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

Fri Aug 21 20:36:03 UTC 2020

On 8/21/20 11:07, serguei.spitsyn at oracle.com wrote:
> Hi Paul,

Sorry, Volker, for using this "indirection".
I hope, Paul redirected my "Hi" to you. :)

Thanks,
Serguei

>
> 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
>>>
>>>
>