jstack, profilers and other tools

[Egor Ushakov]

So does this mean that the older thread dumps way (jstack etc.) is now 
obsolete?
And we should switch our efforts to supporting the new way on jdk 19+?

-Egor

On 12.07.2022 10:36, Alan Bateman wrote:
> On 11/07/2022 19:59, Egor Ushakov wrote:
>> Thanks Alan!
>>
>> I've read jep and the user approach to virtual threads is still 
>> unclear to me:
>> on the one hand for writing the code they are made to look and feel 
>> like "regular" threads, but the approach for tooling seems to be very 
>> different
>> - virtual threads are separated almost everywhere and seems to 
>> require very different observability techniques.
>> I'm fine with supporting all kinds of approaches, just want to make 
>> sure that it will be clear for users how to approach this.
>> For me it is not yet clear.
>
> The threads are distinct and the mental model should be that they have 
> separate stack traces. It would be misleading for APIs or Java tooling 
> to show carrier frames in the virtual thread stack. There will be 
> native tools, esp. those that attach via /proc, that don't know about 
> virtual threads and there isn't much we can do about that.
>
> I think part of your mail is asking why the HotSpot thread dump 
> (triggered by ctrl-\, jcmd Thread.print or the older jstack tool) 
> doesn't include virtual threads. Virtual threads are just objects in 
> the heap and we decided early on that changing this to walk the heap 
> and print tens of thousands of threads would be not be helpful. It has 
> been modified in a small way to identify the threads that are used as 
> carriers but that's about it. The new thread dump that is added 
> includes both platform and virtual threads and is intended to be 
> parsed. Many virtual threads are likely to have the same stack trace 
> and so lends itself to deduplication for example.  In conjunction with 
> JEP 428, tooling will be able to observe the task hierarchy.
>
> Thanks for confirming that the sampling with JFR is working for 
> virtual threads. Hopefully JMC will be updated in time to support 
> virtual threads.
>
> -Alan
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