jstack, profilers and other tools

[Alan Bateman]

On 11/07/2022 18:37, Egor Ushakov wrote:
> Hi all,
>
> I'm trying to prepare IntelliJ for loom and have some trouble 
> understanding how tooling should be modified:
> 1. with debugger it is more or less ok - virtual threads are separated 
> from carrier threads, stack are separate and debugger is responsible 
> for showing all of this.
> 2. thread dumps (jstack as an example) - no virtual threads are shown, 
> carrier threads stacks are truncated even if they are doing some work 
> in the mounted virtual threads. It is not clear for me how users 
> should understand what (even mounted) virtual thread are doing. Should 
> we always switch to the new json format? Should user decide on which 
> format to use? Previously thread dumps way an easy way to grab "what 
> the app is doing" at the moment. Is there a way to achieve this now? 
> Or this way should be abandoned?

The stack trace of the carrier and the virtual thread are intentionally 
separate. So if a virtual thread throws an exception then you won't see 
the carrier stack traces. It's the same thing with thread dumps: the 
stack trace of a carrier thread won't show the stack frames of a mounted 
virtual thread, and vice-versa. There is more on this in the JEP.

The HotSpot thread dump (jcmd Thread.print, jstack) only shows platform 
threads (the threads that the VM knows about). The new thread dump (with 
the HotSpotDiagnisticMXBean API or jcmd Thread.dump_to_file) will print 
both platform and virtual threads. The plain text and JSON formats can 
be used to get a view of what the application is doing. The JSON format 
is intended to be parsed of course.

> 3. Profilers - with jfr I was not able to see any sampling data for 
> virtual threads, where should I find it? 

I'm not aware of any issues. I checked with Markus Grönlund (works on 
JFR) and he's not aware of any issues either. What tool are you are 
using to look at the recording? Do you see the virtual threads when you 
use `jfr print --events JDK.ExecutionSample <recording>`?

-Alan