RFR (M): 8027476: Improve performance of Stringtable unlink, 8027455: Improve symbol table scan times during gc pauses

[Thomas Schatzl]

Hi Bengt,

On Mon, 2014-01-13 at 09:11 +0100, Bengt Rutisson wrote:
> Hi Thomas,
> 
> Looks good. Great work!

Thanks for the review.

> 
> A couple of minor things. I'll leave it up to you if you want to do any 
> changes. I'm fine with the patch as it is.
> 
> 
> G1TraceStringSymbolTableScrubbing is now experiemental. It looks like it 
> is more common to have Trace flags be diagnostic. Do you mind changing 
> to diagnostic?

I would soon like to change this to more detailed output like we do for
update RS in the young GC pause. I thought that keeping stuff
experimental helps to be able to remove the flag later :)

> g1CollectedHeap.cpp
> 
> 5248         StringTable::unlink(_is_alive, &strings_processed, 
> &strings_removed);
> 5249         _strings_processed = strings_processed;
> 5250         _strings_removed = strings_removed;
> 
> Maybe the single threaded case could do?
> 
> StringTable::unlink(_is_alive, &_strings_processed, &_strings_removed);
> 
> Then these local variables could move in inside the 
> G1CollectedHeap::use_parallel_gc_threads() check.
> 
> 5230     size_t strings_processed = 0;
> 5231     size_t strings_removed = 0;
> 5232     size_t symbols_processed = 0;
> 5233     size_t symbols_removed = 0;

That unfortunately does not work, because we need jlong type variables
for the Atomic operations.

However, while looking at Mikael's suggestions I found that the use of
size_t in the change does not give any advantage (as the main limitation
is the underlying hashmap that only allows int typed sizes) except for
too many casts.

I changed this in http://cr.openjdk.java.net/~tschatzl/8027454/webrev.1/
to use int's for the sizes.

Thanks,
  Thomas