RFR/RFA (M): 8185003: JMX: Add a version of ThreadMXBean.dumpAllThreads with a maxDepth argument

[serguei.spitsyn at oracle.com]

Hi Paul,

Thank you for explanation.

Thanks,
Serguei


On 8/21/20 10:54, Volker Simonis wrote:
> On Thu, Aug 20, 2020 at 10:06 PM serguei.spitsyn at oracle.com
> <serguei.spitsyn at oracle.com> wrote:
>> Hi Paul,
>>
>> I was also wondering if there is a compatibility risk involved with the JMM_VERSION change.
>> So, thanks to Volker for asking these questions.
>>
>> One more question.
>> I do not see a backport of the src/jdk.management/share/native/libmanagement_ext/management_ext.c change.
>> Is it intentional, and if so, what is the reason to skip this file?
>>
> "libmanagement_ext/management_ext.c" doesn't exist in jdk8. It was
> introduced with "8042901: Allow com.sun.management to be in a
> different module to java.lang.management" in jdk9. In jdk8 all the
> functionality is in "management/management.h" so there's no need to
> backport the changes from "management_ext.c" .
>
> [1] https://bugs.openjdk.java.net/browse/JDK-8042901
>
>> Thanks,
>> Serguei
>>
>>
>> On 8/20/20 11:30, Volker Simonis wrote:
>>
>> On Wed, Aug 19, 2020 at 6:17 PM Hohensee, Paul <hohensee at amazon.com> wrote:
>>
>> Please review this backport to jdk8u. I especially need a CSR review, since the CSR approval process can be a bottleneck. The patch significantly reduces fleet profiling overhead, and a version of it has been in production at Amazon for over 3 years.
>>
>>
>>
>> Original JBS issue: https://bugs.openjdk.java.net/browse/JDK-8185003
>>
>> Original CSR: https://bugs.openjdk.java.net/browse/JDK-8185705
>>
>> Original patch: http://hg.openjdk.java.net/jdk10/master/rev/68d46cb9be45
>>
>>
>>
>> Backport JBS issue: https://bugs.openjdk.java.net/browse/JDK-8251494
>>
>> Backport CSR: https://bugs.openjdk.java.net/browse/JDK-8251498
>>
>> Backport JDK webrev: http://cr.openjdk.java.net/~phh/8185003/webrev.8u.jdk.05/
>>
>> JDK part looks good to me.
>>
>> Backport Hotspot webrev: http://cr.openjdk.java.net/~phh/8185003/webrev.8u.hotspot.05/
>>
>> HotSpot part looks good to me but see discussion below.
>>
>>
>> Details of the interface changes needed for the backport are in the Description of the Backport CSR 8251498. The actual functional changes are minimal and low risk.
>>
>> I've also reviewed the CSR yesterday which I think is fine. But now,
>> when looking at the implementation, I'm a little concerned about
>> changing JMM_VERSION from " 0x20010203" to "0x20020000" in "jmm.h".
>>
>> This might be especially problematic in combination with the changes
>> in "Management::get_jmm_interface()" which is called by
>> JVM_GetManagement():
>>
>>   void* Management::get_jmm_interface(int version) {
>>   #if INCLUDE_MANAGEMENT
>> -  if (version == JMM_VERSION_1_0) {
>> +  if (version == JMM_VERSION) {
>>       return (void*) &jmm_interface;
>>     }
>>   #endif // INCLUDE_MANAGEMENT
>>     return NULL;
>>   }
>>
>> You've correctly fixed the single caller of "JVM_GetManagement()" in
>> the JDK (in "JNI_OnLoad()" in "management.c"):
>>
>> -    jmm_interface = (JmmInterface*) JVM_GetManagement(JMM_VERSION_1_0);
>> +    jmm_interface = (JmmInterface*) JVM_GetManagement(JMM_VERSION);
>>
>> but I wonder if there are other monitoring/serviceability tools out
>> there which use this interface and which will break after this change.
>> A quick search revealed at least two StackOverflow entries which
>> recommend using "JVM_GetManagement(JMM_VERSION_1_0)" [1,2] and there's
>> a talk and a blog entry doing the same [3, 4].
>>
>> I'm not sure how relevant this is but I think a much safer and
>> backwards-compatible way of doing this downport would be the
>> following:
>>
>> - don't change "Management::get_jmm_interface()" (i.e. still check for
>> "JMM_VERSION_1_0") but return the "new" JMM_VERSION in
>> "jmm_GetVersion()". This won't break anything but will make it
>> possible for clients to detect the new version if they want.
>>
>> - don't change the signature of "DumpThreads()". Instead add a new
>> version (e.g. "DumpThreadsMaxDepth()/jmm_DumpThreadsMaxDepth()") to
>> the "JMMInterface" struct and to "jmm_interface" in "management.cpp".
>> You can do this in one of  the two first, reserved fields of
>> "JMMInterface" so you won't break binary compatibility.
>> "jmm_DumpThreads()" will then be a simple wrapper which calls
>> "jmm_DumpThreadsMaxDepth()" with Integer.MAX_VALUE as depth.
>>
>> - in the jdk you then simply call "DumpThreadsMaxDepth()" in
>> "Java_sun_management_ThreadImpl_dumpThreads0()"
>>
>> I think this way we can maintain full binary compatibility while still
>> using the new feature. What do you think?
>>
>> Best regards,
>> Volker
>>
>> [1] https://urldefense.com/v3/__https://stackoverflow.com/questions/23632653/generate-java-heap-dump-on-uncaught-exception__;!!GqivPVa7Brio!LDD5rfKbGz6KCl0LqcAgrFq7kNLkkoDhhN0ZSgHMDvgGMY5bvKJdpoXIAK6N-KqVsyaF$
>> [2] https://urldefense.com/v3/__https://stackoverflow.com/questions/60887816/jvm-options-printnmtstatistics-save-info-to-file__;!!GqivPVa7Brio!LDD5rfKbGz6KCl0LqcAgrFq7kNLkkoDhhN0ZSgHMDvgGMY5bvKJdpoXIAK6N-Ip7MAQ5$
>> [3] https://urldefense.com/v3/__https://sudonull.com/post/25841-JVM-TI-how-to-make-a-plugin-for-a-virtual-machine-Odnoklassniki-company-blog__;!!GqivPVa7Brio!LDD5rfKbGz6KCl0LqcAgrFq7kNLkkoDhhN0ZSgHMDvgGMY5bvKJdpoXIAK6N-ErSjPdD$
>> [4] https://urldefense.com/v3/__https://2019.jpoint.ru/talks/2o8scc5jbaurnqqlsydzxv/__;!!GqivPVa7Brio!LDD5rfKbGz6KCl0LqcAgrFq7kNLkkoDhhN0ZSgHMDvgGMY5bvKJdpoXIAK6N-Oxb5CQ-$
>>
>> Passes the included (suitably modified) test, as well as the tests in
>>
>>
>>
>> jdk/test/java/lang/management/ThreadMXBean
>>
>> jdk/test/com/sun/management/ThreadMXBean
>>
>>
>>
>> Thanks,
>>
>> Paul
>>
>>