RFR: 8308643: Incorrect value of 'used' jvmstat counter [v2]

[Kim Barrett]

> Please review this change that fixes the space-used performance counters
> provided by ParallelGC and SerialGC.
> 
> The prior fix for a problem in this area (JDK-8268265) wasn't correct.  It
> used a static "last value" variable, for use when sampling is blocked by an
> in-progress GC.  But there are multiple counters, so having one static "last
> value" variable doesn't work.  (Not sure what I and my reviewers were thinking
> at the time. Maybe we overlooked that there are multiple counters.)  The
> solution is to associate a "last value" with each of the counters.
> 
> Also added a test of basic functionality (just accessibility) of the space
> counters.  No regression test of the problem being fixed here, as its hard to
> reliably set up.
> 
> Note: SpaceCounters and CSpaceCounters are very nearly identical.  It's likely
> possible to refactor for more code sharing, which could be done as a followup.
> 
> Testing:
> mach5 tier1

Kim Barrett has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Merge branch 'master' into space-counters
 - test basic functionality
 - fix CSpaceCounters
 - fix SpaceCounters

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14195/files
  - new: https://git.openjdk.org/jdk/pull/14195/files/f8933db3..771d8599

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14195&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14195&range=00-01

  Stats: 104392 lines in 1769 files changed: 77753 ins; 18652 del; 7987 mod
  Patch: https://git.openjdk.org/jdk/pull/14195.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14195/head:pull/14195

PR: https://git.openjdk.org/jdk/pull/14195