RFR 8200696 : Optimal initial capacity of java.lang.Class.enumConstantDirectory

[David Holmes]

Hi Ivan,

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:
>>> Hello!
>>>
>>> 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 [1]:
> "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.

David

> 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,
> Ivan
> 
> [1] 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 
>> universe.length?
>>
>> Thanks,
>> David
>>
>>> 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/
>>>
>>
>