RFR(L): 8195097: Make it possible to process StringTable outside safepoint

[Robbin Ehn]

Hi Coleen,

On 2018-05-29 23:52, coleen.phillimore at oracle.com wrote:
> 
> Hi Robbin,
> 
> StringTable::weak_oops_do() functions seem unused.
> 
> Can you make StringTableDeleteCheck and StringTableDoDelete inherited from 
> StackObj for documentation purposes, and others in this file.

Done, I'll send new version to rfr mail.

Thanks, Robbin

> 
> I've reviewed this code at intervals in progress, so I think this looks good.  
> Reviewed by me.
> 
> thanks,
> Coleen
> 
> On 5/28/18 9:19 AM, Robbin Ehn wrote:
>> Hi all, please review.
>>
>> This implements the StringTable with the ConcurrentHashtable for managing the
>> strings using oopStorage for backing the actual oops via WeakHandles.
>>
>> The unlinking and freeing of hashtable nodes is moved outside the safepoint,
>> which means GC only needs to walk the oopStorage, either concurrently or in a
>> safepoint. Walking oopStorage is also faster so there is a good effect on all
>> safepoints visiting the oops.
>>
>> The unlinking and freeing happens during inserts when dead weak oops are
>> encountered in that bucket. In any normal workload the stringtable self-cleans
>> without needing any additional cleaning. Cleaning/unlinking can also be done
>> concurrently via the ServiceThread, it is started when we have a high ‘dead
>> factor’. E.g. application have a lot of interned string removes the references
>> and never interns again. The ServiceThread also concurrently grows the table if
>> ‘load factor’ is high. Both the cleaning and growing take care to not prolonging
>> time to safepoint, at the cost of some speed.
>>
>> Kitchensink24h, multiple tier1-5 with no issue that I can relate to this
>> changeset, various benchmark such as JMH, specJBB2015.
>>
>> Issue: https://bugs.openjdk.java.net/browse/JDK-8195097
>> Webrev: http://cr.openjdk.java.net/~rehn/8195097/v0/webrev/
>>
>> Thanks, Robbin
>