On Wed, 29 Nov 2023 20:51:02 GMT, Chris Plummer <cjplum...@openjdk.org> wrote:
>> The change impelements dumping of unmounted virtual threads data (stack
>> traces and stack references).
>> Unmounted vthreads can be detected only by iterating over the heap, but
>> hprof stack trace records (HPROF_FRAME/HPROF_TRACE) should be written before
>> HPROF_HEAP_DUMP/HPROF_HEAP_DUMP_SEGMENT.
>> HeapDumper supports segment dump (parallel dump to separate files with
>> subsequent file merge outside of safepoint), the fix switches HeapDumper to
>> always use segment dump: 1st segment contains only non-heap data, other
>> segments are used for dumping heap objects. For serial dumping
>> single-threaded dumping is performed, but 2 segments are created anyway.
>> When HeapObjectDumper detects unmounted virtual thread, it writes
>> HPROF_FRAME/HPROF_TRACE records to the 1st segment ("global writer"), and
>> writes thread object (HPROF_GC_ROOT_JAVA_FRAME) and stack references
>> (HPROF_GC_ROOT_JAVA_FRAME/HPROF_GC_ROOT_JNI_LOCAL) to the HeapObjectDumper
>> segment.
>> As parallel dumpers may write HPROF_FRAME/HPROF_TRACE concurrently and
>> VMDumper needs to write non-heap data before heap object dumpers can write
>> virtual threads data, writing to global writer is protected with
>> DumperController::_global_writer_lock.
>>
>> Testing: run tests which perform heap dump (in different scenarios):
>> - test/hotspot/jtreg/serviceability
>> - test/hotspot/jtreg/runtime/ErrorHandling
>> - test/hotspot/jtreg/gc/epsilon
>> - test/jdk/sun/tools/jhsdb
>
> I think you are dumping all virtual threads found in the heap, even if they
> are unreachable. Although this would be true for platform threads also, there
> is a high likelihood of a lot of unreachable virtual threads in the heap, but
> not so much for platform threads.
>
> Also, have you tested scalability? Not just a large number of live virtual
> threads (like a million), but also combined with a large number of
> unreachable virtual threads.
@plummercj said:
> I think you are dumping all virtual threads found in the heap, even if they
> are unreachable.
There is a check for virtual thread liveness which has to be good enough in
general:
1617 static bool should_dump_vthread(oop vt) {
1618 return java_lang_VirtualThread::state(vt) !=
java_lang_VirtualThread::NEW
1619 && java_lang_VirtualThread::state(vt) !=
java_lang_VirtualThread::TERMINATED;
1620 }
. . .
1919 if (java_lang_VirtualThread::is_instance(o) &&
ThreadDumper::should_dump_vthread(o)) {
1920 _vthread_dumper->dump_vthread(o, writer());
1921 }
-------------
PR Comment: https://git.openjdk.org/jdk/pull/16665#issuecomment-1832794361
