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

[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

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

Commit messages:
 - fix rehash test
 - jump resize

Changes: https://git.openjdk.java.net/jdk/pull/2769/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2769&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261859
  Stats: 45 lines in 2 files changed: 20 ins; 6 del; 19 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