Introduce setup instructions for better parallelism.

doc/hacking.md

@@ -2,6 +2,24 @@
 
 This document gives some helpful instructions which should make your experience as a Ruby core developer easier.
 
+## Setup
+
+### Make
+
+It's common to want to compile things as quickly as possible. Ensuring `make` has the right `--jobs` flag will ensure all processors are utilized when building software projects. To do this effectively, you can set `MAKEFLAGS` in your shell configuration/profile:
+
+	# On macOS with Fish shell:
+	export MAKEFLAGS="--jobs "(sysctl -n hw.ncpu)
+
+	# On macOS with Bash/ZSH shell:
+	export MAKEFLAGS="--jobs $(sysctl -n hw.ncpu)"
+
+	# On Linux with Fish shell:
+	export MAKEFLAGS="--jobs "(nproc)
+
+	# On Linux with Bash/ZSH shell:
+	export MAKEFLAGS="--jobs $(nproc)"
+
 ## Configure Ruby
 
 It's generally advisable to use a build directory.
@@ -10,7 +28,7 @@ It's generally advisable to use a build directory.
 	mkdir build
 	cd build
 	../configure --prefix $HOME/.rubies/ruby-head
-	make -j16 install
+	make install
 
 ### Without Documentation
 
@@ -24,7 +42,7 @@ If you are frequently building Ruby, this will reduce the time it takes to `make
 
 You can create a file in the Ruby source root called `test.rb`. You can build `miniruby` and execute this script:
 
-	make -j16 run
+	make run
 
 If you want more of the standard library, you can use `runruby` instead of `run`.
 
@@ -32,20 +50,20 @@ If you want more of the standard library, you can use `runruby` instead of `run`
 
 There are a set of tests in `bootstraptest/` which cover most basic features of the core Ruby language.
 
-	make -j16 test
+	make test
 
 ### Run Extensive Tests
 
 There are extensive tests in `test/` which cover a wide range of features of the Ruby core language.
 
-	make -j16 test-all
+	make test-all
 
 You can run specific tests by specifying their path:
 
-	make -j16 test-all TESTS=../test/fiber/test_io.rb
+	make test-all TESTS=../test/fiber/test_io.rb
 
 ### Run RubySpec Tests
 
 RubySpec is a project to write a complete, executable specification for the Ruby programming language.
 
-	make -j16 test-all test-rubyspec
+	make test-all test-rubyspec