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

Ivan Gerasimov ivan.gerasimov at oracle.com
Thu Jul 3 18:29:31 UTC 2014

Thanks Martin!

On 03.07.2014 21:12, Martin Buchholz wrote:

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.

More precisely here n == 0, because the capacity gets doubled when increased.

The formula cap = expSize * 3 / 2 is used to estimate the capacity based on the expected number of the items to be inserted.

Sincerely yours, Ivan

On Thu, Jul 3, 2014 at 9:00 AM, Ivan Gerasimov <ivan.gerasimov at oracle.com <mailto: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/ <http://cr.openjdk.java.net/%7Eigerasim/6904367/0/webrev/> Sincerely yours, Ivan

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