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

coleen.phillimore at oracle.com coleen.phillimore at oracle.com
Thu Mar 16 15:52:25 UTC 2017

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.

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

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