Tip 7 – adjusting the JVM HEAP size, why and how to

When running JMeter tests, it may happen that the test is not successful and JMeter crashes with an OutOfMemoryError. This can happen due to all kinds of things, most of the time however you are trying to generate too many threads with an insufficient amount of memory allocated to your Java Virtual Machine. More information on what the HEAP is, how the JVM deals with the HEAP can be found here.

The HEAP size is defined on startup of your java application, in our case when we start JMeter. In other words, adjusting the HEAP means we will need to adjust the startup script. The startup script can be found in your JMeter “bin” directory. In my case, on a linux environment, it can be found here:

/opt/apache-jmeter-3.1/bin

When opening the “jmeter” file in your favorite editor, scroll down until you hit a line stating the following (on Windows based systems open “jmeter.bat”):

# This is the base heap size -- you may increase or decrease it to fit your
# system's memory availability:
HEAP="-Xms512m -Xmx512m"

On Windows systems the line looks slightly different, in the jmeter.bat, due to the nature of BATCH files and DOS working differently from Linux systems

set HEAP=-Xms512m -Xmx512m

When you adjust this line to read something like this:

# This is the base heap size -- you may increase or decrease it to fit your
# system's memory availability:
HEAP="-Xms1024m -Xmx4096m"

the HEAP size will by default be larger, meaning you do not have to think about it anymore when starting JMeter. This does pose one possible challenge, it slows down the startup of JMeter somewhat (at least on most machines I have done it). This seems to be because the JVM will first allocate all the memory you defined in the Xms param, ensuring the JVM is at the appropriate size instantly.

So, we changed the values of Xms and Xmx, what do these two mean?

-Xms ==> minimum heap size. This is the value JAVA will attempt to claim in order to run JMeter. This does NOT imply that it immediately gets this fully allocated by the OS. The OS typically doesn't give an application all blocks it asks for until really needed.
-Xmx ==> maximum heap size. This is the value JAVA will use as a maximum size of the JVM. When JMeter passes this level, an OutOfMemoryError will occur once more.

An alternative, more flexible setup to change the heapsize is to pass the JVM arguments on the commandline when starting JMeter. This of course assumes you are comfortable in commandline and start JMeter from commandline.

[user@machine ]$ JVM_ARGS="-Xms1024m -Xmx2048m" jmeter

Be Aware!

Please always make sure you leave enough memory for your OS and other applications to run on your machine when allocating more HEAP space to a JVM. Not leaving enough memory for your OS to keep running will result in your machine simply being non-responsive and thus your test and testresults being rendered useless.