RFR (XXS) 8008964 - Thread::_metadata_handles uses 1 KB per thread

[Ioi Lam]

Please review a very small fix:

http://cr.openjdk.java.net/~iklam/8008964/metadata_handles_001/

Bug: NPG: Memory regression: Thread::_metadata_handles uses 1 KB per thread.

http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=8008964
     https://jbs.oracle.com/bugs/browse/JDK-8008964

Background:

     Thread::_metadata_handles is a GrowableArray that keeps class metadata
     alive during class redefinitions. See metadataOnStackMark.cpp.

     The initial allocation size is 300 elements per Thread. This seems 
too big.

Summary of fix:

     I changed the initial allocation of 300 elements to 30 elements.

     I added traces in a private build and found that the max array length
     is usually around 100, and most of the threads never use more than
     30 elements.

     For Eclipse:

         metadata_handles expanded from [  0] = 34 time(s) // a total 34 
threads have been created
         metadata_handles expanded from [ 30] =  9 time(s)
         metadata_handles expanded from [ 60] =  2 time(s)
         _metadata_handles_max_len = 114

     For MedRec:

         metadata_handles expanded from [  0] = 156 time(s)
         metadata_handles expanded from [ 30] =  19 time(s)
         metadata_handles expanded from [ 60] =   4 time(s)
         _metadata_handles_max_len = 104

     So it looks like we can safely reduce the initialize of 
Thread::_metadata_handles from 300 to 30,
     a saving of 2160 bytes per live Thread (64-bit VM) or 1080 bytes 
for 32-bit VM.

Tests:

     UTE (vm.runtime.testlist, vm.quick.testlist, 
vm.parallel_class_loading.testlist)

Thanks
- Ioi