Loading classes with many methods is very expensive

Peter Levart peter.levart at gmail.com
Thu Oct 23 13:37:02 UTC 2014

On 10/23/2014 01:57 AM, Stanimir Simeonoff wrote:
> 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.

Java Class.getMethods() implementation is complicated by the fact that, 
although not specified, the order of methods in returned array is 
important. Once it changed, if I remember correctly, and broke many 
programs, so it had to be restored...


> Btw, really nice benchmark.
> Stanimir
> On Thu, Oct 23, 2014 at 1:53 AM, Martin Buchholz <martinrb at google.com>
> wrote:
>> 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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