M 0% /dev
>tmpfs 99M 656K 98M 1% /run
>/dev/vda1 25G 1.4G 23G 6% /
>tmpfs 493M 0 493M 0% /dev/shm
>tmpfs 5.0M 0 5.0M 0% /run/lock
>tmpfs 493M 0 493M 0% /sys/fs/cgroup
>/dev/vda15 105M 3.4M 102M 4% /boot/efi
>tmpfs 99M 0 99M 0% /run/user/1000
The device with / in the Mounted on column is our disk in this case. We have plenty of space available in this example (only 1.4G used). Your usage will probably be different.
Note: Modern machines probably does not need swap space at all (see here).
Now that we know our available hard drive space, we can create a swap file on our filesystem. We will allocate a file of the swap size that we want called swapfile in our root (/) directory.
The best way of creating a swap file is with the fallocate program. This command instantly creates a file of the specified size:
sudo fallocate -l 1G /swapfile
First, we need to lock down the permissions of the file so that only the users with root privileges can read the contents. This prevents normal users from being able to access the file, which would have significant security implications.
Make the file only accessible to root by typing: sudo chmod 600 /swapfile
We can now mark the file as swap space by typing:
sudo mkswap /swapfile
>Output
>Setting up swapspace version 1, size = 1024 MiB (1073737728 bytes)
>no label, UUID=6e965805-2ab9-450f-aed6-577e74089dbf
After marking the file, we can enable the swap file, allowing our system to start utilizing it: sudo swapon /swapfile
Verify that the swap is available by typing:
sudo swapon --show
>Output
>NAME TYPE SIZE USED PRIO
>/swapfile file 1024M 0B -2
We can check the output of the free utility again to corroborate our findings:
free -h
>Output
> total used free shared buff/cache available
>Mem: 985M 84M 220M 680K 680M 722M
>Swap: 1.0G 0B 1.0G
Our recent changes have enabled the swap file for the current session. However, if we reboot, the server will not retain the swap settings automatically. We can change this by adding the swap file to our /etc/fstab file.
Add the swap file information to the end of your /etc/fstab file by typing:
echo '/swapfile none swap sw 0 0' | sudo tee -a /etc/fstab
There are a few options that you can configure that will have an impact on your system's performance when dealing with swap:
-
swappiness, a parameter which configures how often your system swaps data out of RAM to the swap space, expressed as a percentage.We can see the current swappiness value by typing:
cat /proc/sys/vm/swappinessTip: For a Desktop, a swappiness setting of 60 is not a bad value. For a server, you might want to move it closer to 0.
We can set the swappiness to a different value by using the sysctl command.
For instance, to set the swappiness to 10, we could type:
sudo sysctl vm.swappiness=10This setting will persist until the next reboot. We can set this value automatically at restart by adding the line to our /etc/sysctl.conf file:
vm.swappiness=10 -
vfs_cache_pressure, a parameter which configures how much the system will choose to cache inode and dentry information over other data.Basically, this is access data about the filesystem. This is generally very costly to look up and very frequently requested, so it's an excellent thing for your system to cache. You can see the current value by querying the proc filesystem again:
cat /proc/sys/vm/vfs_cache_pressureAs it is currently configured, our system removes inode information from the cache too quickly. We can set this to a more conservative setting like 50 by typing:
sudo sysctl vm.vfs_cache_pressure=50Again, this is only valid for our current session. We can change that by adding it to our configuration file like we did with our swappiness setting:
vm.vfs_cache_pressure=50
To deactivate and remove the swap file, start by deactivating the swap space by typing: sudo swapoff -v /swapfile
Next, remove the swap file entry /swapfile the /etc/fstab file.
Finally, remove the actual swapfile file: sudo rm /swapfile