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, 7 insertions, 1 deletions

diff --git a/ractor_core.h b/ractor_core.h
index 38aded1507..36c0e91c7a 100644
--- a/ractor_core.h
+++ b/ractor_core.h
@@ -103,7 +103,6 @@ struct rb_ractor_sync {
 #if RACTOR_CHECK_MODE > 0
     VALUE locked_by;
 #endif
-    rb_nativethread_cond_t cond;
 
     bool incoming_port_closed;
     bool outgoing_port_closed;
@@ -120,7 +119,12 @@ struct rb_ractor_sync {
     struct ractor_wait {
         enum rb_ractor_wait_status status;
         enum rb_ractor_wakeup_status wakeup_status;
+        rb_thread_t *waiting_thread;
     } wait;
+
+#ifndef RUBY_THREAD_PTHREAD_H
+    rb_nativethread_cond_t cond;
+#endif
 };
 
 // created
@@ -310,11 +314,13 @@ static inline void
 rb_ractor_set_current_ec_(rb_ractor_t *cr, rb_execution_context_t *ec, const char *file, int line)
 {
 #ifdef RB_THREAD_LOCAL_SPECIFIER
+
 # ifdef __APPLE__
     rb_current_ec_set(ec);
 # else
     ruby_current_ec = ec;
 # endif
+
 #else
     native_tls_set(ruby_current_ec_key, ec);
 #endif