What percentage of the empty lists are default-sized? I suspect it is large, in which case we could apply this trick only for the default-sized lists, and eliminate the extra field.
On Mar 26, 2013, at 5:25 PM, Mike Duigou wrote: > Hello all; > > This is a review for optimization work that came out of internal analysis of > Oracle's Java applications. It's based upon analysis that shows that in large > applications as much as 10% of maps and lists are initialized but never > receive any entries. A smaller number spend a large proportion of their > lifetime empty. We've found similar results across other workloads as well. > This patch is not a substitute for pre-sizing your collections and > maps--doing so will *always* have better results. > > This patch extends HashMap and ArrayList to provide special handling for > newly created instances that avoids creating the backing array until needed. > There is a very small additional cost for detecting when to inflate the map > or list that is measurable in interpreted tests but disappears in JITed code. > > http://cr.openjdk.java.net/~mduigou/JDK-7143928/0/webrev/ > > We expect that should this code prove successful in Java 8 it will be > backported to Java 7 updates. > > The unit test may appear to be somewhat unrelated. It was created after > resolving a bug in an early version of this patch to detect the issue > encountered (LinkedHashMap.init() was not being called in readObject() when > the map was empty). > > Mike
