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From Tim Ellison <t.p.elli...@gmail.com>
Subject Re: Performance improvement of java.util.HashMap (Was Re: [jira] Commented: (HARMONY-4064) [classlib][luni] Performance improvement of java.util.HashMap)
Date Thu, 28 Jun 2007 08:58:23 GMT
Jimmy,Jing Lv wrote:
> 2007/6/28, Tim Ellison <t.p.ellison@gmail.com>:
>> Jimmy,Jing Lv wrote:
>> > One thought is right-shift the hashcode and combine bits together (by
>> > AND or OR or Exclusive Or operation) into low order(e.g, (a & (a >>
>> > 8) ^ (a >> 16) | (a >> 24)) & (bucketSize-1), a small test on
it
>> > shows that it does distributed more randomly, but it may be better
>> > algorithms.
>>
>> Why so complex bit shifting?  How did you settle on this?
>>
>> If you are optimizing for the identity hashcode case then you can
>> probably ignore the bottom 'y' bits due to object alignment (and top 'z'
>> bits for address range limits), so just do (a >> 8) & (bucketSize - 1)
>> assuming the objects are 8-bit aligned.
> 
> It sounds interesting and reasonable :)
> But I'm afraid not all hashcode() returns the object's memory
> address,

Understood.  The hashCode() can return any int value, so in principle
you have a random set of 32 bits an no bit manipulation is required.

However, in practice I think we can optimize for typical hashcode
values.  The debate is what is 'typical'?  What is the most common type
of HashMap key? (then we can look at it's hashCode implementation).

> in some other cases, the difference of hashcodes may remain
> in buttom 'y' bits or top 'z' bits, at that time our HashMap will
> perform very slow.

You are right, I should not have thrown away those top bits.  So how
about a *rotate* right by eight bits operation?

> So the best way is to use all 32 bit of a hashcode, and take extra
> operator '|' and '^' does not cost much time.( though I'm not sure if
> the the right-shift paramter '8' is proper)

The operation you suggested was
  a & (a >> 8) ^ (a >> 16) | (a >> 24)

can you please explain how you came up with this?

Regards,
Tim

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