Review Request: 8004352: build-infra: Limit JOBS on large machines

[Erik Joelsson]

The current default for number of parallel build jobs is just equal to 
the number of cores in the system. While this works well on many 
machines, there have been several reports of this not working. I have 
been trying to come up with a better scheme for the default and the 
following is something that I think will do. It has been active in the 
build-infra forest for a while now.

The complaints that were raised were usually concerning one of the 
following:

* A big machine with very many cpus didn't scale well when using all of 
them, so there is no point in trying to, it just made the build less 
stable. Suggestion, introduce a hard cap.
* A machine with lots of cpus but not as much memory would run out of 
memory. Suggestion, limit number of jobs on memory as well as cpus.
* The build eats up all my resources, leaving my browser unusable. 
Suggestion, don't use everything unless asked for.

So this is what I ended up with:
1. Take the min of num_cores and memory_size/1000
2. Cap at 16
3. If more than 4 cores, shave it to 90% rounded down to leave some room.

The user can still override this in two ways. Either by using any of the 
configure arguments:

   --with-num-cores        number of cores in the build system, e.g.
                           --with-num-cores=8 [probed]
   --with-memory-size      memory (in MB) available in the build system, 
e.g.
                           --with-memory-size=1024 [probed]
   --with-jobs             number of parallel jobs to let make run 
[calculated
                           based on cores and memory]

Or by setting JOBS=<number> on the make command line.

Also not that this change will set the CONCURRENT_BUILD_JOBS for hotspot 
to the value of JOBS so that it too will be overridden by the above.

http://cr.openjdk.java.net/~erikj/8004352/webrev.root.01/

/Erik