Github user frreiss commented on a diff in the pull request:

    https://github.com/apache/spark/pull/15005#discussion_r79765892
  
    --- Diff: docs/building-spark.md ---
    @@ -16,24 +16,31 @@ Building Spark using Maven requires Maven 3.3.9 or 
newer and Java 7+.
     
     ### Setting up Maven's Memory Usage
     
    -You'll need to configure Maven to use more memory than usual by setting 
`MAVEN_OPTS`. We recommend the following settings:
    +If you are compiling with Java 7, you'll need to configure Maven to use 
more memory than usual by setting `MAVEN_OPTS`:
     
         export MAVEN_OPTS="-Xmx2g -XX:MaxPermSize=512M 
-XX:ReservedCodeCacheSize=512m"
     
    -If you don't run this, you may see errors like the following:
    +When compiling with Java 8, a similar but smaller set of memory parameters 
is necessary:
    +
    +    export MAVEN_OPTS="-Xmx2g -XX:ReservedCodeCacheSize=512m"
    +
    +If you don't add these parameters to `MAVEN_OPTS`, you may see errors like 
the following:
     
         [INFO] Compiling 203 Scala sources and 9 Java sources to 
/Users/me/Development/spark/core/target/scala-{{site.SCALA_BINARY_VERSION}}/classes...
         [ERROR] PermGen space -> [Help 1]
     
         [INFO] Compiling 203 Scala sources and 9 Java sources to 
/Users/me/Development/spark/core/target/scala-{{site.SCALA_BINARY_VERSION}}/classes...
         [ERROR] Java heap space -> [Help 1]
     
    -You can fix this by setting the `MAVEN_OPTS` variable as discussed before.
    +You can fix these problems by setting the `MAVEN_OPTS` variable as 
discussed before.
     
     **Note:**
     
    -* For Java 8 and above this step is not required.
    -* If using `build/mvn` with no `MAVEN_OPTS` set, the script will automate 
this for you.
    +* This step is not needed for Java 8.
    +* If using `build/mvn` with no `MAVEN_OPTS` set, the script will 
automatically add the above options for Java 7 to the `MAVEN_OPTS` environment 
variable.
    +* The `test` phase of the Spark build will automatically add these options 
to `MAVEN_OPTS`, even when not using `build/mvn`.
    +* You may see warnings like "ignoring option MaxPermSize=1g; support was 
removed in 8.0" when building or running tests with Java 8. These warnings are 
harmless.
    --- End diff --
    
    Developers will see the warning when compiling with `build/mvn`. Updating 
to clarify that.


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