jcmd - executing multiple commands at the same safepoint?

[Kirk Pepperdine]

The stacking at the safe point would be a huge win. Right now thread dump consistency can really confuse people as the tooling will show two threads owning the same lock at seemingly the same time. Reality, it’s just that people didn’t realize you get a safe point for each thread therefor there is motion in the system as you’re collecting the data.

As an aside, it’s amazing how many dev’s aren’t aware of jcmd. Just yesterday after my session at Devoxx I had someone ask me about using jfr from the command line, many in that session had not seen jcmd before. The feedback was, wow, this is very useful and I wished I had of known about it earlier.

— Kirk

> On May 10, 2018, at 7:50 AM, Thomas Stüfe <thomas.stuefe at gmail.com> wrote:
> 
> Hi Kirk,
> 
> On Thu, May 10, 2018 at 8:07 AM, Kirk Pepperdine
> <kirk.pepperdine at gmail.com> wrote:
>> 
>>> On May 10, 2018, at 7:04 AM, Thomas Stüfe <thomas.stuefe at gmail.com> wrote:
>>> 
>>> Oh... maybe. You are right. And I see commas are not good either since
>>> they are used in sub options (e.g. GC.class_stats).
>>> 
>>> Alternative ideas could be using brackets surrounding the
>>> command-and-options (e.g. '{''}'):
>>> 
>>> jcmd  { GC.class_stats columns=InstBytes,KlassBytes }  { VM.metaspace
>>> show-loaders }
>> 
>> are exposed braces safe in shells?
>>> 
>>> I think technically we would not even need delimiters, since the
>>> commands are as keywords limiting enough. E.g. one could just specify:
>>> 
>>> jcmd  GC.class_stats columns=InstBytes,KlassBytes VM.metaspace show-loaders
>> 
>> I’d vote for this form. Simple..
> 
> I think so too.
> 
> Note how it would influence the form of jcmd command files:
> 
> <command 1>
> <command 2>
> would execute two commands sequentially at different safepoints
> 
> <command 1> <command 2>
> would execute two commands at the same safepoint
> 
> Arguably though, if we can stack commands at the command line, the
> jcmd file gets less important.
> 
> I'll play around a bit and wait what others say.
> 
> Thanks, Thomas
> 
>> 
>> — Kirk
>> 
>>> 
>>> and that would be for jcmd too. It is more for the benefit of the user.
>>> 
>>> On Thu, May 10, 2018 at 7:54 AM, Kirk Pepperdine
>>> <kirk.pepperdine at gmail.com> wrote:
>>>> Awesome idea! Would the semicolon would be an issue for shell scripts?
>>>>> On May 10, 2018, at 6:52 AM, Thomas Stüfe <thomas.stuefe at gmail.com> wrote:
>>>>> 
>>>>> Hi all,
>>>>> 
>>>>> just asking around for opinions.
>>>>> 
>>>>> I am playing with the idea of bundling diagnostic commands to have
>>>>> them executed at the same safepoint, in order to get results
>>>>> consistent with each other.  E.g. a heapdump and a corresponding
>>>>> metaspace statistic.
>>>>> 
>>>>> Syntax wise, I think one could specify multiple commands in jcmd
>>>>> separated by comma or semicola:
>>>>> 
>>>>> jcmd <command> <option> <option>; <command 2> <option> <option>
>>>>> 
>>>>> What do you think, would that be a useful addition?
>>>>> 
>>>>> Thanks, Thomas
>>>> 
>>