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September 2024

4 participants
2 discussions

RFC: ZGC: Remove Non-Generational Mode
by Axel Boldt-Christmas 27 Sep '24

27 Sep '24

Hi, I have written a draft JEP for removing the non-generational mode of ZGC. The JEP description is available in JBS: https://bugs.openjdk.org/browse/JDK-8335850 Comments and feedback are welcome. // Axel Boldt-Christmas

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Split Lock Warning with ZGC and -XX:-ClassUnloading on Linux x86_64, JDK 17.0.2
by jianping Lu 11 Sep '24

11 Sep '24

HI ALL: When running JDK 17.0.2 on a Linux x86_64 architecture with ZGC and the JVM option -XX:-ClassUnloading, I encounter split lock warnings from the Linux kernel. This issue appears consistently during garbage collection operations. Here is the specific warning message from the kernel: x86/split lock detection: #AC: ZWorker#0/2154775 took a split_lock trap at address: 0x7f50c6e0433c Upon investigating the assembly at this address, I identified the following instruction: 0x00007f50c6e0433c <+76>: lock cmpxchg %rcx,(%rbx) This is part of the function: Dump of assembler code for function _ZN15ZMarkOopClosure6do_oopEPP7oopDesc: 0x00007f50c6e0433c <+76>: lock cmpxchg %rcx,(%rbx) The split lock warning occurs during the execution of the ZWorker thread, which is responsible for concurrent marking in ZGC. The warning seems to be triggered specifically when class unloading is disabled with -XX:-ClassUnloading. Environment: JDK Version: OpenJDK 17.0.2 GC: ZGC with -XX:-ClassUnloading OS: Linux x86_64 I would like to understand if this behavior is expected when class unloading is disabled or if there are any recommended fixes or workarounds for avoiding the split lock issue during concurrent garbage collection.

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