On Sun, 5 Oct 2025 13:00:44 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.
>
> Jonas Norlinder has updated the pull request incrementally with one
> additional commit since the last revision:
>
> Move from j.l.management to com.sun.management etc.
src/jdk.management/share/classes/com/sun/management/MemoryMXBean.java line 45:
> 43: * GC cycle occurred. This method returns {@code -1} if the
> 44: * platform does not support this operation or if called during
> 45: * shutdown.
Now that the proposal is a JDK-specific management interface it means that this
method can say something about how it relates to, or how it might be used with,
OperatingSystemMXBean::getProcessCpuTime.
I'm also wondering about ThreadMXBean::isThreadCpuTimeSupported. Is it possible
for this to return false but getTotalGcCpuTime to return a value >= 0. The
former concerns Java threads so it might not have any connection to this new
method which concerns CPU usage by non-Java threads, but someone is bound to
ask.
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/27537#discussion_r2405893440
