Hi Alan,
I prepared the variant with lazy initialization of ConcurrentHashMaps
per ClassLoader and performance measurements show no differences. So
here's this variant:
* http://dl.dropbox.com/u/101777488/jdk8-tl/proxy/webrev.03/index.html
I also checked the ClassLoader layout and as it happens the additional
pointer slot increases the ClassLoader object size by 8 bytes in both
addressing modes: 32bit and 64bit. But that's a small overhead compared
to for example the deep-size of AppClassLoader at the beginning of the
main method: 14648 bytes (32bit) / 22432 bytes (64bit).
Regards, Peter
On 01/28/2013 01:57 PM, Peter Levart wrote:
On 01/28/2013 12:49 PM, Alan Bateman wrote:
On 25/01/2013 17:55, Peter Levart wrote:
:
The solution is actually very simple. I just want to validate my
reasoning before jumping to implement it:
- for solving scalability of getProxyClass cache, a field with a
reference to ConcurrentHashMap<List<String>, Class<? extends Proxy>>
is added to j.l.ClassLoader
- for solving scalability of isProxyClass, a field with a reference
to ConcurrentHashMap<Class<? extends Proxy>, Boolean> is added to
j.l.ClassLoader
I haven't had time to look very closely as your more recent changes
(you are clearly doing very good work here). The only thing I wonder
if whether it would be possible to avoid adding to ClassLoader. I
can't say what percentage of frameworks and applications use proxies
but it would be nice if the impact on applications that don't use
proxies is zero.
Hi Alan,
Hm, well. Any application that uses run-time annotations, is
implicitly using Proxies. But I agree that there are applications that
don't use either. Such applications usually don't use many
ClassLoaders either. Applications that use many ClassLoaders are
typically application servers or applications written for modular
systems (such as OSGI or NetBeans) and all those applications are also
full of runtime annotations nowadays. So a typical application that
does not use neither Proxies nor runtime annotations is composed of
bootstrap classloader, AppClassLoader and ExtClassLoader. The
ConcurrentHashMap for the bootstrap classloader is hosted by
j.l.r.Proxy class and is only initialized when the j.l.r.Proxy class
is initialized - so in this case never. The overhead for such
applications is therefore an empty ConcurrentHashMap instance plus the
overhead for a pointer slot in the ClassLoader object multiplied by
the number of ClassLoaders (typically 2). An empty ConcurrentHashMap
in JDK8 is only pre-allocating a single internal Segment:
java.util.concurrent.ConcurrentHashMap@642b6fc7(48 bytes) {
keySet: null
values: null
hashSeed: int
segmentMask: int
segmentShift: int
segments:
java.util.concurrent.ConcurrentHashMap$Segment[16]@8e1dfb1(80 bytes) {
java.util.concurrent.ConcurrentHashMap$Segment@2524e205(40 bytes) {
sync:
java.util.concurrent.locks.ReentrantLock$NonfairSync@17feafba(32 bytes) {
exclusiveOwnerThread: null
head: null
tail: null
state: int
}->(32 deep bytes)
table:
java.util.concurrent.ConcurrentHashMap$HashEntry[2]@1c3aacb4(24 bytes) {
null
null
}->(24 deep bytes)
count: int
modCount: int
threshold: int
loadFactor: float
}->(96 deep bytes)
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
}->(176 deep bytes)
keySet: null
entrySet: null
values: null
}->(224 deep bytes)
...therefore the overhead is approx. 224+4 = 228 bytes (on 32 bit
pointer environments) per ClassLoader. In typical application (with 2
ClassLoader objects) this amounts to approx. 456 bytes.
Is 456 bytes overhead too much?
If it is, I could do lazy initialization of per-classloader CHM
instances, but then the fast-path would incur a little additional
penalty (not to be taken seriously though).
Regards, Peter
P.S. I was inspecting the ClassValue internal implementation. This is
a very nice piece of Java code. Without using any Unsafe magic, it
provides a perfect performant an scalable map of lazily initialized
independent data structures associated with Class instances. There's
nothing special about ClassValue/ClassValueMap that would tie it to
Class instances. In fact I think the ClassValueMap could be made
generic so it could be reused for implementing a ClasLoaderValue, for
example. This would provide a more performant and scalable alternative
to using WeakHashMap<ClassLoader, ...> wrapped by synchronized
wrappers for other uses.
If anything like that happens in the future, the proposed patch can be
quickly adapted to using that infrastructure instead of a direct
reference in the ClassLoader.
-Alan