OpenSearch/docs/reference/setup/sysconfig/swap.asciidoc

[[setup-configuration-memory]]
=== Disable swapping

Most operating systems try to use as much memory as possible for file system
caches and eagerly swap out unused application memory. This can result in parts
of the JVM heap or even its executable pages being swapped out to disk.

Swapping is very bad for performance, for node stability, and should be avoided
at all costs. It can cause garbage collections to last for **minutes** instead
of milliseconds and can cause nodes to respond slowly or even to disconnect
from the cluster. In a resilient distributed system, it's more effective to let
the operating system kill the node.

There are three approaches to disabling swapping. The preferred option is to
completely disable swap. If this is not an option, whether or not to prefer
minimizing swappiness versus memory locking is dependent on your environment.

[[disable-swap-files]]
==== Disable all swap files

Usually Elasticsearch is the only service running on a box, and its memory usage
is controlled by the JVM options. There should be no need to have swap enabled.

On Linux systems, you can disable swap temporarily by running:

[source,sh]
--------------
sudo swapoff -a
--------------

To disable it permanently, you will need to edit the `/etc/fstab` file and
comment out any lines that contain the word `swap`.

On Windows, the equivalent can be achieved by disabling the paging file entirely
via `System Properties → Advanced → Performance → Advanced → Virtual memory`.

[[swappiness]]
==== Configure `swappiness`

Another option available on Linux systems is to ensure that the sysctl value
`vm.swappiness` is set to `1`. This reduces the kernel's tendency to swap and
should not lead to swapping under normal circumstances, while still allowing the
whole system to swap in emergency conditions.

[[mlockall]]
==== Enable `bootstrap.memory_lock`

Another option is to use
http://opengroup.org/onlinepubs/007908799/xsh/mlockall.html[mlockall] on
Linux/Unix systems, or
https://msdn.microsoft.com/en-us/library/windows/desktop/aa366895%28v=vs.85%29.aspx[VirtualLock]
on Windows, to try to lock the process address space into RAM, preventing any
Elasticsearch memory from being swapped out. This can be done, by adding this
line to the `config/elasticsearch.yml` file:

[source,yaml]
--------------
bootstrap.memory_lock: true
--------------

WARNING: `mlockall` might cause the JVM or shell session to exit if it tries to
allocate more memory than is available!

After starting Elasticsearch, you can see whether this setting was applied
successfully by checking the value of `mlockall` in the output from this
request:

[source,js]
--------------
GET _nodes?filter_path=**.mlockall
--------------
// CONSOLE

If you see that `mlockall` is `false`, then it means that the `mlockall`
request has failed.  You will also see a line with more information in the logs
with the words `Unable to lock JVM Memory`.

The most probable reason, on Linux/Unix systems, is that the user running
Elasticsearch doesn't have permission to lock memory.  This can be granted as
follows:

`.zip` and `.tar.gz`::

  Set <<ulimit,`ulimit -l unlimited`>> as root before starting Elasticsearch,
  or set `memlock` to `unlimited` in
  <<limits.conf,`/etc/security/limits.conf`>>.

RPM and Debian::

  Set `MAX_LOCKED_MEMORY` to `unlimited` in the
  <<sysconfig,system configuration file>> (or see below for systems using
  `systemd`).

Systems using `systemd`::

  Set `LimitMEMLOCK` to `infinity` in the <<systemd,systemd configuration>>.

Another possible reason why `mlockall` can fail is that the temporary directory
(usually `/tmp`) is mounted with the `noexec` option. This can be solved by
specifying a new temp directory using the `ES_JAVA_OPTS` environment variable:

[source,sh]
--------------
export ES_JAVA_OPTS="$ES_JAVA_OPTS -Djava.io.tmpdir=/path/to/temp/dir"
./bin/elasticsearch
--------------

or setting this JVM flag in the jvm.options configuration file.