Thanks, that clarifies a lot to me, especially SoftReference.
So with Groovy it is only realistic to have GC of classes (and attached
ClassInfo) kick in once a limit on Metaspace/PermGen (or Heap) is
reached - fine with me, no point to try to "outrun the bear"... :)
A general question (current implementation and most likely APIs to keep
aside): Why does ClassInfo need a reference to the class? To me the use
case would be that you have an Groovy object or a Groovy class and want
to do something with it (call a static or instance method, for example),
so you only need to find ClassInfo from the class and then maybe pass
the class temporarily just for doing things, but don't need it a
reference back from ClassInfo.
I presume I am simply overlooking something rather obvious, but what
would be the use case(s)? Or maybe historical reasons?
I have updated ClassGCTester ("version" 2.0.0) such that the class path
for the URLClassLoader can be specified in more detail.
https://github.com/jexler/classgc
--
Usage: java [java-args] ClassGCTester -cp <class-path> -parent
[tester|null] -classes <classes> [-wait]
-cp: Class path for URLClassLoader that will load the test classes.
Directories and JARs separated by ':' or ';'.
Example: .:classes/:libs/groovy-2.4.6.jar
-parent: Parent class loader for URLClassLoader that will load the
test classes:
'tester': ClassGCTester.class.getClassLoader() - i.e. same
as tester
'null': null - i.e. no parent
-classes: Test classes to load from the URLClassLoader in a loop.
Fully qualified class names separated by ':' or ';'.
Example: SampleInDefaultPackage:net.sample.Sample
-wait: Optional, whether to wait for key pressed before starting
the test.
Allows to attach external tools from the start.
(Like 'jvisualvm' or 'jstat -gc <interval-ms>
<number-of-iterations>' etc.)
Note that this tool usually prints out its PID for convenience.
--
This allows, for example, two produce two of the known
"OutOfMemoryError: Metaspace|PermGen" issues with Groovy 2.4.6, as follows.
Directory Structure:
- GroovyGCTester.class
- groovy-2.4.6.jar
- filling/GroovyFilling.class ("42" compiled, for example, but could be
practically any Groovy class)
If you load Groovy only once with the class loader of ClassGCTester, and
set groovy.use.classvalue=false (the default) you get the error:
$ java -XX:MaxMetaspaceSize=64m -Xmx512m -Dgroovy.use.classvalue=false
-cp .:groovy-2.4.6.jar ClassGCTester -cp filling/ -parent tester
-classes GroovyFilling
--
Secs Test classes Metaspace/PermGen Heap Load time Create
time
#loaded #remaining used committed used
committed average average
[...]
11 8107 8107 38.3m 55.1m 293.8m 455.5m
0.433ms 0.909ms
12 8579 8579 40.1m 57.9m 298.0m 455.5m
0.581ms 0.909ms
13 8665 8665 40.4m 58.5m 309.5m 455.5m
0.577ms 0.908ms
14 9423 9423 43.3m 63.0m 327.0m 455.5m
0.545ms 0.940ms
Exception in thread "main" java.lang.OutOfMemoryError: Metaspace
[...]
--
With true GC kicks in at the limit of 64m:
--
Secs Test classes Metaspace/PermGen Heap Load time Create
time
#loaded #remaining used committed used
committed average average
[...]
12 8579 8579 40.0m 57.9m 300.7m 455.5m
0.485ms 0.915ms
13 9424 9424 43.3m 62.8m 330.0m 455.5m
0.494ms 0.907ms
14 9670 19 7.3m 30.0m 21.0m 367.0m
0.527ms 0.908ms
15 10899 1248 12.0m 31.3m 103.2m 367.0m
0.484ms 0.880ms
[...]
--
If you instead load Groovy with the same URLClassLoader that loads
GroovyFilling each time, it passes with groovy.use.classvalue=false (the
default):
$ java -XX:MaxMetaspaceSize=64m -Xmx512m -Dgroovy.use.classvalue=false
-cp . ClassGCTester -cp groovy-2.4.6.jar:filling/ -parent null -classes
GroovyFilling
--
Secs Test classes Metaspace/PermGen Heap Load time Create
time
#loaded #remaining used committed used
committed average average
[...]
1 9 9 22.4m 23.1m 26.9m 242.0m
1.622ms 112.702ms
2 18 18 38.1m 39.4m 47.9m 308.0m
1.419ms 112.346ms
3 27 27 53.7m 55.4m 112.4m 308.0m
1.367ms 111.185ms
4 36 5 14.0m 28.0m 41.5m 457.0m
1.315ms 111.633ms
5 47 16 33.0m 37.4m 49.0m 474.5m
1.242ms 106.555ms
[...]
--
Whereas you get the error with true:
--
[...]
Secs Test classes Metaspace/PermGen Heap Load time Create
time
#loaded #remaining used committed used
committed average average
0 1 1 8.0m 8.5m 17.2m 245.5m
2.158ms 119.715ms
1 9 9 22.3m 23.0m 24.2m 239.5m
1.596ms 109.466ms
2 18 18 37.8m 39.3m 46.2m 302.0m
1.371ms 109.369ms
3 27 27 53.3m 55.3m 109.8m 302.0m
1.388ms 109.633ms
Exception in thread "main"
Exception: java.lang.OutOfMemoryError thrown from the
UncaughtExceptionHandler in thread "main"
--
For completeness, ClassGCTester was compiled with Java 6 for Java 6,
GroovyFiller with Groovy 2.4.6 (on Java 8, to Java 5) and the tests were
run with Java 8 (Oracle JDK, Mac).
Alain
