Use ConcurrentWeakHashMap for RubyClass.subclasses #5137
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The naively-synchronized WeakHashSet does not guarantee safe iteration in the presence of modification, which triggers a ConcurrentModificationException as seen in #5115. By using a ConcurrentWeakHashMap (designed by Doug Lea et al, see http://www.java2s.com/Code/Java/Collections-Data-Structure/Ahashtablewithemweakkeysemfullconcurrencyofretrievalsandadjustableexpectedconcurrencyforupdates.htm) we can iterate and modify concurrently without errors. However this implementation is based the design of ConcurrentHashMap, which means the in-memory structure will likely be considerably larger than a simple WeakHashMap. I believe it is also based off the pre-Java 8 ConcurrentHashMap, which means any load factor above one thread greatly increases in-memory size, and a load factor of one limits the concurrent use of the collection.
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It's hardy scientific, but using VisualVM to get a heap dump of the following script: a = Object
10000.times { a = Class.new(a) }
puts 'ok'
sleepShows a bit over a 2MB size difference, or about 200 bytes per class. This may increase with more subclasses in each collection, but it should be a much smaller increment than the base hit of the collection. Note that I'm forcing a load factor of 1, so we are getting no concurrency out of this collection...just thread-safe traversal+modification. Before: After: |
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This is a pretty non-invasive patch, and the cost of 200 bytes per class doesn't seem too bad, so I vote that we go with it. |
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The naively-synchronized WeakHashSet does not guarantee safe iteration in the presence of modification, which triggers a ConcurrentModificationException as seen in #5115.
By using a ConcurrentWeakHashMap (designed by Doug Lea et al) we can iterate and modify concurrently without errors.
However this implementation is based the design of ConcurrentHashMap, which means the in-memory structure will likely be considerably larger than a simple WeakHashMap. I believe it is also based off the pre-Java 8 ConcurrentHashMap, which means any load factor above one thread greatly increases in-memory size, and a load factor of one limits the concurrent use of the collection.