Building OpenJDK faster

A basic build instruction was described here. But the full build takes long. Here are the variables that I use to build OpenJDK faster for everyday builds:

• NO_DOCS=true. This causes the build not to generate the javadoc docs for the JDK source code, which isn't very useful for daily engineering.
• NO_IMAGES=true and DEV_ONLY=true. By default, the JDK makefile builds the JDK and the JRE images (as in the state ready for deployment). For everyday debugging purposes, it's not necessary. By setting this variable to true, the image creation is omitted, which saves the build time.
• HOTSPOT_BUILD_JOBS=[ncpu] (for the JVM) and PARALLEL_COMPILE_JOBS=[ncpu](for the JDK). These are for parallel builds. By setting them to the number of the CPU cores available on the build machine, the build process runs in parallel.

There are ways to build either the JVM or the JDK parts of the build, which is a real build time saver.

When I only need to build the JVM part, here's how I build the JVM only:

• Go to the hotspot/make directory.
• Build make targets all_fastdebug copy_fastdebug_jdk export_fastdebug_jdk. Replace 'fastdebug' with 'product' for a product build. The JDK part of the build is copied from the import JDK.
• Look for a java launcher in hotspot/build/.

When I only need to build the JDK part, here's how I build the JDK only:

• Go to the jdk/make direcotry.
• Build make target fastdebug. The JVM part of the build is copied from the import JDK.
• Look for a java launcher in jdk/build/.