RFR: 8359110: Log accumulated GC and process CPU time upon VM exit [v5]

[Jonas Norlinder]

On Mon, 16 Jun 2025 07:40:27 GMT, David Holmes <dholmes at openjdk.org> wrote:

> This seems reasonable though I'm unclear on some details. Could you give a
> high-level description of what times we capture for what threads and when, and
> the calculations involved. Thanks.

What threads:

* All GC threads e.g. director thread, sample threads, worker threads, driver
  threads (except for ZGC where I currently exclude the "runtime" threads which
  we may decide to change, see @tschatzl comments above)

* The VM thread, but only include VM operations related to GC

* The string deduplication thread

What times:

* CPU time for all the threads above

* CPU time for the entire process


When:

* VM thread:

  - If VM operation is GC and `-Xlog:gc` or more is enabled, sample `start`, and `end` CPU time
    for the GC operation. Add `end - start` to `gc_vm_vtime`.

* GC threads:

  - If `-Xlog:gc` or more is enabled, sample all GC threads and store the CPU
    time sum to `gc_threads_vtime` during VM exit.

* String deduplication thread:

  - If string deduplication is enabled and if `-Xlog:gc` or more is enabled
    sample CPU time during VM exit for that thread and store to `string_deduplication_vtime`

* Process:

  - If  `-Xlog:gc` or more is enabled, sample the CPU time for the entire process

Log accumulated CPU time `gc_threads_vtime + gc_vm_vtime + string_deduplication_vtime`
as a percentage of the total process CPU time. We sample the total process CPU 
time with the new method `os::elapsed_process_vtime`.

It should be noted from the above that calling elapsed_gc_vtime assumes that
-Xlog:gc or more is enabled. If one breaks this assumption the GC CPU time will
not include VM thread and may be horribly wrong (e.g. if one runs Serial).

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PR Comment: https://git.openjdk.org/jdk/pull/25779#issuecomment-2976252452