Reducing Waste in Expandable Collections: The Pharo CaseA first version of this paper was presented at the IWST workshop (http://www.esug.org/wiki/pier/Conferences/2014/IWST14), however no proceedings have been made (e.g., our IWST submission is not available to any Digital Library (IEEE or ACM)

Expandable collections are collections whose size may vary as elements are added and removed. Hash maps and ordered collections are popular expandable collections. Expandable collection classes offer an easy-to-use API, however this apparent simplicity is accompanied by a significant amount of wasted resources. We describe some improvements of the collection library to reduce the amount of waste associated with collection expansions. We have designed a new collection library for the Pharo programming language that exhibits better resource management than the standard library. Across a basket of 5 applications, our optimized collection library significantly reduces the memory footprint of the collections: (i) the amount of intermediary internal array storage by 73%, (ii) the number of allocated bytes by 67% and (iii) the number of unused bytes by 72%. This reduction of memory is accompanied with a speedup of about 3% for most of our benchmarks. We analyzed the collection implementations of Java, C#, Scala, and Ruby: these implementations largely behave as in Pharo’s, therefore suffering from the very same limitations. Our result are likely to benefit designers of future programming languages and collection libraries.