jmx-dev RFR: 8368527: JMX: Add an MXBeans method to query GC CPU time

[Kevin Walls]

On Sat, 27 Sep 2025 11:18:58 GMT, Jonas Norlinder <duke at openjdk.org> 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/hotspot/share/include/jmm.h line 85:

> 83:   JMM_GC_COUNT                       = 10,   /* Total number of collections */
> 84:   JMM_GC_CPU_TIME                    = 11,   /* Total accumulated GC CPU time */
> 85:   JMM_JVM_UPTIME_MS                  = 12,   /* The JVM uptime in milliseconds */

It looks a bit odd to me to change the existing define of UPTIME here.  
OK it is not a public interface used between different versions, and people should not be mixing up jmm.h and management implementations...  But usually we would just add the new definition? (items below are not strictly grouped by name)  Maybe this is just a nit, and only makes cross-version comparisons easier.

Looks good overall.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/27537#discussion_r2387698942