Fix MpscLinkedQueue GC issues #7799
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Handle empty queue first, then share most of the implementation for non-empty scenarios (spin and non-spin).
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 ... ```
akarnokd
requested changes
Nov 21, 2024
This reverts commit 4c6c6fa.
This reverts commit b8582b9.
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 ... ```
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Updated with the minimal fix. |
Codecov ReportAll modified and coverable lines are covered by tests ✅
Additional details and impacted files@@ Coverage Diff @@
## 3.x #7799 +/- ##
============================================
- Coverage 99.62% 99.59% -0.03%
- Complexity 6801 6803 +2
============================================
Files 752 752
Lines 47707 47713 +6
Branches 6401 6402 +1
============================================
- Hits 47527 47519 -8
- Misses 84 89 +5
- Partials 96 105 +9 ☔ View full report in Codecov by Sentry. 🚨 Try these New Features:
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akarnokd
approved these changes
Nov 21, 2024
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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.
I noticed this in one of my applications, a heap dump showed an unreachable list of a few hundred thousand nodes all with null values.
Note: There are two commits here. One refactors poll() to (IMO) simplify the different cases, and the second actually fixes the GC issue. If preferred I can just fix the GC issue without the refactoring.Reproducer: