RFR: 8170299: Debugger does not stop inside the low memory notifications code

[David Holmes]

Hi Daniil,

On 4/07/2019 1:04 pm, Daniil Titov wrote:
> Please review the change the fixes the problem with the debugger not stopping in the low memory notification code.
> 
> The problem here is that the ServiceThread that calls these MXBean listeners is hidden from the external view that prevents the debugger from stopping in it.
> 
> The fix introduces new NotificationThread that is visible to the external view and offloads the ServiceThread from sending low memory and other notifications that could result in Java calls ( GC and diagnostic commands notifications) by moving these activities in this new NotificationThread.

There is a long and unfortunate history with this bug.

The original incarnation of this fix was introducing a new thread at the 
Java library level, and I had some concerns about that:

http://mail.openjdk.java.net/pipermail/serviceability-dev/2017-December/022612.html

That effort was resurrected at:

http://mail.openjdk.java.net/pipermail/serviceability-dev/2018-July/024466.html

and

http://mail.openjdk.java.net/pipermail/serviceability-dev/2018-August/024849.html

but was left somewhat in limbo. There was a lot of doubt about the right 
way to fix this bug and whether introducing a new thread was too disruptive.

But introducing a new thread in the VM also has the same set of 
concerns! This needs consideration by the runtime team before going 
ahead. Introducing a new thread likes this needs to be examined in 
detail - particularly the synchronization interactions with other 
threads. It also introduces another monitor designated safepoint-never 
at a time when we are in the process of cleaning up monitors so that 
JavaThreads will only use safepoint-check-always monitors.

Unfortunately I'm about to head out for two weeks vacation, and a number 
of other key runtime folk are also on vacation. but I'd ask that you 
hold off on this until we can look at it in more detail.

Thanks,
David
-----

> Testing: Mach5 tier1,tier2 and tier3 tests succeeded.
> 
> Webrev: https://cr.openjdk.java.net/~dtitov/8170299/webrev.01/
> Bug: https://bugs.openjdk.java.net/browse/JDK-8170299
> 
> Thanks!
> --Daniil
> 
>