RFR: 8307958: Metaspace verification is slow causing extreme class unloading times [v2]

[Axel Boldt-Christmas]

> From JBS:
>>We see very long class unloading times with Generational ZGC in fastdebug builds. MacOS spindumps show that it is caused by Metaspace verification code.
>>
>>I think the main problem is that many of the verification operations are performed at every operation (add, remove, split, merge) and the individual operation verifies much more than a constant number of chunks. This seems to give a quadratic complexity which blows up if you don't class unload often enough.
> 
> The approach here is to resolve this by putting the verification behind the SOMETIMES macro. It is then possible to turn it of completely with `-XX:VerifyMetaspaceInterval=0` while still benefiting from some stochastic verification. 
> 
> The goal here is to have the calls to Verify unchanged. Leaving all explicit gtest verify and Universe::verify calls unaffected.  
> The verification was also left unchanged when committing the memory. 
> 
> All SOMETIMES uses the same frequency, `VerifyMetaspaceInterval`. I looked at the possibility of adding weight to some of the call site, but it did not seem to be worth the complexity. 
> 
> Testing: GHA and Oracle CI (tier1-3 ongoing), various long running stress tests.

Axel Boldt-Christmas has updated the pull request incrementally with two additional commits since the last revision:

 - Always verify gtests -XX:VerifyMetaspaceInterval=1
 - Revert "Fix gtest"
   
   This reverts commit 5991e03fde88d9262778ea460067139b38ed497a.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14084/files
  - new: https://git.openjdk.org/jdk/pull/14084/files/5991e03f..5f0cf5a8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14084&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14084&range=00-01

  Stats: 6 lines in 2 files changed: 0 ins; 4 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/14084.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14084/head:pull/14084

PR: https://git.openjdk.org/jdk/pull/14084