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 being swapped out to disk.

Swapping is very bad for performance and 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.

There are three approaches to disabling swapping:

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

The first 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.mlockall: 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,sh]
--------------
curl 'http://localhost:9200/_nodes?pretty&filter_path=**.mlockall'
--------------

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, by starting Elasticsearch with:

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

or 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
--------------

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

The second option is to completely disable swap. 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: `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`

The second 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.