# Using id2ref for anything? **URL:** \ **Category:** Ruby\ **Created:** [October 25, 2009, 9:42pm UTC](https://www.ruby-forum.com/t/using-id2ref-for-anything/177865 "2009-10-25T21:42:06Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![headius](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/7c8e57/32.png) [@headius](https://www.ruby-forum.com/u/headius)\ **Post date:** [October 25, 2009, 9:42pm UTC](https://www.ruby-forum.com/t/using-id2ref-for-anything/177865/1 "2009-10-25T21:42:06Z")
ObjectSpace.\_id2ref is another of those peculiar methods, an artifact of a particular implementation which, due to its lack of a copying/compacting garbage collector, can always locate in memory an object given its “id”. This is typically _not_ easily possible on other VMs, where objects move around and it may even be difficult to get a unique “id” for a given object since memory locations keep moving and adding a numeric ID would increase object or object handle sizes. On JRuby, \_id2ref is implemented as a pair with Object#object\_id/id. The latter, when called on an object, atomically constructs a numeric ID for the object in question. It then asks our ObjectSpace implementation to insert a weak reference to the object into a table keyed on numeric ID. This allows the resulting ID to be used later for \_id2ref to retrieve the object. Unfortunately object\_id, in its #id form, is often used to get a unique non-#hash key for an object for purposes entirely unrelated to \_id2ref. As a result, any code using object\_id or id on JRuby pays a significantly higher cost than you might expect. If we no longer supported \_id2ref, the only cost would be in producing an ID, probably with a strictly-increasing atomic 64-bit value. There would be no weakref map and no cost of constructing and managing the weakrefs within that map. So I am asking you Rubyists…does this sound like a problem? In the 1.8/1.9 stdlib, the only reference to \_id2ref is one in drb.rb, which could be replaced with a “better way”. None of the gems I have installed use \_id2ref. Originally, weakref.rb used \_id2ref, but we have a native impl of weakref that uses Java’s built-in weakrefs. Google code search only brings up about 353 hits for “lang:ruby \_id2ref”, most of them the already-mentioned cases. One last demonstration of the perf difference between the current Object#object\_id and one that does not use the ObjectSpace weak map: Current: ``` user system ``` total real 1M calls to obj.object\_id 0.658000 0.000000 0.658000 ( 0.658000) 1M calls to Object.new.object\_id 6.636000 0.000000 6.636000 ( 6.636000) Using object’s “identity hash”: ``` user system ``` total real 1M calls to obj.object\_id 0.356000 0.000000 0.356000 ( 0.356000) 1M calls to Object.new.object\_id 0.636000 0.000000 0.636000 ( 0.636000) It’s also interesting to note that even maintaining the contract of object\_id being unique is hard. On the JVM, for example, it is not possible to get a unique numeric id or pointer for a given object unless you manage a weak map of objects on your own… - Charlie ---
**Author:** ![headius](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/7c8e57/32.png) [@headius](https://www.ruby-forum.com/u/headius)\ **Post date:** [October 25, 2009, 10:05pm UTC](https://www.ruby-forum.com/t/using-id2ref-for-anything/177865/2 "2009-10-25T22:05:31Z")
On 10/25/2009 09:41 PM, Charles Oliver N. wrote: > So I am asking you Rubyists…does this sound like a problem? In the > 1.8/1.9 stdlib, the only reference to \_id2ref is one in drb.rb, which > could be replaced with a “better way”. None of the gems I have > installed use \_id2ref. Originally, weakref.rb used \_id2ref, but we > have a native impl of weakref that uses Java’s built-in weakrefs. > Google code search only brings up about 353 hits for “lang:ruby > \_id2ref”, most of them the already-mentioned cases. Charles, thanks for the elaborate report and request! I for my part do not see an issue with removing \_id2ref if a better solution for DRb can be devised. > It’s also interesting to note that even maintaining the contract of > object\_id being unique is hard. On the JVM, for example, it is not > possible to get a unique numeric id or pointer for a given object > unless you manage a weak map of objects on your own… I believe there is an alternative solution which comes at the cost of the memory overhead for every object: place the id in the instance and use a central AtomicLong for “generating” ids. You also save the overhead of map maintenance which would be a central synchronization point. Kind regards robert ---
**Author:** ![headius](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/7c8e57/32.png) [@headius](https://www.ruby-forum.com/u/headius)\ **Post date:** [October 25, 2009, 11:02pm UTC](https://www.ruby-forum.com/t/using-id2ref-for-anything/177865/3 "2009-10-25T23:02:53Z")
Hi, In message “Re: Using id2ref for anything?” on Mon, 26 Oct 2009 05:41:52 +0900, Charles Oliver N. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes: |ObjectSpace.\_id2ref is another of those peculiar methods, an artifact |of a particular implementation which, due to its lack of a |copying/compacting garbage collector, can always locate in memory an |object given its “id”. This is typically _not_ easily possible on |other VMs, where objects move around and it may even be difficult to |get a unique “id” for a given object since memory locations keep |moving and adding a numeric ID would increase object or object handle |sizes. Originally, \_id2ref is an implementation dependent hack for weakref, so that you can remove it, if you can provide the better way. ``` matz. ``` ---
**Author:** ![headius](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/7c8e57/32.png) [@headius](https://www.ruby-forum.com/u/headius)\ **Post date:** [October 26, 2009, 1:18am UTC](https://www.ruby-forum.com/t/using-id2ref-for-anything/177865/4 "2009-10-26T01:18:39Z")
On Sun, Oct 25, 2009 at 5:02 PM, Yukihiro M. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Originally, \_id2ref is an implementation dependent hack for weakref, > so that you can remove it, if you can provide the better way. I suspected as much, since that seemed to be the primary place for it to be used. I guess the remaining question is about the uniqueness of object\_id. I believe on Sun’s JVMs java.lang.System.identityHashcode of an object will be unique for the lifetime of the object, but not unique forever (which I’m sure is the case on MRI). However, I don’t think there’s any guarantee that the identityHashCode will be unique across JVMs, though the documentation says an implementation should make a “best effort” to keep it unique. We will look at removing \_id2ref in 1.5 (or making it do nothing, with a warning) as well as modifying the one stdlib that uses it (DRb, for reasons I have not yet explored). And I will explore whether identityHashcode will be “unique enough” as I suspect it should be. - Charlie ---
**Author:** ![headius](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/7c8e57/32.png) [@headius](https://www.ruby-forum.com/u/headius)\ **Post date:** [October 26, 2009, 12:34am UTC](https://www.ruby-forum.com/t/using-id2ref-for-anything/177865/5 "2009-10-26T00:34:45Z")
On 10/25/09, Charles Oliver N. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > If we no longer supported \_id2ref, the only cost would be in producing > an ID, probably with a strictly-increasing atomic 64-bit value. There > would be no weakref map and no cost of constructing and managing the > weakrefs within that map. > > So I am asking you Rubyists…does this sound like a problem? In my own projects, I use \_id2ref/ **id** in a couple places that I can recall. So, I was about to object to it going away… but on reflection, it seems like \_id2ref can always be replaced by a WeakRef, (at least when running in JRuby). So, removing it shouldn’t really be a problem. ---
**Author:** ![headius](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/7c8e57/32.png) [@headius](https://www.ruby-forum.com/u/headius)\ **Post date:** [October 26, 2009, 1:22am UTC](https://www.ruby-forum.com/t/using-id2ref-for-anything/177865/6 "2009-10-26T01:22:10Z")
On Sun, Oct 25, 2009 at 4:05 PM, Robert K. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > > It’s also interesting to note that even maintaining the contract of > > object\_id being unique is hard. On the JVM, for example, it is not > > possible to get a unique numeric id or pointer for a given object > > unless you manage a weak map of objects on your own… > > I believe there is an alternative solution which comes at the cost of the > memory overhead for every object: place the id in the instance and use a > central AtomicLong for “generating” ids. Â You also save the overhead of map > maintenance which would be a central synchronization point. Yes, that may be too high a cost for us to pay. On 32/64-bit JVMs, adding another field would cost 4 or 8 bytes per object. Stuffing it into the instance variable table would force ivar tables to be created when object\_id is called, which comes with a base cost of 2 words (4 or 8-byte) plus the word for the reference to a Fixnum object (or 4-8 bytes for a reference to an int or long). It’s too high to put on all Objects, especially if identityHashcode is unique enough. - Charlie ---
**Author:** ![headius](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/h/7c8e57/32.png) [@headius](https://www.ruby-forum.com/u/headius)\ **Post date:** [October 26, 2009, 1:19am UTC](https://www.ruby-forum.com/t/using-id2ref-for-anything/177865/7 "2009-10-26T01:19:50Z")
On Sun, Oct 25, 2009 at 6:34 PM, Caleb C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > In my own projects, I use \_id2ref/ **id** in a couple places that I can > recall. So, I was about to object to it going away… but on > reflection, it seems like \_id2ref can always be replaced by a WeakRef, > (at least when running in JRuby). So, removing it shouldn’t really be > a problem. Yes, all cases of \_id2ref could be implemented yourself by building a weak map from your own user-generated (or from object\_id) to objects. So I think there’s probably no good reason we need to have \_id2ref support if we have our own weakref implementation. - Charlie