On 12/13/2012 11:46 AM, David Holmes wrote:
Hi Peter,
I dropped Joel from the direct cc as this isn't annotation related.
On 13/12/2012 8:13 PM, Peter Levart wrote:
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.
Okay - though I just submitted the previous version to get some
performance data :(
Never mind, it's not a big difference, see below...
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<T> 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.
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?
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/reflection_data_benchmark_results_i7-2600K.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.hol...@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
