RFR: [6904367]: (coll) IdentityHashMap is resized before exceeding the expected maximum size

Martin Buchholz martinrb at google.com
Thu Jul 3 17:12:24 UTC 2014

Martin's law of expanding capacity:

Always grow by using the form

newCapacity = oldCapacity + oldCapacity >> n

for some suitable constant n.  This will be efficient and more overflow
resistant than the alternative

newCapacity = oldCapacity * (2**n + 1) / (2**n)

Here n == 1.

On Thu, Jul 3, 2014 at 9:00 AM, Ivan Gerasimov <ivan.gerasimov at oracle.com>

> Hello!
> IdentityHasMap has a couple of small issues.
> The first one is a performance issue: If you create a map, specifying 42
> as the expected number of element, you'll be able to insert only 41
> elements into the preallocated table. Inserting the 42th element will
> trigger resizing of the storage.
> Another issue is that resizing occurs only after the element insertion.
> In the extreme case it can lead to the situation when the element is
> successfully inserted and then en exception is thrown due the map been
> unable to grow.
> Would you please help review the fix?
> BUGURL: https://bugs.openjdk.java.net/browse/JDK-6904367
> WEBREV: http://cr.openjdk.java.net/~igerasim/6904367/0/webrev/
> Sincerely yours,
> Ivan

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