RFR JDK-8232222: Set state to 'linked' when an archived class is restored at runtime
Ioi Lam
ioi.lam at oracle.com
Mon Jun 1 22:56:34 UTC 2020
On 5/31/20 11:14 PM, David Holmes wrote:
> 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.
>
I think the CSR should also include the benefit of doing this. It's not
a lot of code change, but now we have to maintain two different code
paths for post_class_prepare to be called.
JVMTI agents will typically introduce quite a bit of overhead in
start-up, so a reduction in the range of 0.2~0.4ms seems a drop to the
bucket. I'd rather keep the VM simple unless we have a strong reason to
make it more complicated.
Thanks
- Ioi
> 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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