RFR: 8297638: Memory leak in case of many started-dead threads

[Jaikiran Pai]

On Thu, 19 Jan 2023 01:04:39 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> src/jdk.jdi/share/classes/com/sun/tools/jdi/VMState.java line 177:
>> 
>>> 175:     private void removeUnreachableListeners() {
>>> 176:         boolean found = false;
>>> 177:         while (listenersReferenceQueue.poll() != null) {
>> 
>> Hello Chris, I have no prior knowledge of this code, so this is just a casual observation - should this change here be a `if` instead of a `while`? I see that this `removeUnreachableListeners()` gets called whenever a new listener is being added in `addListener(...)`. With the use of `while`, is this intentionally waiting for the GC to kick in and add at least one reference object to this queue?
>
> No, because we have to clear the queue. This code could have been written as:
> 
>         boolean found = listenersReferenceQueue.poll();
>         while (listenersReferenceQueue.poll() != null);
> 
> But you are partly right in that `removeUnreachableListeners()` doesn't really do anything until after a GC has triggered adding some objects to the queue. When there is one, there are probably a bunch, and they'll all get handled by the code below the above code. If there are none on the queue, then this method doesn't really do anything and has almost no overhead.

Thank you for that explanation Chris. What you state makes sense. I originally misread that while construct.

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PR: https://git.openjdk.org/jdk/pull/12081