Tomcat OutOfMemoryError: Heap space/PermGen space

Have You ever seen such error in Your logs? Probably yes. It simply means that available memory for Tomcat was consumed and nothing left. To be precise: available memory for JVM which Tomcat uses to run.

What causes this error? It can be caused by memory leaks in the application or the applications has big requirements "by design", even if there are no memory leaks. For the first case You should use some profiling tools to find and fix memory leaks - perhaps this might help without need to change memory settings for Tomcat JVM. If You are sure that Your application has no memory leaks, the only way is to increase memory used by Tomcat JVM. See below how to do that - please note that I described modifying Tomcat memory settings when it is installed as a service under Windows OS.

32-bit Windows

The main problem here is that 32-bit OS is able to see no more than 3,2GB of RAM, even if You have 4GB or more physically installed. This is upper limit of memory that can be use - in theory. However, in practise You will not be able to use more than 1 to 1,5GB of RAM for Tomcat's JVM - the rest of memory is used by OS itself and installed software.

64-bit Windows

Let's assume that You need to assign more than 1,5GB of RAM for Tomcat JVM. Therefore You need 64-bit OS with at least 4GB of RAM (for example 64-bit Windows Professional supports up to 192GB of RAM). Of course You must also use 64-bit JDK - when You use 32-bit JDK under 64-bit OS, You will again face the limit of 3,2GB RAM. You also must use 64-bit version of Tomcat (as it contains Windows service wrapper to use with 64-bit JVMs on 64-bit Windows platforms). Below You will find complete list of all needed steps:

Step 1: make sure You are using 64-bit Windows OS. 

Step 2: install 64-bit JDK and set $JAVA_HOME to the installation directory. 

Step 3: download .zip file with 64-bit Tomcat for Windows (file: apache-tomcat-[version]-windows-x64.zip). 

Step 4: extract Tomcat and go into Tomcat's /bin directory.

Step 5: open service.bat file and locate the line with --JvmMS and --JvmMX parameters, and modify it with new values memory, eg:

%EXECUTABLE%" //US//%SERVICE_NAME% ++JvmOptions "-Djava.io.tmpdir=%CATALINA_BASE%\temp" ++JvmOptions "-XX:MaxPermSize=1024m" --JvmMs 2048 --JvmMx 4096

Note: PermSize is set in different way than heap which has predefined JvmMS and JvmMX flags.

Step 6: open Windows console with administrator rights, then go into Tomcat /bin and execute command:

service.bat install tomcat6

Step 7: start service with command:

net start tomcat6

Note 1: You can skip point 5 and execute directly point 6. After that just start tomcat6w.exe which is a GUI tool for managing the service. You can set memory values there.

Note 2: as an alternative to above steps, You can download Tomcat service installer (file: apache-tomcat-[version].exe) which performs service installation, and then use tomcat6w.exe tool for tune memory settings.

That's all. Your Tomcat service should use provided memory settings.