Difference between revisions of "Swap"

Swap space is used to extend the amount of memory(RAM) available for running programs.  Without swap space, if you run out of memory applications will fail or be terminated.  With available swap space, Linux can take less frequently accessed memory pages and write them to disk, allowing more space for running applications.

There are two different ways in which swap can be managed in Manjaro

This article is not a comprehensive guide to swap.  It is intended to serve as a basic primer.  More detailed information can be found in the [[#See_Also|articles linked below]]

In most circumstances swap is not absolutely required unless you are running out of memory(RAM).  However, even with plenty of available memory, it is often used as a safety net or due to specific application requirements.

 

The amount of swap you need is highly variable based on the specific applications and workload.  There is no way to provide a universal on swap size recommendation without monitoring usage over a period of time.  A reasonable place to start would be the smaller of your total system memory(RAM) or 8GiB.

If you plan to enable hibernation it is recommended to have at least as much swap as you have RAM.

  NAME      TYPE      SIZE USED PRIO

  /dev/sda3 partition 8.4G  0B   -2

A swap partition is the traditional way of managing swap.  In this scenario, a dedicated partition or partitions are created for holding swap.

 

To create a swap partition, you first need an available partition. This can be created in any disk management tool and should be set as type Linux Swap.

 

 

Using a swap partition has one major disadvantage. Changing the size of swap or adding swap requires repartitioning the disk.  In current Linux kernels, it is possible to use a swap file instead of a dedicated partition.

As of kernel 5.0, swapfiles are supported on btfrs.  They still require some special handling in addition to the above steps.

zswap is compressed swap kept in memory(RAM). It keeps the most frequently used pages in RAM and writes less frequently used pages to the swap space on disk.  In many workloads, this will result in increased swap performance.

 

Although swap seems like a great way to expand memory, excessive swap use will cause severe performance degradation.

 

 

 

{{ic|vfs_cache_pressure}} is a percentage value that controls the tendency of the kernel to reclaim the memory which is used for caching of directory and inode objects.  The default value is 100.  Increasing this value will increase the rate in which these objects are removed from the RAM cache.  Decreasing it will allow these objects to be cached in memory longer, consuming additional RAM over time.  Depending on your specific workload, increasing or decreasing this value too far can have significant negative impacts on system performance.  Experimentation is needed to find the appropriate balance and the default value is reasonable.  In general, it is more common to optimize swappiness before experimenting with vfs_cache_pressure.

To set these values you can use the command {{ic|sysctl}}.  For example, to set the swappiness value to 20 you could use:

  sudo sysctl vm.swappiness=20

There is no preset defined answer on the optimal values for these parameters.  Experimentation is needed to find the optimum configuration for your hardware and workload.