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

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 * (2n + 1) / (2n)

Here n == 1.

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

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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