RFR(L) 8153224 Monitor deflation prolong safepoints

[Robbin Ehn]

Hi Dan, Carsten,

>>> However, moving deflate_idle_monitors() from the safepoint cleanup phase
>>> to before the actual garbage collection can wait until we do the work to
>>> decouple triggering of monitor deflation to be independent of the the
>>> safepoint cleanup phase.
>>>

If I got the context correct here:
All cleanups are done before the VM op is execute.
It is the last thing we do in SS::begin(), so anything added to 
SS::do_cleanup_tasks/ParallelSPCleanupTask is done first in any safepoint.

/Robbin

>>
>> SGTM.
>>
>>> Speaking of optimizations, it sure would be nice if little changes to
>>>> java threads could be combined and performed on the way out of the
>>>> safepoint in one go instead of having lots of iterations of the thread list
>>>> in various places. Some people have thousands of threads and each traversal
>>>> of the thread list hurts.
>>>>
>>>>
>>>> Do you have a specific example in mind?
>>>>
>>>
>>> No concrete example for a public mailing list. :(. But do notice that
>>> independent tasks that require traversals of the thread list are already
>>> fused in ParallelSPCleanupThreadClosure
>>> <http://hg.openjdk.java.net/jdk-updates/jdk12u/file/fe6c633292ff/src/hotspot/share/runtime/safepoint.cpp#l586>.
>>> If you made deflate_thread_local_monitors
>>> set jt->omShouldDeflateIdleMonitors to true, then you wouldn't need to
>>> iterator over all java threads in do_safepoint_work.
>>>
>>>
>>> I think I see what you mean... so when ParallelSPCleanupThreadClosure::
>>> do_thread() calls deflate_thread_local_monitors():
>>>
>>> 2250   if (AsyncDeflateIdleMonitors) {
>>> 2251     // Nothing to do when idle ObjectMonitors are deflated using a
>>> 2252     // JavaThread unless a special cleanup has been requested.
>>>
>>> Replace L2251-2 with:
>>>           // Mark the JavaThread for idle monitor cleanup unless a
>>>           // special cleanup has been requested.
>>> 2253     if (!is_cleanup_requested()) {
>>>
>>> Add these three lines:
>>>             if (thread->omInUseCount > 0) {
>>>               // This JavaThread is using monitors so mark it.
>>>               thread->omShouldDeflateIdleMonitors = true;
>>>             }
>>> 2254       return;
>>> 2255     }
>>>
>>> That will allow this block to go away:
>>>
>>> 1695   // Request deflation of per-thread idle monitors by each
>>> JavaThread:
>>> 1696   for (JavaThreadIteratorWithHandle jtiwh; JavaThread *jt =
>>> jtiwh.next(); ) {
>>> 1697     if (jt->omInUseCount > 0) {
>>> 1698       // This JavaThread is using monitors so check it.
>>> 1699       jt->omShouldDeflateIdleMonitors = true;
>>> 1700     }
>>> 1701   }
>>>
>>> Please let me know if I understand what you meant...
>>>
>>
>> This is exactly what I meant.
>>
>>
>> Good. This will be in the next round of code review.
>>
> 
> Nice.
> 
> Carsten
>