RFR 8200696 : Optimal initial capacity of java.lang.Class.enumConstantDirectory
david.holmes at oracle.com
Wed Apr 4 01:41:58 UTC 2018
On 4/04/2018 10:01 AM, Ivan Gerasimov wrote:
> Hi David!
> On 4/3/18 4:49 PM, David Holmes wrote:
>> Hi Ivan,
>> On 4/04/2018 9:22 AM, Ivan Gerasimov wrote:
>>> Yet another occurrence of not-optimally pre-sized HashMap.
>>> When java.lang.Class.enumConstantDirectory is created, the initial
>>> capacity is set to be (2 * universe.length), which is more than
>>> necessary in some cases.
>>> Choosing the capacity optimally will allow us to save a few bytes
>>> with some enum classes.
>> How are you defining optimal?
> By optimal I meant the minimum value that will not make the internal
> storage to be reallocated during insertion.
> Here's a quotation from the documentation :
> "If the initial capacity is greater than the maximum number of entries
> divided by the load factor, no rehash operations will ever occur."
> Given the default load factor 0.75, the formula (expectedSize / 0.75 +
> 1) gives us the optimal initial capacity.
Isn't it simpler to just do:
new HashMap(universe.length, 1.0f);
and change the load factor? Otherwise you have to allow for an extra 25%
of space, and that is before it expands to be a power of 2 IIUC.
> The testlibrary class testlibrary/OptimalCapacity.java is used to make
> sure that the chosen initial capacity is neither too low (so that no
> reallocations happen) nor too high (no memory is wasted).
> With kind regards,
>  https://docs.oracle.com/javase/10/docs/api/java/util/HashMap.html
>> The bug report only gives one example where the current calculation
>> produces a too large value. If the HashMap itself rounds up the
>> initial capacity to a power of 2, how much extra space do we need
>> compared to universe.length? Do we just need to pass
>> (universe.length+1) to get the smallest power of 2 greater than
>>> Would you please help review this trivial fix?
>>> BUGURL: https://bugs.openjdk.java.net/browse/JDK-8200696
>>> WEBREV: http://cr.openjdk.java.net/~igerasim/8200696/00/webrev/
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