The Java Virtual Machine (JVM) running IntelliJ IDEA allocates some predefined amount of memory. The default value depends on the platform. If you are experiencing slowdowns, you may want to increase the memory heap.
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If you want to configure the heap size for the build process that compiles your code, open Settings/PreferencesCtrl+Alt+S, select Build, Execution, Deployment | Compiler, and specify the necessary amount of memory in the Build process heap size field.
From the main menu, select Help | Change Memory Settings.
Set the necessary amount of memory that you want to allocate and click Save and Restart.
This action changes the value of the -Xmx option used by the JVM and restarts IntelliJ IDEA with the new setting.
The Change Memory Settings action is available starting from IntelliJ IDEA version 2019.2. For previous versions or if the IDE crashes, you can change the value of the -Xmx option manually as described in JVM options.
IntelliJ IDEA also warns you if the amount of free heap memory after a garbage collection is less than 5% of the maximum heap size:
Click Configure to increase the amount of memory allocated by the JVM. If you are not sure what would be a good value, use the one suggested by IntelliJ IDEA.
Click Save and Restart and wait for IntelliJ IDEA to restart with the new memory heap setting.
Enable the memory indicator
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IntelliJ IDEA can show you the amount of used memory in the status bar. Use it to judge how much memory to allocate.
Right-click the status bar and select Memory Indicator.
Toolbox App
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If you are using the Toolbox App, you can change the maximum allocated heap size for a specific IDE instance without starting it.
Open the Toolbox App, click next to the relevant IDE instance, and select Settings.
In the instance settings dialog, expand Configuration and specify the heap size in the Maximum heap size field.
If the IDE instance is currently running, the new settings will take effect only after you restart it.
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If you are using a standalone instance not managed by the Toolbox App, and you can't start it, it is possible to manually change the -Xmx option that controls the amount of allocated memory. Create a copy of the default JVM options file and change the value of the -Xmx option in it.