RFR JDK-7143928 : (coll) Optimize for Empty ArrayList and HashMap
brian.goetz at oracle.com
Wed Mar 27 05:53:08 UTC 2013
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.
> 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).
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