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

David Holmes david.holmes at oracle.com
Fri Dec 14 04:59:49 UTC 2012

Hi Peter,

On 13/12/2012 10:27 PM, Peter Levart wrote:

On 12/13/2012 11:46 AM, David Holmes wrote:

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? Sorry I don't understand the problem you are seeing here. If we find a cached public fields, for example, we will return it. Else we start the process of calculating anew. If someone else manages to fill in the cache then we will get it when we call privateGetDeclaredFields. The results are expected to be idempotent so I don't see what the problem is. It's true, yes. For the outside caller there is no observable difference. It can only happen that we retrieve declaredFields from a newer snapshot (say v.2) of ReflectionData and then compute publicFields and install them into an older ReflectionData instance v.1 which is already obsolete. But for the outside observer there's no difference. From performance standpoint, the additional call to reflectionData() that we save is on the slow-path so it's not worth it.

Okay - so I'll ignore this :)

The split of reflectionData() into two methods does have an impact though. FWIW my micro-benchmark shows that without the split, only the following public method is observed to be faster then in the original JDK8 code:

- getFields - 4x With the split of reflectionData() but without these unneeded overloaded privateGetDeclaredFields methods, the following methods are faster: - getMethods - 1.7x (1, 2 threads) ... 1x (4...32 threads) - getFields - 4x - getDeclaredConstructor - 6x ... 11x - getDeclaredMethod - 3x But for performance tests that you already initiated, it is important to note that original patch is good enough since it appears that no public method is any slower than in the original JDK8 code. Speaking of that, I don't know much about the constraints of the JIT compiler, but it seems from the results above, that it can in-line multiple levels of method calls and that the total size of the methods matter. If this is true then it might be possible to split several private methods in java.lang.Class into pairs of short fast-path and longer slow-path. Is this worth the maintenance cost?

Method size certainly does affect inlining. Generally however we would only refactor this way if we find a bottleneck that needs to be broken - splitting methods adds additional meta-data overhead etc.

As you've already done the investigation here I think the split is a reasonable one to make. But I wonder in general applications how often we'll even end up compiling this method.

Thanks, David

Regards, Peter

Thanks, David ------

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

https://raw.github.com/plevart/jdk8-tl/JEP-149.c/test/reflectiondatabenchmarkresultsi7-2600K.txt

they indicate no noticeable performance decreases. Some methods are in fact faster (more in-linable?): - getFields - 4x - getDeclaredConstructor - 4x ... 10x - getDeclaredMethod - 3x 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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