7127687: MethodType leaks memory due to interning

[Jim Laskey]

What we are trying to do is intern the MethodType. Maps are designed to provide the key -> value relationship (one way.) If we only used the key and a dummy value in the Map, it would be equivalent to a write only collection, since there is no method to return a found key.  We would have to set the value to sometime meaningful like the existing MethodType. Doing so creates the hard reference to the MethodType and transitively to the unloadable Classes. 

We encountered this problem in Nashorn where we generate object classes at runtime. MethodTypes that referred to these classes "hung on" to them indefinitely.  When running JavaScript test suites, we soon exhausted the perm gen.

I tried using WeakHashMap initially but then realized the implications. WeakHashSet is useless, since it uses WeakHashMap, making it not weak at all. 

This fix has been well tested, as it is integrated in our test system (4 times a day on several different platforms.)


Sent from my iPhone

On 2012-03-28, at 11:51 PM, Vitaly Davidovich <vitalyd at gmail.com> wrote:

> We only care about keys here anyway, right? WeakHashMap with a dummy value object (some static reference) should achieve the same thing since its Entry will have the key referenced weakly and the value strongly (but we don't care since the value is a dummy static ref).  Maybe I misunderstood the intent though ...
> 
> Sent from my phone
> 
> On Mar 28, 2012 9:02 PM, "Jim Laskey" <jlaskey at me.com> wrote:
> The WeakHashMap leads to a non-weak reference to the class, since only the key is weak. Same is true for public versions of WeakHashSet. The collection used here is truly weak. 
> 
> Sent from my iPhone 4
> 
> On 2012-03-28, at 9:42 PM, Vitaly Davidovich <vitalyd at gmail.com> wrote:
> 
>> Hi John,
>> 
>> I think you can use diamond generic inference when declaring the weak intern set.
>> 
>> Also any reason you didn't use WeakHashMap directly with dummy value to simulate the set? Or wrap the WeakHashMap and synchronize the accessors to it?
>> 
>> Sent from my phone
>> 
>> On Mar 28, 2012 7:52 PM, "John Rose" <john.r.rose at oracle.com> wrote:
>> http://cr.openjdk.java.net/~jrose/7127687/webrev.00/
>> 
>> 7127687: MethodType leaks memory due to interning
>> Summary: Replace internTable with a weak-reference version.
>> 
>> This is a point fix for JDK 8, and will (pending approval) also be back-ported to JDK 7u.
>> 
>> — John
>> 
>> Notes on process:  This code is part of JSR 292.  Therefore the review comments will be collected in mlvm-dev, and changes will be integrated via hsx/hotspot-comp.
>> 
>> At least one reviewer must be an official Reviewer the JDK 8 Project [1], but other reviewers are most welcome.
>> 
>> [1] http://openjdk.java.net/census#jdk8
>> 
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