On Tue, 30 Sep 2025 04:21:44 GMT, Bernd <[email protected]> wrote: >> Hi all, >> >> This PR augments the CPU time sampling measurement capabilities that a user >> can perform from Java code with the addition of >> `MemoryMXBean.getGcCpuTime()`. With this patch it will be possible for a >> user to measure process and GC CPU time during critical section or >> iterations in benchmarks to name a few. This new method complements the >> existing `OperatingSystemMXBean.getProcessCpuTime()` for a refined >> understanding. >> >> `CollectedHeap::gc_threads_do` may operate on terminated GC threads during >> shutdown, but thanks to JDK-8366865 by @walulyai we can piggyback on the new >> `Universe::is_shutting_down`. I have implemented a stress-test >> `test/jdk/java/lang/management/MemoryMXBean/GetGcCpuTime.java` that may >> identify reading CPU time of terminated threads. Synchronizing on >> `Universe::is_shutting_down` and `Heap_lock` resolves this problem. >> >> FWIW; To my understanding we don't want to add a >> `Universe::is_shutting_down` check in gc_threads_do as this may introduce a >> performance penalty that is unacceptable, therefore we must be careful about >> the few places where external users call upon gc_threads_do and may race >> with a terminating VM. >> >> Tested: test/jdk/java/lang/management/MemoryMXBean/GetGcCpuTime.java, >> jdk/javax/management/mxbean hotspot/jtreg/vmTestbase/nsk/monitoring on Linux >> x64, Linux aarch64, Windows x64, macOS x64 and macOS aarch64 with release >> and fastdebug. > > src/java.management/share/classes/java/lang/management/MemoryMXBean.java line > 273: > >> 271: * Returns the CPU time used by all garbage collection threads. >> 272: * >> 273: * <p> This include time since genesis, so the value can be > > The “so the” is not too obvious to me. Maybe simplify it a bit “This includes > time since genesis and counts activities even before the first collection > cycle.”?
Thanks @ecki for pointing that out :) I make a simplification whenever the CSR discussion (https://bugs.openjdk.org/browse/JDK-8368529) have reached a consensus where this method should live. > src/java.management/share/classes/java/lang/management/MemoryMXBean.java line > 279: > >> 277: * deduplication thread (if enabled). This method returns >> 278: * {@code -1} if the platform does not support this operation >> 279: * or if called during shutdown. > > Can we explicitly state here that this is concurrent time as well as worker > time spent during pauses but not accumulated pause times? Just want to ensure that I read this correct, please correct any mistake below: * concurrent time = `CPU time` * accumulate times = `wall-clock time` Is your suggestion that we should clarify that the method may only account for `CPU time` and not `wall-clock time` during pauses? ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/27537#discussion_r2390466153 PR Review Comment: https://git.openjdk.org/jdk/pull/27537#discussion_r2390453810
