M:N thread scheduler for Ractors

This patch introduce M:N thread scheduler for Ractor system.

In general, M:N thread scheduler employs N native threads (OS threads)
to manage M user-level threads (Ruby threads in this case).
On the Ruby interpreter, 1 native thread is provided for 1 Ractor
and all Ruby threads are managed by the native thread.

From Ruby 1.9, the interpreter uses 1:1 thread scheduler which means
1 Ruby thread has 1 native thread. M:N scheduler change this strategy.

Because of compatibility issue (and stableness issue of the implementation)
main Ractor doesn't use M:N scheduler on default. On the other words,
threads on the main Ractor will be managed with 1:1 thread scheduler.

There are additional settings by environment variables:

`RUBY_MN_THREADS=1` enables M:N thread scheduler on the main ractor.
Note that non-main ractors use the M:N scheduler without this
configuration. With this configuration, single ractor applications
run threads on M:1 thread scheduler (green threads, user-level threads).

`RUBY_MAX_CPU=n` specifies maximum number of native threads for
M:N scheduler (default: 8).

This patch will be reverted soon if non-easy issues are found.

[Bug #19842]

1 files changed, 14 insertions, 0 deletions

diff --git a/bootstraptest/test_thread.rb b/bootstraptest/test_thread.rb
index 0c163d0db0..18b4fcd2e9 100644
--- a/bootstraptest/test_thread.rb
+++ b/bootstraptest/test_thread.rb
@@ -242,6 +242,20 @@ assert_equal 'true', %{
   end
 }
 
+assert_equal 'true', %{
+  Thread.new{}.join
+  begin
+    Process.waitpid2 fork{
+      Thread.new{
+        sleep 0.1
+      }.join
+    }
+    true
+  rescue NotImplementedError
+    true
+  end
+}
+
 assert_equal 'ok', %{
   open("zzz_t1.rb", "w") do |f|
     f.puts <<-END