[[heap-size]]
=== Setting the heap size

By default, Elasticsearch tells the JVM to use a heap with a minimum and maximum
size of 1 GB. When moving to production, it is important to configure heap size
to ensure that Elasticsearch has enough heap available.

Elasticsearch will assign the entire heap specified in
<<jvm-options,jvm.options>> via the `Xms` (minimum heap size) and `Xmx` (maximum
heap size) settings.

The value for these setting depends on the amount of RAM available on your
server. Good rules of thumb are:

* Set the minimum heap size (`Xms`) and maximum heap size (`Xmx`) to be equal to
  each other.

* The more heap available to Elasticsearch, the more memory it can use for
  caching. But note that too much heap can subject you to long garbage
  collection pauses.

* Set `Xmx` to no more than 50% of your physical RAM, to ensure that there is
  enough physical RAM left for kernel file system caches.

* Don’t set `Xmx` to above the cutoff that the JVM uses for compressed object
  pointers (compressed oops); the exact cutoff varies but is near 32 GB. You can
  verify that you are under the limit by looking for a line in the logs like the
  following:
+
    heap size [1.9gb], compressed ordinary object pointers [true]

* Even better, try to stay below the threshold for zero-based compressed oops;
  the exact cutoff varies but 26 GB is safe on most systems, but can be as large
  as 30 GB on some systems. You can verify that you are under the limit by
  starting Elasticsearch with the JVM options `-XX:+UnlockDiagnosticVMOptions
  -XX:+PrintCompressedOopsMode` and looking for a line like the following:
+
--
    heap address: 0x000000011be00000, size: 27648 MB, zero based Compressed Oops

showing that zero-based compressed oops are enabled instead of

    heap address: 0x0000000118400000, size: 28672 MB, Compressed Oops with base: 0x00000001183ff000
--

Here are examples of how to set the heap size via the jvm.options file:

[source,txt]
------------------
-Xms2g <1>
-Xmx2g <2>
------------------
<1> Set the minimum heap size to 2g.
<2> Set the maximum heap size to 2g.

It is also possible to set the heap size via an environment variable. This can
be done by commenting out the `Xms` and `Xmx` settings in the
<<jvm-options,`jvm.options`>> file and setting these values via `ES_JAVA_OPTS`:

[source,sh]
------------------
ES_JAVA_OPTS="-Xms2g -Xmx2g" ./bin/elasticsearch <1>
ES_JAVA_OPTS="-Xms4000m -Xmx4000m" ./bin/elasticsearch <2>
------------------
<1> Set the minimum and maximum heap size to 2 GB.
<2> Set the minimum and maximum heap size to 4000 MB.

NOTE: Configuring the heap for the <<windows-service,Windows service>> is
different than the above. The values initially populated for the Windows service
can be configured as above but are different after the service has been
installed. Consult the <<windows-service,Windows service documentation>> for
additional details.
