Memory swapping in linux

Swap

This page provides an introduction to swap space and paging on GNU/Linux. It covers creation and activation of swap partitions and swap files.

Linux divides its physical RAM (random access memory) into chunks of memory called pages. Swapping is the process whereby a page of memory is copied to the preconfigured space on the hard disk, called swap space, to free up that page of memory. The combined sizes of the physical memory and the swap space is the amount of virtual memory available.

Support for swap is provided by the Linux kernel and user-space utilities from the util-linux package.

Swap space

Swap space can take the form of a disk partition or a file. Users may create a swap space during installation or at any later time as desired. Swap space can be used for two purposes, to extend the virtual memory beyond the installed physical memory (RAM), and also for suspend-to-disk support.

If it is beneficial to extend the virtual memory with swap depends on the amount of installed physical memory. If the amount of physical memory is less than the amount of memory required to run all the desired programs, then it may be beneficial to enable swap. This avoids out of memory conditions, where the Linux kernel OOM killer mechanism will automatically attempt to free up memory by killing processes. To increase the amount of virtual memory to the required amount, add the necessary difference (or more) as swap space.

The biggest drawback of enabling swap is its lower performance, see section #Performance. Hence, enabling swap is a matter of personal preference: some prefer programs to be killed over enabling swap and others prefer enabling swap and slower system when the physical memory is exhausted.

The factual accuracy of this article or section is disputed.

Reason: Linus Torvalds says that swap partitions are preferable: «all the normal distributions set things up with swap partitions, not files, because honestly, swapfiles tend to be slower and have various other complexity issues.» (Discuss in Talk:Swap)

Or to show physical memory as well as swap usage:

Swap partition

A swap partition can be created with most GNU/Linux partitioning tools. Swap partitions are designated as type 82 on MBR and 0657FD6D-A4AB-43C4-84E5-0933C84B4F4F on GPT.

To set up a partition as Linux swap area, the mkswap(8) command is used. For example:

To enable the device for paging:

To enable this swap partition on boot, add an entry to /etc/fstab :

UUID=device_UUID none swap defaults 0 0

where the device_UUID is the UUID of the swap space.

Note: The fstab-entry is optional if the swap partition is located on a device using GPT. See #Activation by systemd.

Warning: Enabling discard on RAID setups using mdadm will cause system lockup on boot and during runtime, if using swapon.

Activation by systemd

systemd activates swap partitions based on two different mechanisms. Both are executables in /usr/lib/systemd/system-generators . The generators are run on start-up and create native systemd units for mounts. The first, systemd-fstab-generator , reads the fstab to generate units, including a unit for swap. The second, systemd-gpt-auto-generator inspects the root disk to generate units. It operates on GPT disks only, and can identify swap partitions by their type GUID, see systemd#GPT partition automounting for more information.

Disabling swap

To deactivate specific swap space:

Alternatively use the -a switch to deactivate all swap space.

Since swap is managed by systemd, it will be activated again on the next system startup. To disable the automatic activation of detected swap space permanently, run systemctl —type swap to find the responsible .swap unit and mask it.

Swap file

As an alternative to creating an entire partition, a swap file offers the ability to vary its size on-the-fly, and is more easily removed altogether. This may be especially desirable if disk space is at a premium (e.g. a modestly-sized SSD).

Swap file creation

Use dd to create a swap file the size of your choosing. For example, creating an 8 GiB swap file:

# dd if=/dev/zero of=/swapfile bs=1M count=8k status=progress

Note: Using dd to allocate a swap file is the most portable solution, see swapon(8) § Files with holes for details.

Set the right permissions (a world-readable swap file is a huge local vulnerability):

After creating the correctly sized file, format it to swap:

Finally, edit the fstab configuration to add an entry for the swap file:

/swapfile none swap defaults 0 0

For additional information, see fstab#Usage.

Note: The swap file must be specified by its location on the file system, not by its UUID or LABEL.

Swap file removal

To remove a swap file, it must be turned off first and then can be removed:

# swapoff /swapfile # rm -f /swapfile

Finally, remove the relevant entry from /etc/fstab .

Compressed block device in RAM

zswap is available by default if you use a swap file or partition, but you can avoid swap files or partitions altogether by using a compressed block device in RAM, using zram. See Improving performance#zram or zswap for more information on the differences between using zram or zswap.

Swap encryption

Performance

Swap operations are usually significantly slower than directly accessing data in RAM. Disabling swap entirely to improve performance can sometimes lead to a degradation, since it decreases the memory available for virtual file system (VFS) caches, causing more frequent and costly disk usage.

Swap values can be adjusted to help performance:

Swappiness

When memory usage reaches a certain threshold, the kernel starts looking at active memory and seeing what it can free up. File data can be written out to the file system (if changed), unloaded and re-loaded later; other data must be written to swap before it can be unloaded.

Note: There is a common misconception that swappiness affects the memory threshold or prevents using swap space, but it only affects the preference for freeing up file pages over swap. See: this article for a more detailed explanation or the the kernel source code where it is used.

To check the current swappiness value:

Alternatively, the files /sys/fs/cgroup/memory/memory.swappiness (cgroup v1-specific) or /proc/sys/vm/swappiness can be read in order to obtain the raw integer value.

To temporarily set the swappiness value:

To set the swappiness value permanently, create a sysctl.d(5) configuration file. For example:

/etc/sysctl.d/99-swappiness.conf

To have the boot loader set swappiness when loading the kernel, add a kernel parameter, e.g. sysctl.vm.swappiness=10 .

To test and more on why this may work, take a look at this article. For a more recent counter-argument, see this article.

VFS cache pressure

Another sysctl parameter that affects swap performance is vm.vfs_cache_pressure , which controls the tendency of the kernel to reclaim the memory which is used for caching of VFS caches, versus pagecache and swap. Increasing this value increases the rate at which VFS caches are reclaimed[1]. For more information, see the Linux kernel documentation.

Priority

If you have more than one swap file or swap partition you should consider assigning a priority value (0 to 32767) for each swap area. The system will use swap areas of higher priority before using swap areas of lower priority. For example, if you have a faster disk ( /dev/sda ) and a slower disk ( /dev/sdb ), assign a higher priority to the swap area located on the fastest device. Priorities can be assigned in fstab via the pri parameter:

/dev/sda1 none swap defaults,pri=100 0 0 /dev/sdb2 none swap defaults,pri=10 0 0

Or via the —priority parameter of swapon:

# swapon --priority 100 /dev/sda1

If two or more areas have the same priority, and it is the highest priority available, pages are allocated on a round-robin basis between them.

Striping

There is no necessity to use RAID for swap performance reasons. The kernel itself can stripe swapping on several devices, if you just give them the same priority in the /etc/fstab file. Refer to The Software-RAID HOWTO for details.

See also

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What is Swap Memory in Linux?

For every operating system, there is a dedicated amount of RAM available that makes the processing of a program possible. However, the amount of this RAM is limited which is why RAM cannot hold a bulk of data in it. Therefore, there should be a backup option available which can support RAM whenever it runs out of memory.

This concept holds for the Windows operating system as well as for Linux. In Windows OS, whenever RAM has an insufficient amount of memory to hold a process, it borrows some amount of memory from the secondary storage. This borrowed memory is known as Virtual Memory. Similarly, whenever RAM runs out of memory in Linux, it borrows some memory from the secondary storage to store its inactive content.

In this way, the RAM finds sufficient space to hold a new process within it. Here, the borrowed space from the hard disk is called Swap Memory. In this article, we will try to learn the concept of swap memory in detail.

Working of Swap Memory:

As explained above, swap memory is the dedicated amount of hard drive that is used whenever RAM runs out of memory. There is a memory management program in Linux that takes care of this process. Whenever RAM is short of memory, the memory management program looks for all those inactive blocks of data present in RAM that have not been used for a long time.

When it successfully finds those blocks, it shifts them into the swap memory. In this way, the space of RAM is freed up and hence it can be utilized for some other programs that need processing on an urgent basis. The concept of swapping is very much similar to the concept of paging used in the Windows operating system.

Types of Swap Memory:

Typically there are two different types of swap memory which are mentioned below:

• Swap Partition- This is the default type of swap memory which is in fact, a hard drive partition that is dedicated to swapping.
• Swap File- This is a self-created type of swap memory. Whenever there is no sufficient amount of space left in the hard drive to create a swap partition, a swap file is manually created for swapping the inactive contents of RAM into it.

What should be the Ideal Frequency of Swapping?

Linux allows us to set the frequency of swapping on our own i.e. how frequently the process of swapping should take place. You can set the value of swapping between 0 and 100 depending upon your requirements. A low-frequency value of swapping means that the process of swapping will take place very rarely only when it is needed whereas a high-frequency value of swapping means that the swapping process will occur quite often. However, the default and recommended value of swapping frequency is 60.

Benefits of using Swap Memory:

By learning the working of swap memory, we can easily perceive the benefits of using it. However, some of the main benefits of using swap memory are listed below:

• It can easily hold those inactive blocks of RAM that are hardly used once or twice and then they are never used. The freed up RAM can then be used to hold more programs that have a higher priority.
• It prevents RAM from running out of space.
• It acts as a backup for enhancing the actual space of RAM.
• It allows you to run heavy applications more conveniently that require a large amount of RAM.
• During the process of hibernation, all the contents of RAM are written on the swap memory. Therefore, it is essentially required for the hibernation process to take place successfully.
• It enhances the overall performance of your system.

Conclusion:

In this article, we have learned the usage and working swap memory along with its numerous benefits. Swap memory acts as a backup option for RAM when it runs short of space. We all know that we cannot have an infinite amount of RAM however; we do realize that today’s high-end applications require a large amount of RAM to operate smoothly. Therefore, we must have a sufficient amount of RAM to avoid our applications from crashing.

Also, there is a cost associated with adding in more RAM whereas there is no cost of using swap memory. Moreover, additional RAM can also be plugged in up to a certain limit depending upon your hardware. Hence, the only option we are left with is to use swap memory which can make our system work very efficiently without any cost.

About the author

Karim Buzdar

Karim Buzdar holds a degree in telecommunication engineering and holds several sysadmin certifications. As an IT engineer and technical author, he writes for various web sites. He blogs at LinuxWays.

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