RFR: 8357828: Add a timestamp to jcmd diagnostic commands [v2]

[Ivan Bereziuk]

On Tue, 20 Jan 2026 08:57:52 GMT, Ivan Bereziuk <duke at openjdk.org> wrote:

>> `jcmd` provides great diagnostics but many commands lack a timestamp in their output.
>> Adding a timestamp to the output of some would add value for those debugging JVM data.
>> 
>> Some diagnostic commands already provide timestamps. For example `Thread.print` already prints one of "yyyy-MM-dd HH:mm:ss" format.
>> 
>> With this MR I propose to introduce time-stamping to all diagnostic `jcmd` commands in a form of an additional common flag "-T":
>> 
>> jcmd [pid | main-class] [-T] command... | PerfCounter.print | -f filename
>>                         ^^^^
>> 
>> * The choice for time format is "yyyy-MM-dd HH:mm:ss" as it is already used in `Thread.print`.
>> * `Thread.print` prints timestamp irrespectively from "-T" flag presence to preserve backwards compatibility.
>> 
>> I haven't added a timestamp to the following diagnostic command:
>> * `VM.uptime` - command run with `-date` argument will also print a timestamp;
>> * `VM.system_properties` - as the output already lists a timestamp. Not sure if we need more timestamps here.
>> * `Thread.dump_to_file` - the content dumped to a file already has a timestamp;
>
> Ivan Bereziuk has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 15 additional commits since the last revision:
> 
>  - remove TimeStamp::No as it's not used. virtual should be flipped to override in bulk (afressed clang warning)
>  - Merge branch 'master' into 8357828_add_timestamp_to_jcmd
>  - changes to jcmd.md
>  - undo changes to reorder_help_cmd()
>  - cleanup
>  - add timestamp to VM.version. Add test
>  - updated jcmd usage text
>  - undo the changes to the modified earlier tests as timestamp presence is fully backwards compatible
>  - introduce -T as a commong flag
>  - Merge branch 'master' into 8357828_add_timestamp_to_jcmd
>  - ... and 5 more: https://git.openjdk.org/jdk/compare/e1bc2481...5f1cefe0

I've made timestamping optional with "-T" flag. The flag is part of the command sent to the target JVM.

jcmd [pid | main-class] [-T] command... | PerfCounter.print | -f filename
                        ^^^^

The target diagnostic command handler can decide how to act on it. 

@dholmes-ora 
> ... what form should this timestamp take? Shouldn't it be configurable to allow matching with what may be presented by Unified Logging e.g. VM uptime rather than wall-clock time?

> And rather than add timestamp printing code to each dcmd it would make more sense to me to have a global dcmd flag that says "print a timestamp" (with a given format).

`Thread.print` already prints a timestamp, hence explicit "-T" has no effect for this particular command. This also affected my choice for using format "yyyy-MM-dd HH:mm:ss". At least as a default.
Do you think we need to be able changing the timestamp format (say "-T=UTC")?

Should we add time-stamping to STDOUT from commands that are of "action" type? E.g. "GC.run".

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PR Comment: https://git.openjdk.org/jdk/pull/27368#issuecomment-3772022851