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Fix MpscLinkedQueue GC issues #7799

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Fix MpscLinkedQueue GC issues #7799

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Commits on Nov 21, 2024

  1. Refactor MpscLinkedQueue.poll 

    Handle empty queue first, then share most of the
implementation for non-empty scenarios (spin and
non-spin).
    @olivergillespie
    olivergillespie committed Nov 21, 2024
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  2. Unlink dead nodes in MpscLinkedQueue 

    Similar to
https://github.com/JCTools/JCTools/blob/master/jctools-core/src/main/java/org/jctools/queues/MpscLinkedQueue.java#L120,
null out the next pointer in the discarded consumer node
when polling from the queue. If not, we leave behind a (potentially long)
chain of connected garbage nodes. If we're unlucky (for example one of
the early nodes is promoted to old generation, triggering nepotism),
this can cause GC issues as now we have a long linked list which must be
marked by young collections.

Reproducer:

```
import io.reactivex.rxjava3.internal.queue.MpscLinkedQueue;

public class MpscLinkedQueueGC {
    public static void main(String[] args) {
        MpscLinkedQueue<Integer> queue = new MpscLinkedQueue<>();
        for (int i = 0; i < 10; i++) System.gc(); // tenure consumer node
        while (true) {
            queue.offer(123);
            queue.poll();
        }
    }
}
```

```
Before fix:

$ java -Xlog:gc -Xmx1G -cp build/classes/java/main MpscLinkedQueueGC.java
...
[1.261s] GC(20) Pause Young (Normal) (G1 Preventive Collection) 115M->115M(204M) 209.335ms
[1.385s] GC(23) Pause Young (Normal) (G1 Evacuation Pause) 148M->149M(204M) 31.491ms
[1.417s] GC(24) Pause Young (Normal) (G1 Evacuation Pause) 157M->158M(204M) 19.333ms
[1.453s] GC(25) Pause Young (Normal) (G1 Evacuation Pause) 166M->167M(599M) 22.678ms
[1.966s] GC(26) Pause Young (Normal) (G1 Evacuation Pause) 249M->249M(497M) 305.238ms
...

After fix:
$ java -Xlog:gc -Xmx1G -cp build/classes/java/main MpscLinkedQueueGC.java
...
[1.169s] GC(14) Pause Young (Normal) (G1 Evacuation Pause) 304M->2M(506M) 0.755ms
[1.558s] GC(15) Pause Young (Normal) (G1 Evacuation Pause) 304M->2M(506M) 0.689ms
[1.948s] GC(16) Pause Young (Normal) (G1 Evacuation Pause) 304M->2M(506M) 0.800ms
[2.337s] GC(17) Pause Young (Normal) (G1 Evacuation Pause) 304M->2M(506M) 0.714ms
...
```
    @olivergillespie
    olivergillespie committed Nov 21, 2024
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  3. Revert "Unlink dead nodes in MpscLinkedQueue" 

    This reverts commit 4c6c6fa.
    @olivergillespie
    olivergillespie committed Nov 21, 2024
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  4. Revert "Refactor MpscLinkedQueue.poll" 

    This reverts commit b8582b9.
    @olivergillespie
    olivergillespie committed Nov 21, 2024
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  5. Unlink dead nodes in MpscLinkedQueue 

    Similar to
https://github.com/JCTools/JCTools/blob/master/jctools-core/src/main/java/org/jctools/queues/MpscLinkedQueue.java#L120,
null out the next pointer in the discarded consumer node
when polling from the queue. If not, we leave behind a (potentially long)
chain of connected garbage nodes. If we're unlucky (for example one of
the early nodes is promoted to old generation, triggering nepotism),
this can cause GC issues as now we have a long linked list which must be
marked by young collections.

Reproducer:

```
import io.reactivex.rxjava3.internal.queue.MpscLinkedQueue;

public class MpscLinkedQueueGC {
    public static void main(String[] args) {
        MpscLinkedQueue<Integer> queue = new MpscLinkedQueue<>();
        for (int i = 0; i < 10; i++) System.gc(); // tenure consumer node
        while (true) {
            queue.offer(123);
            queue.poll();
        }
    }
}
```

```
Before fix:

$ java -Xlog:gc -Xmx1G -cp build/classes/java/main MpscLinkedQueueGC.java
...
[1.261s] GC(20) Pause Young (Normal) (G1 Preventive Collection) 115M->115M(204M) 209.335ms
[1.385s] GC(23) Pause Young (Normal) (G1 Evacuation Pause) 148M->149M(204M) 31.491ms
[1.417s] GC(24) Pause Young (Normal) (G1 Evacuation Pause) 157M->158M(204M) 19.333ms
[1.453s] GC(25) Pause Young (Normal) (G1 Evacuation Pause) 166M->167M(599M) 22.678ms
[1.966s] GC(26) Pause Young (Normal) (G1 Evacuation Pause) 249M->249M(497M) 305.238ms
...

After fix:
$ java -Xlog:gc -Xmx1G -cp build/classes/java/main MpscLinkedQueueGC.java
...
[1.169s] GC(14) Pause Young (Normal) (G1 Evacuation Pause) 304M->2M(506M) 0.755ms
[1.558s] GC(15) Pause Young (Normal) (G1 Evacuation Pause) 304M->2M(506M) 0.689ms
[1.948s] GC(16) Pause Young (Normal) (G1 Evacuation Pause) 304M->2M(506M) 0.800ms
[2.337s] GC(17) Pause Young (Normal) (G1 Evacuation Pause) 304M->2M(506M) 0.714ms
...
```
    @olivergillespie
    olivergillespie committed Nov 21, 2024
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