How to specify JVM maximum heap size "-Xmx" for running an application with "run" action in SBT?

Question

My application does large data arrays processing and needs more memory than JVM gives by default. I know in Java it's specified by "-Xmx" option. How do I set SBT up to use particular "-Xmx" value to run an application with "run" action?

Answer 1

For forked processes you should look at Build.scala

To modify the java options for forked processes you need to specify them in the Build.scala (or whatever you've named your build) like this:

val buildSettings = Defaults.defaultSettings ++ Seq(
   //…
   javaOptions += "-Xmx1G",
   //…
)

This will give you the proper options without modifying JAVA_OPTS globally, and it will put custom JAVA_OPTS in an sbt generated start-script

For non forked processes it's most convenient to set the config via sbtopts or sbtconfig depending on your sbt version.

Since sbt 0.13.6 .sbtconfig is deprecated. Modify /usr/local/etc/sbtopts along these lines:

-J-Xms512M
-J-Xmx3536M
-J-Xss1M
-J-XX:+CMSClassUnloadingEnabled
-J-XX:+UseConcMarkSweepGC
-J-XX:MaxPermSize=724M
-J-agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=5005

You can also create an .sbtopts file in the root of your SBT project using the same syntax as in the /usr/local/etc/sbtopts file. This makes the project self-contained.

Before sbt 0.13.6 you could set the options in .sbtconfig for non forked processes:

1. Check where sbt is:

   $ which sbt
   /usr/local/bin/sbt
   

2. Look at the contents:

   $ cat /usr/local/bin/sbt
   #!/bin/sh
   test -f ~/.sbtconfig && . ~/.sbtconfig
   exec java ${SBT_OPTS} -jar /usr/local/Cellar/sbt/0.12.1/libexec/sbt-launch.jar "$@"
   

3. Set the correct jvm options to prevent OOM (both regular and PermGen):

   $ cat ~/.sbtconfig
   SBT_OPTS="-Xms512M -Xmx3536M -Xss1M 
    -XX:+CMSClassUnloadingEnabled 
    -XX:+UseConcMarkSweepGC -XX:MaxPermSize=724M"
   

If you want to set SBT_OPTS only for the current run of sbt you can use env SBT_OPTS=".." sbt as suggested by Googol Shan. Or you can use the option added in Sbt 12: sbt -mem 2048. This gets unwieldy for longer lists of options, but it might help if you have different projects with different needs.

Note that CMSClassUnloadingEnabled in concert with UseConcMarkSweepGC helps keep the PermGen space clean, but depending on what frameworks you use you might have an actual leak on PermGen, which eventually forces a restart.