Review request: 8008549: NPG: SystemDictionary::find(...) unnecessarily keeps class loaders alive

Stefan Karlsson stefan.karlsson at
Thu Feb 21 08:57:20 PST 2013

This change fixes a bug, which showed up as a regression in our Jetty 
footprint regression testing. The bug prevents the young GCs from 
reclaiming some, which in turn holds on to 7 KB 
of native memory each.

Description from the bug report:
Before the PermGen removal a class loader could be garbage collected by 
a young GC if it didn't load any classes and all references to it were 

After the PermGen removal, there's an unnecessary call to 
register_loader in the SystemDictionary::find(...) function:
ClassLoaderData* loader_data = register_loader(class_loader, CHECK_NULL);

which prevents these kind of class loader from being garbage collected 
by the young GCs.

Fortunately, we don't have to register a ClassLoaderData for the class 
loader in the find(...) function. The rational is that if we haven't 
already setup a ClassLoaderData for the class loader, then it won't have 
an entry in the dictionary, and we can safely return NULL.

This bug is one of the reasons for the memory footprint regressions seen 
JDK-8001590 <>: NPG: 
Footprint increase when running Jetty

StefanK & Erik Helin
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