Add Thread.delay

Author: ysbaddaden

src/crystal/system/thread.cr

@@ -179,6 +179,18 @@ class Thread
     Crystal::System::Thread.sleep(time)
   end
 
+  # Delays execution for a brief moment.
+  @[NoInline]
+  def self.delay(backoff : Int32) : Int32
+    if backoff < 7
+      backoff.times { Intrinsics.pause }
+      backoff &+ 1
+    else
+      Thread.yield
+      0
+    end
+  end
+
   # Returns the Thread object associated to the running system thread.
   def self.current : Thread
     Crystal::System::Thread.current_thread

src/fiber/execution_context/multi_threaded/scheduler.cr

@@ -66,15 +66,17 @@ module Fiber::ExecutionContext
         # fiber but thread B didn't saved its context yet); we must wait until
         # the context switch assembly saved all registers on the stack and set
         # the fiber as resumable.
+        attempts = 0
+
         until fiber.resumable?
           if fiber.dead?
             raise "BUG: tried to resume dead fiber #{fiber} (#{inspect})"
           end
 
           # OPTIMIZE: if the thread saving the fiber context has been preempted,
           # this will block the current thread from progressing... shall we
-          # abort and reenqueue the fiber after MAX iterations?
-          Intrinsics.pause
+          # abort and reenqueue the fiber after MAX attempts?
+          attempts = Thread.delay(attempts)
         end
 
         swapcontext(fiber)