RFR: 8268857: Merge three vm operations in thread_dump

[David Holmes]

On Wed, 16 Jun 2021 03:33:14 GMT, Denghui Dong <ddong at openjdk.org> wrote:

> Hi,
> 
> Could I have a review of this change that merges three vm operations(VM_PrintThreads, VM_PrintJNI, VM_FindDeadlocks) in thread_dump and signal_thread_entry.
> 
> `jstack` is a very common command, even in the production environment.
> 
> In addition to reduce the cost of entering safepoint, I think this patch also could ensure the consistency of the results of VM_PrintThreads and VM_FindDeadlocks.

Sorry but I do not agree with this change. I understand your rationale but you have overlooked that by combining these three safepoint operations you have created a much longer safepoint pause which may lock out other more important safepoint operations, and you may also be holding the HeapLock across this extended safepoint.

The operations that you have coalesced are not considered time critical but can themselves be very time consuming e.g deadlock detection.

David

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PR: https://git.openjdk.java.net/jdk/pull/4504