bottleneck by java.lang.Class.getAnnotations() - rebased patch

[Peter Levart]

On 12/12/2012 11:59 AM, Joel Borggrén-Franck wrote:
> Hi all,
>
> First, thanks all of you that are involved in this!
>
> I agree with David here, lets split this up (for now) and do reflective objects in the context of jep-149 and annotations separately.
>
> As you may know there are even more annotation coming in with JSR 308 annotations on type (use), so I want to complete that work first and then do the effort of reducing contention and overhead for both type and regular annotations and also fixing up the behaviour for redefine/retransform class.
>
> One other point to consider is that some of the fields in java/lang/reflect/ classes are known by the VM so not all changes in Java-land are actually doable. Glancing over your patches very quickly I don't think you have done anything to upset the VM, but then I am not an expert in this area.
>
> Also, with the VM permgen changes we might have to rethink our assumptions in order to reduce total overhead. For example as I understand it previously we could just ship the same pointer into permgen for the annotations arrays, now that isn't possible so we create a new copy of the array for every Field/Method/Constructor instance. Perhaps there is some clever way of eliminating those copies.
You mean the "byte[] annotations" arrays in the Field/Method/Constructor 
instances returned by the following java.lang.Class native methods:

     private native Field[]       getDeclaredFields0(boolean publicOnly);
     private native Method[]      getDeclaredMethods0(boolean publicOnly);
     private native Constructor<T>[] getDeclaredConstructors0(boolean 
publicOnly);

If caching is effective (useCaches == true) then, in the absence of 
class redefinition, each of these methods is called at most two times 
(once for each value of publicOnly flag). If I understand the semantics 
of this flag correctly, it is used solely for the purpose of filtering 
the returned instances to include all or only public members.

The java part of caching could be modified to only call each of these 
methods once (with publicOnly==false) and derive the "publicOnly" 
variant of the array by doing the filtering in java. This way only a 
single instance of a particular Field/Method/Constructor will be 
retained by caches and there will be no duplication of annotation arrays.

Regards, Peter
>
> So while I think your patches generally makes sense, I think it is prudent to delay this for annotations until all our new annotation features are in.
>
> cheers
> /Joel
>
> On Dec 10, 2012, at 7:18 AM, David Holmes <david.holmes at oracle.com> wrote:
>
>> Hi Peter,
>>
>> Sorry for the delay on this.
>>
>> Generally your VolatileData and my ReflectionHelper are doing a similar job. But I agree with your reasoning that all of the cached SoftReferences are likely to be cleared at once, and so a SoftReference to a helper object with direct references, is more effective than a direct reference to a helper object with SoftReferences. My initial stance with this was very conservative as the more change that is introduced the more uncertainty there is about the impact.
>>
>> I say the above primarily because I think, if I am to proceed with this, I will need to separate out the general reflection caching changes from the annotation changes. There are a number of reasons for this:
>>
>> First, I'm not at all familiar with the implementation of annotations at the VM or Java level, and the recent changes in this area just exacerbate my ignorance of the mechanics. So I don't feel qualified to evaluate that aspect.
>>
>> Second, the bulk of the reflection caching code is simplified by the fact that due to current constraints on class redefinition the caching is effectively idempotent for fields/methods/constructors. But that is not the case for annotations.
>>
>> Finally, the use of synchronization with the annotations method is perplexing me. I sent Joe a private email on this but I may as well raise it here - and I think you have alluded to this in your earlier emails as well: initAnnotationsIfNecessary() is a synchronized instance method but I can not find any other code in the VM that synchronizes on the Class object's monitor. So if this synchronization is trying to establish consistency in the face of class redefinition, I do not see where class redefinition is participating in the synchronization!
>>
>> So what I would like to do is take your basic VolatileData part of the patch and run with it for JEP-149 purposes, while separating the annotations issue so they can be dealt with by the experts in that particular area.
>>
>> I'm sorry it has taken so long to arrive at a fairly negative position, but I need someone else to take up the annotations gauntlet and run with it.
>>
>> Thanks,
>> David
>>
>> On 3/12/2012 5:41 PM, Peter Levart wrote:
>>> Hi David, Alan, Alexander and others,
>>>
>>> In the meanwhile I have added another annotations space optimization to
>>> the patch. If a Class doesn't inherit any annotations from a superclass,
>>> which I think is a common case, it assigns the same Map instance to
>>> "annotations" as well as "declaredAnnotations" fields. Previously - and
>>> in the original code - this only happened for java.lang.Object and
>>> interfaces.
>>>
>>> Here's the updated webrev:
>>>
>>> http://dl.dropbox.com/u/101777488/jdk8-tl/JEP-149/webrev.02/index.html
>>>
>>> I have also rewritten the performance micro-benchmarks. With the
>>> addition of repeating annotations, one performance aspect surfaces: when
>>> asking for a particular annotation type on a Class and that annotation
>>> is not present, the new repeating annotations support method
>>> AnnotationSupport.getOneAnnotation asks for @ContainedBy meta-annotation
>>> on the annotation type. This can result in an even more apparent
>>> synchronization hot-spot with original code that uses synchronized
>>> initAnnotationsIfNecessary(). This aspect is tested with the 3rd test.
>>> Other 2 tests test the same thing as before but are more stable now,
>>> since now they measure retrieval of 5 different annotation types from
>>> each AnnotatedElement, previously they only measured retrieval of 1
>>> which was very sensitive to HashMap irregularities (it could happen that
>>> a particular key mapped to a bucket that was overloaded in one test-run
>>> and not in another)...
>>>
>>> Here're the new tests:
>>>
>>> https://raw.github.com/plevart/jdk8-tl/JEP-149/test/src/test/ReflectionTest.java
>>>
>>> And the corresponding results when run on an i7 CPU on Linux:
>>>
>>> https://raw.github.com/plevart/jdk8-tl/JEP-149/test/benchmark_results_i7-2600K.txt
>>>
>>>
>>> Regards, Peter
>>>
>>>
>>>
>>> On 12/03/2012 02:15 AM, David Holmes wrote:
>>>> On 1/12/2012 4:54 AM, Alan Bateman wrote:
>>>>> On 30/11/2012 18:36, Peter Levart wrote:
>>>>>> :
>>>>>>
>>>>>> So, what do you think? What kind of tests should I prepare in addidion
>>>>>> to those 3 so that the patch might get a consideration?
>>>>> I think this is good work and thanks for re-basing your patch. I know
>>>>> David plans to do a detail review. I think it will require extensive
>>>>> performance testing too, perhaps with some large applications.
>>>> Indeed I do plan a detailed review and have initiated some initial
>>>> performance tests.
>>>>
>>>> I am also swamped but will try to get to the review this week - and
>>>> will also need to check the referenced annotations updates.
>>>>
>>>> David
>>>>
>>>>> -Alan
>>>>>