How to execute Rake file tasks in parallel - build

Question

I'm using Rake to build a C language build system.
When compiling multiple files,
I want to compile multiple files in parallel, using multiple cores of the CPU.
Can you give me some advice on how to write a Rake file?
I would like to achieve the make -j in Rake.
As a restriction, I prefer not to install a new Gem.
For clarity, here is a simplified Rake file.
CC = "gcc"
task :default => "hello"
file "hello" => ["hello.o", "message.o"] do
sh "#{CC} -o hello hello.o message.o"
end
file "hello.o" => "hello.c" do　(1)
sh "#{CC} -c hello.c"
end
file "message.o" => "message.c" do　(2)
sh "#{CC} -c message.c"
end
For tasks, I can use multitask.
However, for file tasks, I don't know how to describe it.
I would like to run the file tasks (1) and (2) concurrently.
my environment:
ruby 2.6.4p104 (2019-08-28 revision 67798) [i386-cygwin]
I thank you in advance.

Answer 1

You can create threads and you can run your files in new thread.
For example
CC = "gcc"
task :default => "hello"
file "hello" => ["hello.o", "message.o"] do
Thread.new do
sh "#{CC} -o hello hello.o message.o"
end
end
file "hello.o" => "hello.c" do
Thread.new do
sh "#{CC} -c hello.c"
end
end
file "message.o" => "message.c" do
Thread.new do
sh "#{CC} -c message.c"
end
end

Answer 2

I found out that rake -m considers all tasks as multitasking. I defined individual tasks only for the tasks that I do not want to be executed in parallel. By specifying something like "rake -m -j 8", I confirmed that the build time was reduced.