RFR (M): 8217778: StringTable cleanup miscalculates amount of dead objects

[coleen.phillimore at oracle.com]

On 1/28/19 11:02 PM, Kim Barrett wrote:
>> On Jan 28, 2019, at 4:46 PM, coleen.phillimore at oracle.com wrote:
>>
>>
>> Hi Thomas,  Can you forward the review thread for JDK-8213229?
>>
>> I wonder if we could just trigger_unloading after any GC has run and ignore dead string counts.  Are they ever zero?
> We’re trying to only trigger the ServiceThread’s StringTable cleanup when there
> are enough nulled-out entries to make it worth the effort.  My guess would be that
> interned strings rarely die young.  In which case that approach would probably
> trigger the ServiceThread unnecessarily frequently.  Maybe instead avoid the
> counting and unconditionally trigger after a (possibly concurrent) full collection?
> That doesn’t do so well for the (possibly rare?) application that does frequently
> intern and then drop strings though.  Or don’t trigger the ServiceThread more
> often than some (perhaps configurable?  Ugh!  A new CLOption) frequency.

I definitely don't want another command line option, but triggering 
after a collection that might do class unloading makes sense.  But yeah, 
some cases might have lots of interned strings outside of class 
loading.  I thought that the stringTable insert code noted dead entries 
on the way, but that's only used by the symbolTable.
>
> Also, I wonder if the SymbolTable has the same miscalculation.  The SymbolTable
> code was somewhat cribbed from StringTable, after all.
>

Yes, the SymbolTable code was similar but the Symbols are 90-99% brought 
in by class loading, so triggering during unloading unconditionally make 
sense there.   And even when we counted, if there was 1 Symbol free, 
we'd clean the table.  It's concurrent, and hasn't been seen to cause 
undo slowdown on the applications we've benchmarked, so this is good.

See:  8213753: Move timing of ProtectionDomainTable and 
ResolvedMethodTable cleanup to service thread

I should split this RFE because this also moves SymbolTable cleaning to 
only trigger during unloading.

Coleen