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.
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
This doesn’t require a restart of Elasticsearch.
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.
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.
Another option is to use
mlockall on
Linux/Unix systems, or
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:
bootstrap.memory_lock: true
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:
GET _nodes?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:
-
.zipand.tar.gz -
Set
ulimit -l unlimitedas root before starting Elasticsearch, or setmemlocktounlimitedin/etc/security/limits.conf. - RPM and Debian
-
Set
MAX_LOCKED_MEMORYtounlimitedin the system configuration file (or see below for systems usingsystemd). -
Systems using
systemd -
Set
LimitMEMLOCKtoinfinityin the systemd configuration.
Another possible reason why mlockall can fail is that
the JNA temporary directory (usually a sub-directory of /tmp) is mounted with the noexec option. This can be solved by specifying
a new temporary directory for JNA using the ES_JAVA_OPTS environment variable:
export ES_JAVA_OPTS="$ES_JAVA_OPTS -Djna.tmpdir=<path>" ./bin/elasticsearch
or setting this JVM flag in the jvm.options configuration file.