Loading classes with many methods is very expensive

Karen Kinnear karen.kinnear at oracle.com
Thu Oct 23 13:48:13 UTC 2014


Which hotspot algorithm is the one you identified as slow? Would that be
for loading the class or for Class.getMethods?


On Oct 23, 2014, at 9:37 AM, Peter Levart wrote:

> 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...
> Peter
>> 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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