--enable-javac-server now default on in jdk9/dev

[Erik Joelsson]

Hello,

Yesterday, JDK-8144039 was pushed to jdk9/dev. This should make building 
JDK 9 faster by enabling the javac server feature by default. If you 
experience trouble with this, the feature can be disabled using 
--disable-javac-server. If we get too many problem reports that we 
aren't able to address, we will of course consider reverting this 
change. Please keep an eye out and report issues on build-dev.

You might wonder what the javac-server is. It's important to note that 
this is not the full sjavac feature that we have sort of supported for a 
long time in the "new" build. It's only the client/server model from 
sjavac, and none of the dependency checking mechanism. The build speedup 
comes from running all javac compilations in the same hot JVM. This was 
implemented by Andreas Lundblad in JDK-8135131.

In my testing, I have seen considerable build speedups on all tried 
configurations. On my linux workstion (16 cores (32 with ht)) "make 
exploded-image" went from 04:09 to 03:20. On machines with less cpus the 
performance increase is generally larger.

/Erik