Loading classes with many methods is very expensive

Stanimir Simeonoff stanimir at riflexo.com
Wed Oct 22 23:57:30 UTC 2014

Class.java can easily be improved by adding an auxiliary  HasMap<String,
Integer/int[]> indexing the position in the array by name and then checking
only few overloaded signatures in case of a match.
The auxiliary map can be deployed only past some threshold of methods, so
it should not affect the common case.

Alternatively sorting the array and using binary search is quite trivial to
implement as well.

Btw, really nice benchmark.


On Thu, Oct 23, 2014 at 1:53 AM, Martin Buchholz <martinrb at google.com>

> Here at Google we have both kinds of scalability problems - loading classes
> from a classpath with 10,000 jars, and loading a single class file packed
> with the maximal number of methods.  This message is about the latter.
> If you have a class with ~64k methods with a superclass that also has ~64k
> methods, class loading that single class will cost you ~30sec and calling
> Class.getMethods another ~10sec.  Both are unacceptably slow. I think both
> are due to O(N^2) algorithms, the first in hotspot, and the second in
> Class.java.
> I have the start of a fix for Class.java, but it makes the common case
> slower.  A really good fix is harder to find.  In general, I think
> Class.java could benefit from some performance-oriented rework.  Is anyone
> else working on class loading performance, especially in hotspot?
> Here's the benchmark (that could perhaps be integrated into openjdk even
> without a fix)
> http://cr.openjdk.java.net/~martin/webrevs/openjdk9/Class.getMethods-benchmark/test/java/lang/Class/getMethods/ManyMethodsBenchmark.java.html
> Base class load time: 186.44 ms
> getDeclaredMethods: Methods: 65521, Total time: 43.27 ms, Time per method:
> 0.0007 ms
> getMethods        : Methods: 65530, Total time: 60.82 ms, Time per method:
> 0.0009 ms
> Derived class load time: 33440.13 ms
> getDeclaredMethods: Methods: 65521, Total time: 39.71 ms, Time per method:
> 0.0006 ms
> getMethods        : Methods: 65530, Total time: 11582.54 ms, Time per
> method: 0.1768 ms

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