Environment

• JRuby version (jruby -v): jruby 1.7.26 (1.9.3p551) 2016-08-26 69763b8 on Java HotSpot(TM) 64-Bit Server VM 1.8.0_101-b13 +jit [darwin-x86_64]
• Operating system and platform (e.g. uname -a): Darwin **.local 17.2.0 Darwin Kernel Version 17.2.0: Fri Sep 29 18:27:05 PDT 2017; root:xnu-4570.20.62~3/RELEASE_X86_64 x86_64

Description

Extensively Using ruby extend functionality blocks threads which in turns causes server performance issues.
extend method calls jruby addSubclass function which is a bottleneck.

Background

Many libraries that provide JSON serialization make use of extend. Under heavy load, this causes performance degradation.

Code Snippet to Reproduce Issue:

module Bar
  def hello
    p "hello from bar!"
  end
end

class Foo
  def hello
    p "hello!"
  end
end

threads = 1000.times.collect do
  Thread.new do
    10000.times do |i|
      foo = Foo.new
      foo.extend(Bar)
    end
  end
end

threads.each {|t| t.join }

Run the above code snippet and capture thread dump. You will see a lot of threads waiting to enter synchronized this block.

here is an example thread dump.