But you can support any requested initial size if stored in the size field when list is empty.
On Wed, Mar 27, 2013 at 8:02 AM, Mike Duigou <mike.dui...@oracle.com> wrote: > This seems like a good idea. I will follow up with the performance people > to see if their findings include the requested initial size. > > Mike > > On Mar 26 2013, at 22:53 , Brian Goetz wrote: > > > 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 > > > >
