RFR(S): 8087143: Reduce Symbol::_identity_hash to 2 bytes

[Yumin Qi]

Updated webrev at 03 again. Please let me know if it is OK.

Thanks
Yumin

On 6/24/2015 9:21 AM, Yumin Qi wrote:
> Aleksey and Ioi,
>
> On 6/24/15, 2:55 AM, Aleksey Shipilev wrote:
>> Hi Ioi,
>>
>> On 06/23/2015 10:28 PM, Ioi Lam wrote:
>>> I assume by "address" you mean
>>>
>>> ((unsigned)((this)>>  (LogMinObjAlignmentInBytes + 3)));
>> Yes.
>>
>>> I think for performance reasons we are storing a random number in the
>>> Symbol instead of computing the hashcode base on the string value.
>>> The intention of this RFE not to change that, but just use fewer
>>> random bits to accomplish the same goal. Yes, we are assuming a
>>> particular hashtable implementation, but I don't think we are going
>>> to change that any time soon.
>> ...
>>
>>> And if we ever change the hash function
>>> to heavily depend on the top 16 bits of the hash code, we can
>>> certainly revert back to a 32-bit random number.
>> This sounds awfully similar to introducing the technical debt.
>> Succumbing to this practice means you will have to make an avalanche
>> modification once you need to do a little change in hash table
>> implementation, given somebody actually remembers to change the hash
>> function. I'm not keen of introducing these dependencies without a
>> *very* good reason to.
>>
>> In this example, if there is a hash function that returns a 32-bit
>> value, please make sure the value is random in all 32 bits. If that
>> proves hard, then the memory footprint savings (what, 1K out of 3626K,
>> if I read the data right?) does not worth the hassle of complicating the
>> hash function beyond reasonable, and introducing undocumented
>> dependencies that will eventually backfire.
> Currently the _identity_hash = os::random() which the random seed is 
> 1234567.
> I think to make it better to have more random, we need to have a seed 
> as 65535? Since 0 is same as 65536 with a short and so on.
>
> I'm not familiar with the random algorithm (os:random())--- suppose it 
> will generate a random between 0 and _rand_seed?
>
>
> Thanks
> Yumin
>
>> Thanks,
>> -Aleksey