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:
The first 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
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
If you see that
false, then it means that the
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:
ulimit -l unlimitedas root before starting Elasticsearch, or set
- RPM and Debian
unlimitedin the system configuration file (or see below for systems using
- Systems using
infinityin the systemd configuration.
Another possible reason why
mlockall can fail is that the temporary directory
/tmp) is mounted with the
noexec option. This can be solved by
specifying a new temp directory using the
ES_JAVA_OPTS environment variable:
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.
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
sudo swapoff -a. To disable it permanently, you will need
to edit the
/etc/fstab file and comment out any lines that contain the
On Windows, the equivalent can be achieved by disabling the paging file entirely
System Properties → Advanced → Performance → Advanced → Virtual memory.
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.