RFR(L) 8227745: Enable Escape Analysis for Better Performance in the Presence of JVMTI Agents

[David Holmes]

Hi Richard,

Some further queries/concerns:

src/hotspot/share/runtime/objectMonitor.cpp

Can you please explain the changes to ObjectMonitor::wait:

!   _recursions = save      // restore the old recursion count
!                 + jt->get_and_reset_relock_count_after_wait(); // 
increased by the deferred relock count

what is the "deferred relock count"? I gather it relates to

"The code was extended to be able to deoptimize objects of a frame that 
is not the top frame and to let another thread than the owning thread do 
it."

which I don't like the sound of at all when it comes to ObjectMonitor 
state. So I'd like to understand in detail exactly what is going on here 
and why.  This is a very intrusive change that seems to badly break 
encapsulation and impacts future changes to ObjectMonitor that are under 
investigation.

---

src/hotspot/share/runtime/thread.cpp

Can you please explain why JavaThread::wait_for_object_deoptimization 
has to be handcrafted in this way rather than using proper transitions.

We got rid of "deopt suspend" some time ago and it is disturbing to see 
it being added back (effectively). This seems like it may be something 
that handshakes could be used for.

Thanks,
David
-----

On 12/12/2019 7:02 am, David Holmes wrote:
> On 12/12/2019 1:07 am, Reingruber, Richard wrote:
>> Hi David,
>>
>>    > Most of the details here are in areas I can comment on in detail, 
>> but I
>>    > did take an initial general look at things.
>>
>> Thanks for taking the time!
> 
> Apologies the above should read:
> 
> "Most of the details here are in areas I *can't* comment on in detail ..."
> 
> David
> 
>>    > The only thing that jumped out at me is that I think the
>>    > DeoptimizeObjectsALotThread should be a hidden thread.
>>    >
>>    > +  bool is_hidden_from_external_view() const { return true; }
>>
>> Yes, it should. Will add the method like above.
>>
>>    > Also I don't see any testing of the DeoptimizeObjectsALotThread. 
>> Without
>>    > active testing this will just bit-rot.
>>
>> DeoptimizeObjectsALot is meant for stress testing with a larger 
>> workload. I will add a minimal test
>> to keep it fresh.
>>
>>    > Also on the tests I don't understand your @requires clause:
>>    >
>>    >   @requires ((vm.compMode != "Xcomp") & vm.compiler2.enabled &
>>    > (vm.opt.TieredCompilation != true))
>>    >
>>    > This seems to require that TieredCompilation is disabled, but 
>> tiered is
>>    > our normal mode of operation. ??
>>    >
>>
>> I removed the clause. I guess I wanted to target the tests towards the 
>> code they are supposed to
>> test, and it's easier to analyze failures w/o tiered compilation and 
>> with just one compiler thread.
>>
>> Additionally I will make use of 
>> compiler.whitebox.CompilerWhiteBoxTest.THRESHOLD in the tests.
>>
>> Thanks,
>> Richard.
>>
>> -----Original Message-----
>> From: David Holmes <david.holmes at oracle.com>
>> Sent: Mittwoch, 11. Dezember 2019 08:03
>> To: Reingruber, Richard <richard.reingruber at sap.com>; 
>> serviceability-dev at openjdk.java.net; 
>> hotspot-compiler-dev at openjdk.java.net; 
>> hotspot-runtime-dev at openjdk.java.net
>> Subject: Re: RFR(L) 8227745: Enable Escape Analysis for Better 
>> Performance in the Presence of JVMTI Agents
>>
>> Hi Richard,
>>
>> On 11/12/2019 7:45 am, Reingruber, Richard wrote:
>>> Hi,
>>>
>>> I would like to get reviews please for
>>>
>>> http://cr.openjdk.java.net/~rrich/webrevs/2019/8227745/webrev.3/
>>>
>>> Corresponding RFE:
>>> https://bugs.openjdk.java.net/browse/JDK-8227745
>>>
>>> Fixes also https://bugs.openjdk.java.net/browse/JDK-8233915
>>> And potentially https://bugs.openjdk.java.net/browse/JDK-8214584 [1]
>>>
>>> Vladimir Kozlov kindly put webrev.3 through tier1-8 testing without 
>>> issues (thanks!). In addition the
>>> change is being tested at SAP since I posted the first RFR some 
>>> months ago.
>>>
>>> The intention of this enhancement is to benefit performance wise from 
>>> escape analysis even if JVMTI
>>> agents request capabilities that allow them to access local variable 
>>> values. E.g. if you start-up
>>> with -agentlib:jdwp=transport=dt_socket,server=y,suspend=n, then 
>>> escape analysis is disabled right
>>> from the beginning, well before a debugger attaches -- if ever one 
>>> should do so. With the
>>> enhancement, escape analysis will remain enabled until and after a 
>>> debugger attaches. EA based
>>> optimizations are reverted just before an agent acquires the 
>>> reference to an object. In the JBS item
>>> you'll find more details.
>>
>> Most of the details here are in areas I can comment on in detail, but I
>> did take an initial general look at things.
>>
>> The only thing that jumped out at me is that I think the
>> DeoptimizeObjectsALotThread should be a hidden thread.
>>
>> +  bool is_hidden_from_external_view() const { return true; }
>>
>> Also I don't see any testing of the DeoptimizeObjectsALotThread. Without
>> active testing this will just bit-rot.
>>
>> Also on the tests I don't understand your @requires clause:
>>
>>    @requires ((vm.compMode != "Xcomp") & vm.compiler2.enabled &
>> (vm.opt.TieredCompilation != true))
>>
>> This seems to require that TieredCompilation is disabled, but tiered is
>> our normal mode of operation. ??
>>
>> Thanks,
>> David
>>
>>> Thanks,
>>> Richard.
>>>
>>> [1] Experimental fix for JDK-8214584 based on JDK-8227745
>>>       
>>> http://cr.openjdk.java.net/~rrich/webrevs/2019/8214584/experiment_v1.patch 
>>>
>>>