From: Eric Hodel <drbrain@...7.net>
Date: 2004-07-07T15:39:34+09:00
Subject: Re: Druby and garbage collection

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Han Holl (han.holl@pobox.com) wrote:

> Hello,
>=20
> I must be doing something quite wrong, but I'm stumped by this:
>=20
> The server:
>=20
> require 'drb'
>=20
>=20
> class Server
>   def pgproxy
>     return DatabaseProxy.new
>   end
> end

This is very, very unsafe.  Your DatabaseProxy may destroy itself at any
time, since the reference is pulled out of ObjectSpace.  You probably
want to use TimerIdConv instead of the default DRbIdConv.

The default IdConv uses ObjectSpace._id2ref to retrieve remote objects.
Without some reference to these objects your remote processes may be
left hanging even immediately after object creation.  TimerIdConv ages
objects and allows them to expire N minutes after the last access.

> class DatabaseProxy
>   include DRbUndumped
>=20
>   def exec(q, *methods)
>       a =3D Array.new(10000)
>       STDERR.puts "Created #{a.object_id}"; STDERR.flush
>       ObjectSpace::define_finalizer(a, proc {|id|
>         STDERR.puts "Finalizer #{id}"; STDERR.flush
>       })

This is the wrong way to define a finalizer.  The proc must be created
outside of where self exists, otherwise the block holds onto self, and
self can never be gc'd.  (This is a big gotcha!  Understanding why this
is wrong took me quite some time to figure out.)

>       a
>   end
>   def gc
>       GC.start
>   end
>
> end

STDERR.sync =3D true

class DBProxy

  @@finalizer =3D proc do |id|
    STDERR.puts "Finalizing #{id}"
  }

  def exec(q, *methods)
    a =3D Array.new(10000)

    STDERR.puts "Created #{a.object_id}"

    ObjectSpace.define_finalizer a, @@finalizer

    return a
  end

end

> DRb.start_service("drbunix:/tmp/testdrb", Server.new)
> DRb.thread.join
> ### end of server.rb
>=20
> And the client:
>=20
>=20
> require 'drb'
>=20
> DRb.start_service
> b =3D DRbObject.new(nil, "drbunix://tmp/testdrb").pgproxy
> 100.times do=20
>   a =3D b.exec('aa')
> end
> b.gc
> ### end of client.rb
>=20
> If I run them I don't see any garbage collection going on in the
> server process, and the process size grows considerably.
> Who is holding references to these arrays ?

To answer again, the finalizer is.

Defining a finalizer where you have a reference to the object you want
to finalize creates a circular reference which prevents the finalizer
from working.  You must declare your finalizer in its own scope.
Either creating a static finalizer or by invoking a method that will
create one for you will allow your finalizer to work correctly.

--=20
Eric Hodel - drbrain@segment7.net - http://segment7.net
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