RFR: 8184765: Dynamically resize SystemDictionary

[David Holmes]

Hi Karen,

On 13/10/2017 8:42 PM, Karen Kinnear wrote:
> A quick note - we are resizing the hash table, i.e. number of linked lists, so as to speed up lookup if it is too small, or waste less space if it is
> too large. Each dictionary may have multiple entries off of it - one per loaded class - so there is no such thing as running out
> of space based on the sizing (just based on total OOM). I have not yet looked at the change - but just wanted to clarify the
> goal and concept for folks first.

Thanks for clarifying - though AFAICS this proposal only grows the 
dictionary. Is the "waste less space" to be achieved by starting with a 
smaller dictionary initially? Do we have a way to control the initial size?

Thanks,
David

> thanks
> Karen
> 
>> On Oct 11, 2017, at 4:10 PM, coleen.phillimore at oracle.com wrote:
>>
>>
>> Gerard, some preliminary comments.
>>
>> http://cr.openjdk.java.net/~gziemski/8184765_rev1/src/hotspot/share/classfile/systemDictionary.cpp.udiff.html
>>
>> *+{*
>> *!_MutexLocker mu(SystemDictionary_lock, THREAD_);*
>> *!_Klass* probe = dictionary->find(_d_hash, name, protection_domain);*
>> *if (probe != NULL) return probe;*
>> **
>> *+ }*
>>
>> I don't think you need this because dictionary->find() should have the NoSafepointVerifier, so the index will not change.
>>
>> http://cr.openjdk.java.net/~gziemski/8184765_rev1/src/hotspot/share/classfile/classLoaderData.cpp.udiff.html
>>
>> I think we want a global _some_dictionary_needs_resizing to avoid walking through the CLDG.
>>
>> And have Dictionary have a field _resizing_needed to avoid the calculation during the safepoint, which is set when adding an entry.
>>
>> _resizing_needed = *(number_of_entries() > (_resize_load_trigger*table_size());
>> *CLDG::_any_resizing_needed |= _resizing_needed;   // or something like that.
>>
>> I can write more about the rationale of this change in the bug report, if needed.
>>
>> Thank you for doing this change.
>> Coleen
>>
>>
>> On 10/10/17 4:40 PM, Gerard Ziemski wrote:
>>> hi all,
>>>
>>> Please review this change that adds dynamic resizing of system dictionaries.
>>>
>>> The biggest change is refactoring of the code that protects calculating of the table entry’s index using SystemDictionary_lock
>>>
>>> A few notes:
>>>
>>> - dynamic resizing is off when either dumping or using shared spaces
>>> - we remove the experimental runtime option “PredictedLoadedClassCount” and add “DynamicallyResizeSystemDictionaries” to turn the resizing off (it’s on by default)
>>> - the jtreg test uses stream of bytes to dynamically load numbered classes from memory instead of disk (thank you Ioi)
>>>
>>> bug:    https://bugs.openjdk.java.net/browse/JDK-8184765
>>> webrev: http://cr.openjdk.java.net/~gziemski/8184765_rev1
>>>
>>>
>>> cheers
>>>
>>
>