RFR JDK-8232222: Set state to 'linked' when an archived class is restored at runtime
David Holmes
david.holmes at oracle.com
Mon Jun 1 06:14:50 UTC 2020
Hi Jiangli,
On 29/05/2020 9:02 am, Jiangli Zhou wrote:
> (Looping in serviceability-dev at openjdk.java.net ...)
>
> Hi David and Ioi,
>
> On Wed, May 27, 2020 at 11:15 PM David Holmes <david.holmes at oracle.com> wrote:
>>
>> Hi Jiangli,
>>
>> On 28/05/2020 11:35 am, Ioi Lam wrote:
>>>
>>>
>>> On 5/27/20 6:17 PM, Jiangli Zhou wrote:
>>>> On Wed, May 27, 2020 at 1:56 PM Ioi Lam <ioi.lam at oracle.com> wrote:
>>>>> On 5/26/20 6:21 PM, Jiangli Zhou wrote:
>>>>>
>>>>>> Focusing on the link state for archived classes in this thread, I
>>>>>> updated the webrev to only set archived boot classes to 'linked' state
>>>>>> at restore time. More investigations can be done for archived classes
>>>>>> for other builtin loaders.
>>>>>>
>>>>>> https://bugs.openjdk.java.net/browse/JDK-8232222
>>>>>> http://cr.openjdk.java.net/~jiangli/8232222/webrev.02/
>>>>>>
>>>>>> Please let me know if there is any additional concerns to the change.
>>>>>>
>>>>>> Best regards,
>>>>>> Jiangli
>>>>>>
>>>>> Hi Jiangli,
>>>>>
>>>>> I think the change is fine. I am wondering if this
>>>>>
>>>>> 2530 if (!BytecodeVerificationLocal &&
>>>>> 2531 loader_data->is_the_null_class_loader_data()) {
>>>>> 2532 _init_state = linked;
>>>>> 2533 }
>>>>>
>>>>>
>>>>> can be changed to
>>>>>
>>>>> if (!BytecodeVerificationLocal &&
>>>>> loader_data->is_the_null_class_loader_data() &&
>>>>> !JvmtiExport::should_post_class_prepare())
>>>>>
>>>>> That way, there's no need to change systemDictionary.cpp.
>>>>>
>>>>>
>>>> I was going to take the suggestion, but realized that it would add
>>>> unnecessary complications for archived boot classes with class
>>>> pre-initialization support. Some agents may set
>>>> JvmtiExport::should_post_class_prepare(). It's worthwhile to support
>>>> class pre-init uniformly for archived boot classes with
>>>> JvmtiExport::should_post_class_prepare() enabled or disabled.
>>>
>>> This would introduce behavioral changes when JVMTI is enabled:
>>>
>>> + The order of JvmtiExport::post_class_prepare is different than before
>>> + JvmtiExport::post_class_prepare may be called for a class that was not
>>> called before (if the class is never linked during run time)
>>> + JvmtiExport::post_class_prepare was called inside the init_lock, now
>>> it's called outside of the init_lock
>>
>> I have to say I share Ioi's concerns here. This change will impact JVM
>> TI agents in a way we can't be sure of. From a specification perspective
>> I think we are fine as linking can be lazy or eager, so there's no
>> implied order either. But this would be a behavioural change that will
>> be observable by agents. (I'm less concerned about the init_lock
>> situation as it seems potentially buggy to me to call out to an agent
>> with the init_lock held in the first place! I find it hard to imagine an
>> agent only working correctly if the init_lock is held.)
>>
>
> Totally agree that we need to be very careful here (that's also part
> of the reason why I separated this into an individual RFE for the
> dedicated discussion). David, thanks for the analysis from the spec
> perspective! Agreed with the init_lock comment also. In the future, I
> think we can even get rid of the needs for init_lock completely for
> some of the pre-initialized classes.
>
> This change has gone through extensive testing since the later part of
> last year and has been in use (with the default CDS) with agents that
> do post_class_prepare. Hopefully that would ease some of the concerns.
That is good to know, but that is just one sample of a set of agents.
>> This would need a CSR request and involvement of the serviceabilty folk,
>> to work through any potential issues.
>>
>
> I've looped in serviceability-dev at openjdk.java.net for this
> discussion. Chris or Serguei could you please take a look of the
> change, http://cr.openjdk.java.net/~jiangli/8232222/webrev.02/,
> specifically the JvmtiExport::post_class_prepare change in
> systemDictionary.cpp.
>
> Filing a CSR request sounds good to me. The CSR looks after source,
> binary, and behavioral compatibility. From a behavior point of view,
> the change most likely does not cause any visible effects to a JVMTI
> agent (based on what's observed in testing and usages). What should be
> included in the CSR?
The CSR request should explain the behavioural change that will be
observable by agents, and all of the potential compatibility issues that
might arise from that - pointing out of course that as the spec (JVMS
5.4**) allows for eager or lazy linking, agents shouldn't be relying on
the exact timing or order of events.
** I note this section has some additional constraints regarding
dynamically computed constants that might also come into play with this
pre-linking for CDS classes.
Cheers,
David
-----
>> Ioi's suggestion avoids this problem, but, as you note, at the expense
>> of disabling this optimisation if an agent is attached and wants class
>> prepare events.
>>
>
> Right, if we handle that case conditionally, we would alway need to
> store the cached static field values separately since the dump time
> cannot foresee if the runtime can set boot classes in 'linked' state
> (and 'fully_initialized' state with the planned changes) at restore
> time. As a result, we need to handle all pre-initialized static fields
> like what we are doing today, which is storing them in the archived
> class_info_records then installing them to the related fields at
> runtime. That causes both unwanted memory and CPU overhead at runtime.
>
> I also updated the webrev.02 in place with typo fixes. Thanks!
>
> Best regards,
> Jiangli
>
>> Thanks,
>> David
>>
>>> Thanks
>>> - Ioi
>>>
>>>>
>>>>> BTW, I was wondering where the performance came from, so I wrote an
>>>>> investigative patch:
>>>>>
>>>>> diff -r 0702191777c9 src/hotspot/share/oops/instanceKlass.cpp
>>>>> --- a/src/hotspot/share/oops/instanceKlass.cpp Thu May 21 15:56:27
>>>>> 2020 -0700
>>>>> +++ b/src/hotspot/share/oops/instanceKlass.cpp Wed May 27 10:48:57
>>>>> 2020 -0700
>>>>> @@ -866,6 +866,13 @@
>>>>> return true;
>>>>> }
>>>>>
>>>>> + if (UseSharedSpaces && !BytecodeVerificationLocal &&
>>>>> is_shared_boot_class()) {
>>>>> + Handle h_init_lock(THREAD, init_lock());
>>>>> + ObjectLocker ol(h_init_lock, THREAD, h_init_lock() != NULL);
>>>>> + set_init_state(linked);
>>>>> + return true;
>>>>> + }
>>>>> +
>>>>> // trace only the link time for this klass that includes
>>>>> // the verification time
>>>>> PerfClassTraceTime vmtimer(ClassLoader::perf_class_link_time(),
>>>>>
>>>>>
>>>>> Benchmarking results (smaller numbers are better):
>>>>>
>>>>> (baseline vs your patch)
>>>>>
>>>>> baseline jiangli baseline
>>>>> jiangli
>>>>> 1: 58514375 57755638 (-758737) ----- 40.266
>>>>> 40.135 (
>>>>> -0.131) -
>>>>> 2: 58506426 57754623 (-751803) ----- 40.367
>>>>> 39.417 (
>>>>> -0.950) -----
>>>>> 3: 58498554 57759735 (-738819) ----- 40.513
>>>>> 39.970 (
>>>>> -0.543) ---
>>>>> 4: 58491265 57751296 (-739969) ----- 40.439
>>>>> 40.268 (
>>>>> -0.171) -
>>>>> 5: 58500588 57750975 (-749613) ----- 40.569
>>>>> 40.080 (
>>>>> -0.489) --
>>>>> 6: 58497015 57744418 (-752597) ----- 41.097
>>>>> 40.147 (
>>>>> -0.950) -----
>>>>> 7: 58494335 57749909 (-744426) ----- 39.983 40.214
>>>>> ( 0.231) +
>>>>> 8: 58500401 57750305 (-750096) ----- 40.235 40.417
>>>>> ( 0.182) +
>>>>> 9: 58490728 57767463 (-723265) ----- 40.354
>>>>> 39.928 (
>>>>> -0.426) --
>>>>> 10: 58497858 57746557 (-751301) ----- 40.756
>>>>> 39.706 (
>>>>> -1.050) -----
>>>>> ============================================================
>>>>> 58499154 57753091 (-746062) ----- 40.457
>>>>> 40.027 (
>>>>> -0.430) --
>>>>> instr delta = -746062 -1.2753%
>>>>> time delta = -0.430 ms -1.0619%
>>>>>
>>>>>
>>>>> (baseline vs my patch)
>>>>>
>>>>> baseline ioi baseline ioi
>>>>> 1: 58503574 57821124 (-682450) ----- 40.554 39.783 (
>>>>> -0.771) -----
>>>>> 2: 58499325 57819459 (-679866) ----- 40.092 40.325
>>>>> ( 0.233) ++
>>>>> 3: 58492362 57811978 (-680384) ----- 40.546
>>>>> 39.826 (
>>>>> -0.720) -----
>>>>> 4: 58488655 57828878 (-659777) ----- 40.270 40.550
>>>>> ( 0.280) ++
>>>>> 5: 58501567 57830179 (-671388) ----- 40.382
>>>>> 40.145 (
>>>>> -0.237) --
>>>>> 6: 58496552 57808774 (-687778) ----- 40.702
>>>>> 40.527 (
>>>>> -0.175) -
>>>>> 7: 58482701 57808925 (-673776) ----- 40.268
>>>>> 39.849 (
>>>>> -0.419) ---
>>>>> 8: 58493831 57807810 (-686021) ----- 40.396
>>>>> 39.940 (
>>>>> -0.456) ---
>>>>> 9: 58489388 57811354 (-678034) ----- 40.575
>>>>> 40.078 (
>>>>> -0.497) ---
>>>>> 10: 58482512 57795489 (-687023) ----- 40.084 40.247
>>>>> ( 0.163) +
>>>>> ============================================================
>>>>> 58493046 57814396 (-678650) ----- 40.386
>>>>> 40.126 (
>>>>> -0.260) --
>>>>> instr delta = -678650 -1.1602%
>>>>> time delta = -0.260 ms -0.6445%
>>>>>
>>>>>
>>>>> (your patch vs my patch)
>>>>>
>>>>> jiangli ioi jiangli ioi
>>>>> 1: 57716711 57782622 ( 65911) ++++ 41.042 40.302 (
>>>>> -0.740) -----
>>>>> 2: 57709666 57780196 ( 70530) ++++ 40.334 40.965 (
>>>>> 0.631) ++++
>>>>> 3: 57716074 57803315 ( 87241) +++++ 40.239 39.823 (
>>>>> -0.416) ---
>>>>> 4: 57725152 57782719 ( 57567) +++ 40.430 39.805 (
>>>>> -0.625) ----
>>>>> 5: 57719799 57787187 ( 67388) ++++ 40.138 40.003 (
>>>>> -0.135) -
>>>>> 6: 57721922 57769193 ( 47271) +++ 40.324 40.207 (
>>>>> -0.117) -
>>>>> 7: 57716438 57785212 ( 68774) ++++ 39.978 40.149 (
>>>>> 0.171) +
>>>>> 8: 57713834 57778797 ( 64963) ++++ 40.359 40.210 (
>>>>> -0.149) -
>>>>> 9: 57711272 57786376 ( 75104) ++++ 40.575 40.724 (
>>>>> 0.149) +
>>>>> 10: 57711660 57780548 ( 68888) ++++ 40.291 40.091 (
>>>>> -0.200) -
>>>>> ============================================================
>>>>> 57716252 57783615 ( 67363) ++++ 40.370 40.226 (
>>>>> -0.144) -
>>>>> instr delta = 67363 0.1167%
>>>>> time delta = -0.144 ms -0.3560%
>>>>>
>>>>>
>>>>> These numbers show that the majority of the time spent (678650
>>>>> instructions) inside InstanceKlass::link_class_impl is spent from the
>>>>> PerfClassTraceTime. Walking of the class hierarchy and taking the
>>>>> h_init_lock only takes about 67363 instructions).
>>>>>
>>>>> Due to this finding, I filed two more RFEs:
>>>>>
>>>>> https://bugs.openjdk.java.net/browse/JDK-8246019
>>>>> PerfClassTraceTime slows down VM start-up
>>>>>
>>>> It's related to JDK-8246020, and I've commented on the bug (see
>>>> JDK-8246020 comments). UsePerfData for perf data collection is common
>>>> in cloud usages. It's better to keep UsePerfData enabled by default.
>>>>
>>>>> https://bugs.openjdk.java.net/browse/JDK-8246015
>>>>> Method::link_method is called twice for CDS methods
>>>>
>>>> That was addressed as part of the initial change for JDK-8232222:
>>>> http://cr.openjdk.java.net/~jiangli/8232222/weberv.02/src/hotspot/share/oops/instanceKlass.cpp.frames.html
>>>>
>>>>
>>>> It's cleaner to handle it separately, so I removed it from the latest
>>>> version. I've assigned JDK-8246015 to myself and will address it
>>>> separately. Thanks for recording the separate bug.
>>>>
>>>> Thanks!
>>>> Jiangli
>>>>
>>>>>
>>>>> Thanks
>>>>> - Ioi
>>>
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