On 03/28/2013 06:38 PM, Mike Duigou wrote:
We have heard back from the performance folks that 85% of empty lists are
created at default size. The proposed patch is going to be revised to do the
inflation trick only for default sized maps which will eliminate the need for a
new field. Something creative involving the use of the existing size field can
certainly be considered for a future optimization.
This is actually a better approach, since it leaves a gate to non-lazy
allocation. As with all lazy things, delaying failure can have
undesirable effects. Catching OOME at HashMap construction vs. 1st put...
Regards, Peter
On Mar 28 2013, at 04:59 , Doug Lea wrote:
On 03/27/13 12:17, Martin Buchholz wrote:
But you can support any requested initial size if stored in the size field
when list is empty.
In other words: Mike, please do not add a field if your goal is
to save space!
Part of the analysis was that with current object layouts the added fields did
not change the memory footprint of either class. This, of course only applies
to current versions of Hotspot and it's object layout.
Also, I hope you or the performance testing folks
have extensive and accurate enough tests to show that the change
not only helps the empty case but does not hurt the vastly more
common non-empty case.
They do. I believe that the object layout hides any increase in size.
Considering that this is the most
commonly used java.util class, there should be empirical
evidence that this is a net improvement.
I will try to find how much of the analysis I can share.
My guess is that it can be done (we did something similar for
ConcurrentHashMap) but it takes a lot of care.
-Doug
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
