[PATCH] JFR thread sampling issue

[Milan Mimica]

Hi Markus

I'm gonna try to resurrect thins (once again).

Did you get a chance to check in more details? At a glance, the code takes
care not to wrap around the thread list.



On Fri, 30 Nov 2018 at 15:21, Markus Gronlund <markus.gronlund at oracle.com>
wrote:

> Hi Milan,
>
> Thanks for pointing to this and providing a patch suggestion.
>
> Overall it looks reasonable, but I need to take a closer look in regards
> to how longer sample sessions interplays with the thread smr list - we
> should ensure that an attempt does not wrap around the thread list.
>
> Sorry for the delay on this issue - your overall suggestion will be
> incorporated in some form, but we need to be careful that it does not have
> hidden side effects. I will get back to you on this.
>
> Thanks again
> Markus
>
> -----Original Message-----
> From: Milan Mimica <milan.mimica at gmail.com>
> Sent: den 19 november 2018 13:42
> To: hotspot-jfr-dev at openjdk.java.net
> Subject: [PATCH] JFR thread sampling issue
>
> Hello
>
> I was regularly using JFR to profile my service and thread sampling
> results have been very useful. Unfortunately, when I have switched to Java
> 9 and afterwards, it stopped working. The recording would catch too few
> samples for the results to be useful. My service has about 500 threads,
> most of them being on some blocked state.
>
> With the release of Java 11, I have started digging into the JFR source
> code and I think I have found the problem. I think
> JfrThreadSampler::task_stacktrace is supposed to find at most 5 threads
> that are in state _thread_in_Java, or at most 1 thread that is in
> _thread_in_native, but instead it just picks next 5 (or 1) threads and then
> ignores them of they are not in right state. Without the insight into code
> change prior to JDK 11 I can just guess, but there are some clues that lead
> me to think that's how it was supposed to work. One of the clues is that
> sample_task.do_sample_thread returns a result that is otherwise unused.
>
> I'm attaching a patch.
> Tested test-jdk_jfr_sanity on fastdebug, and in my production.
>
> I'm waiting for my Author status approval so I'll be able to create a
> proper changeset. I believe I also need a ticket for this.
>
>
> --
> Milan Mimica
>


-- 
Milan Mimica