What's the purpose of hashtable::reverse()

[Mikael Gerdin]

Hi Coleen,

On 2017-03-16 16:52, coleen.phillimore at oracle.com wrote:
>
>
> On 3/16/17 6:20 AM, Ioi Lam wrote:
>> Thanks Mikael for the detective work. The bug comment says:
>>
>> "the String and symbol tables are hashtables where the hashbuckets
>> are linked lists -- in reverse chronological order of there creation.
>> This
>> means that, in some circumstances, some symbols are unnecessarily touched
>> when looking for older most likely needed symbols. This can be
>> addressed by
>> simply reversing the order of the lists before creating the archive."
>>
>> Since JDK9, CDS no longer stores the string and symbol tables using
>> the "regular" hashtable.
>>
>> The only "regular" hashtable use by CDS is the shared dictionary (for
>> name->class lookup), but this table is rehashed at some point, so
>> calling reverse() will no longer has the desired effect of moving
>> popular Klasses (such as java/lang/Object) to the front of a linked-list.
>>
>> So this optimization is irrelevant now. I've created an RFE to remove it.
>
> Great, thank you!
>
> More optimizations that we have no means of measurement.

That was stated in the evaluation of this issue some 13 years ago (time 
flies ;)

> ###@###.### 2004-02-13
> The measured performance benefit of these changes is less than expected and
> it will be difficult to justify changes to tiger this late without better
> numbers.

/Mikael

>
> Coleen
>>
>> https://bugs.openjdk.java.net/browse/JDK-8176863
>>
>> Thanks
>> - Ioi
>>
>> On 3/16/17 5:17 PM, Mikael Gerdin wrote:
>>> Hi Ioi,
>>>
>>> I found this old bug where I think the reversing was added:
>>> https://bugs.openjdk.java.net/browse/JDK-4994483
>>>
>>> /Mikael
>>>
>>> On 2017-03-16 07:48, Ioi Lam wrote:
>>>> This seems to be used only by CDS dumping, and if I remove the call
>>>> nothing bad seems to happen. Does anyone know what hashtable::reverse()
>>>> could possibly achieve?
>>>>
>>>> Also, there's another variant that seems to be dead code that no one
>>>> calls.
>>>>
>>>>     template <class T, MEMFLAGS F> void Hashtable<T, F>::reverse(void*
>>>> boundary)
>>>>
>>>> - Ioi
>>>>
>>>>
>>
>