RFR: 8319876: Reduce memory consumption of VM_ThreadDump::doit [v3]

[Long Yang]

> I would like to fix this.
> 
> Create 1000 threads, and the stack depth of each thread is 1000. 
> After running jmx.dumpAllThreads(true, true), the RSS reaches 5.2GiB. 
> After optimization, the RSS is 250MiB.
> 
> I would appreciate it if anyone could review this.

Long Yang has updated the pull request incrementally with one additional commit since the last revision:

  Use VMThread::vm_thread() to avoid the need to call Thread::current()

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16598/files
  - new: https://git.openjdk.org/jdk/pull/16598/files/0ee0e17d..b485290b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16598&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16598&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16598.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16598/head:pull/16598

PR: https://git.openjdk.org/jdk/pull/16598