Java applications like JIRA and Confluence run in a "Java virtual machine" (JVM), instead of directly within an operating system. When started, the Java virtual machine is allocated a certain amount of memory, which it makes available to applications like JIRA. By default, Java virtual machines are allocated 64 MB of memory, no matter how many gigabytes of memory your server may actually have available. 64 MB is inadequate for medium to large JIRA installations, and so this needs to be increased. Seeing OutOfMemoryErrors in the logs is symptomatic of this.
Step 1: Diagnosis
Assess Root Cause
Often, there is a root cause for OutOfMemory Errors that may be better to address than just increasing memory. See JIRA Crashes Due to 'OutOfMemoryError Java heap space' for a discussion.
Determine JIRA usage patterns
To determine the JIRA usage patterns:
This server has been allocated a maximum of 768 MB and a minimum of 256 MB (typically defined in the
setenv script which is executed by running the
start-jira script). If you are trying to see whether your settings are being picked up by JIRA, this is where to look. Here, you can see that JIRA has reserved 742 MB, of which 190 MB is actually in use. If this JIRA instance were running out of memory, it would have reserved the maximum available (768 MB), and would be using an amount close to this.
Determine available system memory
Setting the -Xmx above the available amount on the server runs the risk of OutOfMemoryErrors due to lack of physical memory. If that occurs the system will use swap space, which greatly decreases performance.
As a rule of thumb, if you have fewer than 5000 issues, JIRA should run well with the default 768 MB. Granting JIRA too much memory can impact performance negatively, so it is best to start with 768 MB and make modest increases as necessary. As another data point, 40,000 works well with 768 MB to 1 GB.
Step 2: Increase Available Memory
Windows (starting from .bat file)
Setting Properties for Windows Services via Command Line
Setting Properties for Windows Services via the Windows Registry
In some versions of Windows, there is no option to add Java variables to the service. In these cases, you must add the properties by viewing the option list in the registry.