Asp Forum - WeakRef Hash - comp.lang.ruby

James Gray

1/23/2006 4:17:00 PM

I need a Hash-like structure, using WeakRef, so that the key value
pairs can be garbage collected as needed. I only need to support []
() and []=(), not the whole range of Hash functions. If the pair has
been collected, I just need a simple nil returned.

I've tried implementing this a couple of different ways, but keep
running into trouble. Does anyone have any tips?

James Edward Gray II

10 Answers

Robert Klemme

1/23/2006 5:16:00 PM

James Edward Gray II wrote:
> I need a Hash-like structure, using WeakRef, so that the key value
> pairs can be garbage collected as needed. I only need to support []
> () and []=(), not the whole range of Hash functions. If the pair has
> been collected, I just need a simple nil returned.
>
> I've tried implementing this a couple of different ways, but keep
> running into trouble. Does anyone have any tips?

Can't you just do something like this?

require 'delegate'
require 'weakref'
WeakHash = DelegateClass(Hash)
class WeakHash
def []=(key,val)
__getobj__[WeakRef.new(key)] = WeakRef.new(val)
end

def [](key)
__getobj__[WeakRef.new(key)]
end

def each(&b)
__getobj__.each do |k,v|
b[k.__getobj__, v.__getobj__] unless k.__getobj__.nil?
end
self
end

def cleanup
delete_if {|k,v| k.__getobj__.nil?}
end
end

Kind regards

robert

James Gray

1/23/2006 6:02:00 PM

On Jan 23, 2006, at 10:17 AM, James Edward Gray II wrote:

> I need a Hash-like structure, using WeakRef, so that the key value
> pairs can be garbage collected as needed. I only need to support []
> () and []=(), not the whole range of Hash functions. If the pair
> has been collected, I just need a simple nil returned.
>
> I've tried implementing this a couple of different ways, but keep
> running into trouble. Does anyone have any tips?

The following code, adapted from an old post by Guy Decoux seems to
do the trick:

class WeakCache
def initialize( cache = Hash.new )
@cache = cache
end

def []( key )
value_id = @cache[key]
return ObjectSpace._id2ref(value_id) unless value_id.nil?
nil
end

def []=( key, value )
ObjectSpace.define_finalizer(value, lambda { @cache.delete
(key) })
@cache[key] = value.object_id
end
end

__END__

I'm still interested in seeing a WeakRefHash though, if anyone has
rolled something similar in the past...

James Edward Gray II

Mauricio Fernández

1/24/2006 12:01:00 AM

On Tue, Jan 24, 2006 at 03:02:04AM +0900, James Edward Gray II wrote:
> The following code, adapted from an old post by Guy Decoux seems to
> do the trick:
>
> class WeakCache
> def initialize( cache = Hash.new )
> @cache = cache
> end
[...]
> def []=( key, value )
> ObjectSpace.define_finalizer(value, lambda { @cache.delete(key) })
==============================
This keeps a reference to value so it will never be reclaimed:

class WeakCache
attr_reader :cache
def initialize( cache = Hash.new )
@cache = cache
end

def []( key )
value_id = @cache[key]
return ObjectSpace._id2ref(value_id) unless value_id.nil?
nil
end

def []=( key, value )
ObjectSpace.define_finalizer(value, lambda { @cache.delete(key) })
@cache[key] = value.object_id
end
end

RUBY_VERSION # => "1.8.4"
RUBY_RELEASE_DATE # => "2005-12-24"
c = WeakCache.new
puts c.cache.size
100000.times{|i| c[i] = i.to_s}
GC.start
puts c.cache.size
__END__
# >> 0
# >> 100000

Compare to this:

class WeakCache
attr_reader :cache
def initialize( cache = Hash.new )
@cache = cache
end

def []( key )
value_id = @cache[key]
return ObjectSpace._id2ref(value_id) unless value_id.nil?
nil
end

def make_lambda(key)
lambda{|value| @cache.delete(key) }
end

def []=( key, value )
ObjectSpace.define_finalizer(value, make_lambda(key))
@cache[key] = value.object_id
end
end

RUBY_VERSION # => "1.8.4"
RUBY_RELEASE_DATE # => "2005-12-24"
c = WeakCache.new
puts c.cache.size
100000.times{|i| c[i] = i.to_s}
GC.start
puts c.cache.size
__END__
# >> 0
# >> 3

That is very inefficient though, you want something more like

class WeakCache
attr_reader :cache
def initialize( cache = Hash.new )
@cache = cache
@rev_cache = {}
@reclaim_method = method(:reclaim_value)
end

def []( key )
value_id = @cache[key]
return ObjectSpace._id2ref(value_id) unless value_id.nil?
nil
end

def reclaim_value(value_id)
@cache.delete @rev_cache[value_id]
@rev_cache.delete value_id
end

def []=( key, value )
@rev_cache[value.object_id] = key
@cache[key] = value.object_id
ObjectSpace.define_finalizer(value, @reclaim_method)
end
end

RUBY_VERSION # => "1.8.4"
RUBY_RELEASE_DATE # => "2005-12-24"
c = WeakCache.new
puts c.cache.size
100000.times{|i| c[i] = i.to_s}
GC.start
puts c.cache.size
__END__
# >> 0
# >> 4

--
Mauricio Fernandez

James Gray

1/24/2006 2:07:00 AM

On Jan 23, 2006, at 6:01 PM, Mauricio Fernandez wrote:

> On Tue, Jan 24, 2006 at 03:02:04AM +0900, James Edward Gray II wrote:
>> The following code, adapted from an old post by Guy Decoux seems to
>> do the trick:
>>
>> class WeakCache
>> def initialize( cache = Hash.new )
>> @cache = cache
>> end
> [...]
>> def []=( key, value )
>> ObjectSpace.define_finalizer(value, lambda { @cache.delete
>> (key) })
>
> ==============================
> This keeps a reference to value so it will never be reclaimed:

[snip sensational examples]

Thank you for the excellent lesson!

James Edward Gray II

Jeff Schwab

1/24/2006 12:09:00 PM

Mauricio Fernandez wrote:

> class WeakCache
> attr_reader :cache
> def initialize( cache = Hash.new )
> @cache = cache
> end
>
> def []( key )
> value_id = @cache[key]
> return ObjectSpace._id2ref(value_id) unless value_id.nil?
> nil
> end
>
> def make_lambda(key)
> lambda{|value| @cache.delete(key) }
> end
>
> def []=( key, value )
> ObjectSpace.define_finalizer(value, make_lambda(key))
> @cache[key] = value.object_id
> end
> end
>
> RUBY_VERSION # => "1.8.4"
> RUBY_RELEASE_DATE # => "2005-12-24"
> c = WeakCache.new
> puts c.cache.size
> 100000.times{|i| c[i] = i.to_s}
> GC.start
> puts c.cache.size
> __END__
> # >> 0
> # >> 3
>
>
>
> That is very inefficient though, you want something more like
>
>
> class WeakCache
> attr_reader :cache
> def initialize( cache = Hash.new )
> @cache = cache
> @rev_cache = {}
> @reclaim_method = method(:reclaim_value)
> end
>
> def []( key )
> value_id = @cache[key]
> return ObjectSpace._id2ref(value_id) unless value_id.nil?
> nil
> end
>
> def reclaim_value(value_id)
> @cache.delete @rev_cache[value_id]
> @rev_cache.delete value_id
> end
>
> def []=( key, value )
> @rev_cache[value.object_id] = key
> @cache[key] = value.object_id
> ObjectSpace.define_finalizer(value, @reclaim_method)
> end
> end
>
> RUBY_VERSION # => "1.8.4"
> RUBY_RELEASE_DATE # => "2005-12-24"
> c = WeakCache.new
> puts c.cache.size
> 100000.times{|i| c[i] = i.to_s}
> GC.start
> puts c.cache.size
> __END__
> # >> 0
> # >> 4
>
>

I like your technique, but suppose the same value is stored for more
than one key in a WeakCache? When the value is finalized, only the last
key will be removed from the cache. I think it might be better to let
the reverse cache hold more than one key per value.

Robert Klemme

1/24/2006 7:07:00 PM

2006/1/24, James Edward Gray II <james@grayproductions.net>:
> On Jan 24, 2006, at 9:51 AM, Robert Klemme wrote:
>
> > Can't you just do something like this?
> >
> > require 'delegate'
> > require 'weakref'
> > WeakHash = DelegateClass(Hash)
> > class WeakHash
> > def []=(key,val)
> > __getobj__[WeakRef.new(key)] = WeakRef.new(val)
> > end
> >
> > def [](key)
> > __getobj__[WeakRef.new(key)]
> > end
> >
> > def each(&b)
> > __getobj__.each do |k,v|
> > b[k.__getobj__, v.__getobj__] unless k.__getobj__.nil?
> > end
> > self
> > end
> >
> > def cleanup
> > delete_if {|k,v| k.__getobj__.nil?}
> > end
> > end
>
> I didn't see this message while the gateway was down. Sorry about that.
>
> I can't use this code as is. I assume you didn't set up delegation
> quite right:

What problem exactly do you have with it?

> class WeakHash < DelegateClass(Hash)
> def initialize
> super(Hash.new)
> end
>
> # ...
> end

No need to inherit to fix this. You can simply do

require 'delegate'
Wh=DelegateClass(Hash)
class Wh
def initialize(h={})
__setobj__ h
end
end

> Some questions the above raises for me:
>
> 1. What will Ruby do if a key is GCed, but not a value?

This was just rough outline code. For a production version I'd
probably change it to consider a pair as gone if either of them is
GC'ed.

> 2. How does this code deal with a value being GCed when the key
> remains?

It will yield nil - but see 1.

> 3. Don't we need to shut off GC in places, to keep references from
> disappearing before we can use them?

No. Because while the yield takes place instances are hard referenced
and cannot be gc'ed.

> Thanks for the help.

Ywc

Kind regards

robert

--
Have a look: http://www.flickr.com/photos/fu...

Robert Klemme

1/24/2006 8:47:00 PM

2006/1/24, James Edward Gray II <james@grayproductions.net>:
> On Jan 24, 2006, at 1:06 PM, Robert Klemme wrote:
>
> >> I can't use this code as is. I assume you didn't set up delegation
> >> quite right:
> >
> > What problem exactly do you have with it?
>
> When I placed the code you posted in a file and tried to create
> WeahHash, the program crashed. (Wrong number of arguments to
> initialize().) I assume it would have worked if I passed the Hash
> manually though.

Yep.

> >> class WeakHash < DelegateClass(Hash)
> >> def initialize
> >> super(Hash.new)
> >> end
> >>
> >> # ...
> >> end
> >
> > No need to inherit to fix this. You can simply do
> >
> > require 'delegate'
> > Wh=DelegateClass(Hash)
> > class Wh
> > def initialize(h={})
> > __setobj__ h
> > end
> > end
>
> Here's another question for you: What are we gaining by delegating
> to an object here, as opposed to subclassing?

Note, the difference between yours and mine was that you first
delegated and then subclassed the delegating class. I'd say do either
of both in this case but not both. Generally I'd prefer delegation in
this use case because we really have a case of wrapping and unwrapping
during insertion and retrieval ops. IMHO inheritance is often used in
many cases where it's not appropriate (although in this case you could
argue that a WeakHash is really a Hash - but then again, it doesn't
share some of Hash's basic properties, namely that the contents do not
change suddenly). There's no hard written rule to be found here.

> >> Some questions the above raises for me:
> >>
> >> 1. What will Ruby do if a key is GCed, but not a value?
> >
> > This was just rough outline code. For a production version I'd
> > probably change it to consider a pair as gone if either of them is
> > GC'ed.
> >
> >> 2. How does this code deal with a value being GCed when the key
> >> remains?
> >
> > It will yield nil - but see 1.
> >
> >> 3. Don't we need to shut off GC in places, to keep references from
> >> disappearing before we can use them?
> >
> > No. Because while the yield takes place instances are hard referenced
> > and cannot be gc'ed.
>
> Good answers all around. Thanks.

In fact to be really sure, one should probably do something like this:

def each
__getobj__.each do |wk,wv|
k,v=wk.__getobj__, wv.__getobj__
yield unless k.nil? || v.nil?
end
end

Cheers

robert

--
Have a look: http://www.flickr.com/photos/fu...

Mauricio Fernández

1/24/2006 9:14:00 PM

On Tue, Jan 24, 2006 at 01:17:07AM +0900, James Edward Gray II wrote:
> I need a Hash-like structure, using WeakRef, so that the key value
> pairs can be garbage collected as needed. I only need to support []
> () and []=(), not the whole range of Hash functions. If the pair has
> been collected, I just need a simple nil returned.
>
> I've tried implementing this a couple of different ways, but keep
> running into trouble. Does anyone have any tips?

I summarized the solutions seen in this thread so far, and presented a couple
new ones at
http://eigenclass.org/hiki.rb?weakhash+a...

There are other ways to implement WeakHash, I'll probably add some to the
above page (later).

HTH
--
Mauricio Fernandez

Robert Klemme

1/24/2006 9:37:00 PM

2006/1/24, Mauricio Fernandez <mfp@acm.org>:
> I summarized the solutions seen in this thread so far, and presented a couple
> new ones at
> http://eigenclass.org/hiki.rb?weakhash+a...
>
> There are other ways to implement WeakHash, I'll probably add some to the
> above page (later).

Nice work. Btw, it's expected that my lookup code is slow because it
has to create a weakref for every insertion. We could try to optimize
this by providing a single weakref for the hash that is used for
lookups.

class WR < WeakRef
attr_accessor :__getobj__
end

class RKWeakHash
def initialize(...)
@lookup = WR.new nil
end

def [](x)
@lookup.__getobj__ = x
__getobj__[@lookup]
end
end

Unfortunately this is not thread safe, unless one invests more and
stores lookup thread local...

robert
--
Have a look: http://www.flickr.com/photos/fu...

James Gray

1/25/2006 3:12:00 AM

On Jan 24, 2006, at 2:47 PM, Robert Klemme wrote:

> Note, the difference between yours and mine was that you first
> delegated and then subclassed the delegating class.

My usage was straight out of the delegate docs:

http://www.ruby-doc.org/stdlib/libdoc/delegate/rdoc/...

Technically, I wrote those docs, but I certainly didn't invent that
idiom. I mainly copied it from Matz's usage in TempFile. (I believe
he wrote that, my apologies if I mis-credited there.)

James Edward Gray II

comp.lang.ruby

WeakRef Hash

James Gray

Robert Klemme

James Gray

Mauricio Fernández

James Gray

Jeff Schwab

Robert Klemme

Robert Klemme

Mauricio Fernández

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