RFR JDK-7143928 : (coll) Optimize for Empty ArrayList and HashMap

[Peter Levart]

Hi Mike,

Have you considered the possibility to re-constitute the initial 
capacity from threshold and loadFactor?

Regards, Peter

On 03/27/2013 05:28 PM, Mike Duigou wrote:
> I started looking at crafty reuse of size but found too many direct references to size to attempt getting this right in the current iteration. Reusing size is definitely still available to someone who wants to dive in and prepare an implementation.
>
> Mike
>
> On Mar 27 2013, at 09:17 , Martin Buchholz wrote:
>
>> 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.duigou at 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
>>