RFR: 8261859: gc/g1/TestStringDeduplicationTableRehash.java failed with "RuntimeException: 'Rehash Count: 0' found in stdout" [v2]

[Kim Barrett]

> Please review this fix of an intermittently failing string deduplication test.
> 
> There are several problems.
> 
> (1) Because of environmental or other unrelated changes, the test might
> simply fail.  The test was considering it a failure if any GC reported a
> zero rehash count.  But if the first GC triggered a resize, that would
> suppress the requested "rehash a lot", and could report a zero rehash count,
> failing the test.  So the test criteria is changed to require at least one
> non-zero rehash count rather than no zero rehash counts.  Since rehashes are
> normally unlikely, and the primary point is to exercise the rehash code,
> having some reported non-zero rehash count is sufficient.
> 
> (2) Reporting only occurs if the string dedup thread was triggered and had
> work to do.  If the initial collections all need resizes, and none of the
> subsequent ones have any work for the thread to do, then again we won't have
> any reported rehashes.  The test is changed to always generate some new
> strings for later GCs to discover and queue for deduplication processing,
> causing the dedup thread to run and reporting at the end.
> 
> (3) The table resizing mechanism was only doing one step (doubling or
> halving) of size change per collection.  If the number of table entries is
> large (small), several GCs might be required for the table to grow (shrink)
> to the desired size.  Once again, this could suppress table rehashes,
> causing the test to fail.  It also wastes effort because the table needs to
> be resized multiple times when one right-sized resize would be sufficient.
> Resizing now computes the "final" size based on the number of entries and
> load factors, and may increase or decrease the table size by multiple powers
> of 2 in one resizing operation.
> 
> Testing:
> Manual execution of the string dedup tests and examining their logs.
> 
> Manual execution of the resize and rehash string dedup tests with a small
> initial table size, to similate an environment with a larger initial set of
> strings that triggers early resize.
> 
> mach5 tier1

Kim Barrett has updated the pull request incrementally with one additional commit since the last revision:

  limit shrink, add asserts for new size

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2769/files
  - new: https://git.openjdk.java.net/jdk/pull/2769/files/9af2f27c..d71efe36

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2769&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2769&range=00-01

  Stats: 10 lines in 1 file changed: 3 ins; 4 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2769.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2769/head:pull/2769

PR: https://git.openjdk.java.net/jdk/pull/2769