ReflectionData space optimization in java.lang.Class (JEP-149) (original) (raw)

Peter Levart peter.levart at gmail.com
Thu Dec 13 10:13:26 UTC 2012

Hi Mandy, David and others,

Here's the updated version of the patch for ReflectionData:

http://dl.dropbox.com/u/101777488/jdk8-tl/JEP-149.c/webrev.02/index.html

I split long lines as proposed by Mandy. I also split reflectionData() method in two methods, so the fast-path method is smaller and might get better chances of being in-lined.

I also found code-paths that evaluated reflectionData() method more than once for each external call. It's the methods:

private Field[] privateGetDeclaredFields(boolean publicOnly)

and

private Method[] privateGetDeclaredMethods(boolean publicOnly)

which are called from:

private Field[] privateGetPublicFields(Set<Class<?>> traversedInterfaces)

and

private Method[] privateGetPublicMethods()

respectively. I therefore introduced overloaded variants of the former methods taking a ReflectionData parameter like the following:

 private Field[] privateGetDeclaredFields(boolean publicOnly) {
     return privateGetDeclaredFields(publicOnly, reflectionData());
 }
 // A variant called from methods that already obtained

ReflectionData instance private Field[] privateGetDeclaredFields(boolean publicOnly, ReflectionData rd) { ...

the same for privateGetDeclaredMethods. This is not for performance reasons (though it might help) but for correctness. Each external call should be a separate "transaction" and should work on the same ReflectionData instance. The "transaction" is only guaranteed on the level of a particular java.lang.Class instance though. Some methods also invoke other Class instances (to gather inherited public methods / fields) and those invocations might be separate transactions in the face of concurrent class re-definitions. But we are not going to implement a database here, are we?

I prepared some micro benchmarks for individual public methods. Here're the results:

https://raw.github.com/plevart/jdk8-tl/JEP-149.c/test/reflection_data_benchmark_results_i7-2600K.txt

they indicate no noticeable performance decreases. Some methods are in fact faster (more in-linable?):

Here's the code for micro-benchmarks:

https://raw.github.com/plevart/jdk8-tl/JEP-149.c/test/src/test/ReflectionDataTest.java

Regards, Peter

On 12/12/2012 11:59 PM, Mandy Chung wrote:

Hi Peter,

On 12/12/12 4:31 AM, Peter Levart wrote: Hi all,

Ok, no problem. So here's a patch that summarizes what I did in the previous patch, but only for reflective fields (Field[], Method[], Constructor[]) not including annotations: http://dl.dropbox.com/u/101777488/jdk8-tl/JEP-149.c/webrev/index.html Finally able to make time to review this patch. It's good work. While it's good to see the synchronization issue with annotations be fixed, separating the cache for reflection and annotation helps. As David replied, he will take your patch and run with it for JEP-149. The change looks good. Nit: there are several long lines L2235,2244,2245,2249,2269 etc that should be broken into separate lines. The remaining open question is the performance difference in the reflectionData() method and how well it will be jit'ed. In the common case, there is no class redefinition where classCachesOnClassRedefinition() is essentially like an nop. I believe David will look at the footprint and performance numbers as he has initiated the performance testing (need to do it with this new patch). Thanks Mandy The annotations part is unchanged semantically, but I had to: - modify private method clearCachesOnClassRedefinition to only include invalidation of annotations and declaredAnnotations fields. I also renamed it to clearAnnotationCachesOnClassRedefinition - renamed lastRedefinedCount to lastAnnotationsRedefinedCount and, since this field is now only accessed while holding a lock (from synchronized initAnnotationsIfNecessary), I removed the volatile keyword. That's it. While looking at this unchanged part of code some more, I see other races as well. For example: two concurrent accesses to annotations combined with redefinition of a class can result in NPE. Here's a serial execution: Thread 1: getAnnotation(annotationType); initAnnotationsIfNecessary(); VM: classRedefinedCount++; Thread 2: getAnnotation(annotationType); initAnnotationsIfNecessary(); clearAnnotationCachesOnClassRedefinition(); annotations = null; Thread 1: return AnnotationSupport.getOneAnnotation(annotations, annotationClass); // 'annotations' field can be null

So this needs to be fixed sooner or later. Joel! Are your JSR 308 canges involving java.lang.Class too? Regards, Peter 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. 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

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