On Sat, 27 Sep 2025 11:18:58 GMT, Jonas Norlinder <[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.
Thanks, it’s a useful function. (I would not have expected it in the memory
bean but in the go bean, but I guess both is fine.) Some more comments inline
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.”?
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?
src/java.management/share/classes/java/lang/management/MemoryMXBean.java line
282:
> 280: *
> 281: * @return the total CPU time for all garbage collection
> 282: * threads in nanoseconds.
Not sure did I miss the discussion, other methods like
getTotalCompilationTime() return millis, is it ok or required to use new units
here?
test/jdk/java/lang/management/MemoryMXBean/GetGcCpuTime.java line 73:
> 71: while (true) {
> 72: long gcCpuTimeFromThread =
> mxMemoryBean.getGcCpuTime();
> 73: if (gcCpuTimeFromThread < -1) {
None of the tests actually test if it is ever != -1 or if it is monotonically
increasing or under which conditions (go? Platform? Build flags?) it is
unsupported?
-------------
PR Review: https://git.openjdk.org/jdk/pull/27537#pullrequestreview-3282521865
PR Review Comment: https://git.openjdk.org/jdk/pull/27537#discussion_r2389806062
PR Review Comment: https://git.openjdk.org/jdk/pull/27537#discussion_r2389808338
PR Review Comment: https://git.openjdk.org/jdk/pull/27537#discussion_r2389815933
PR Review Comment: https://git.openjdk.org/jdk/pull/27537#discussion_r2389822026
