RFR: 8158871: Long response times with G1 and StringDeduplication

[Per Liden]

Hi,

Could I please have a couple of reviews in this bug fix.

Summary: When entries are pruned from the StringDedupTable, the actual 
freeing of those entries are done while both holding the 
StringDedupTable_lock and being joined to the SuspendibleThreadSet. This 
causes both String.intern() and safepoints to be blocked until the 
freeing is completed. As described in the bug report, this can cause a 
very noticeable hick up in application response times when a huge amount 
(millions) of entries are pruned at once. The patch fixes this by moving 
all freeing out from the StringDedupTable_lock and the 
SuspendibleThreadSet is only joined temporarily to bulk-extract the list 
of entries to free, which is an O(1) operation.

Webrev: http://cr.openjdk.java.net/~pliden/8158871/webrev.0/

Bug: https://bugs.openjdk.java.net/browse/JDK-8158871

Testing: All jtreg string dedup tests pass. Manually provoked the 
problematic condition and verified that the actual freeing of entries 
doesn't block safepoints or String.intern().

thanks,
Per