RFR: 8280784: VM_Cleanup unnecessarily processes all thread oops

[Stefan Karlsson]

While looking at ZGC latencies in a benchmark with >20000 Java threads, I noticed that the Cleanup VM operation could take up to 500 ms. It turned out that the time was spent processing the oops in all Java threads. Since none of the safepoint cleanup tasks use the oops in the threads, I propose that we stop processing the oops in this VM Operation.

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

Commit messages:
 - 8280784: VM_Cleanup unnecessarily processes all thread oops

Changes: https://git.openjdk.java.net/jdk/pull/7246/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7246&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280784
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7246.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7246/head:pull/7246

PR: https://git.openjdk.java.net/jdk/pull/7246