RFR: 8280784: VM_Cleanup unnecessarily processes all thread oops

[Aleksey Shipilev]

On Thu, 27 Jan 2022 14:35:08 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

> 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.

Looks fine, but doesn't that apply to other "empty" VMOps: `VM_None`, `VM_ForceSafepoint`, `VM_ThreadSuspend`, etc? Probably some code commoning is in order there...

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Marked as reviewed by shade (Reviewer).

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