EVALUATION

The problem was identified as the pessimistic setting of _unallocated_block
which was subsequently used for determining the range of the card table
to be scanned for the perm gen. With DefNew, _unallocated_block is not
used by default. Nevertheless, even with +BlockOffsetArrayUseUnallocatedBlock,
it was found that DefNew did not suffer a large penalty from scanning a
mostly empty (unused) perm gen. The parallel scanning code appears to
suffer a large penalty as in the Noviforum test case, which should be
independently investigated.

For the case of Posco, in the short term, though, it appears to suffice
to use a more discriminating definition of used_region() which exploits
the fact that perm gen may be using all of the unused perm space as a
single large linear allocation block.

For a set of diffs that takes the above approach, see the
suggested fix section.


###@###.### 2004-12-21 18:55:15 GMT

For a simpler, cleaner fix from Jon, see suggested fix section.


###@###.### 2004-12-21 19:46:54 GMT
                                     

SUGGESTED FIX

see http://analemma.sfbay/net/spot/scratch/ysr/bugs/webrev.posco
for a preliminary webrev wrt the Mustang (1.6) train. This may be
used as a starting point for the fix, and is easily portable in
its current form back to the 1.4.2 release train.
###@###.### 2004-12-21 18:55:15 GMT

Jon M. suggested the following cleaner, simpler fix,
being the deletion of a single line from the CFL space
c'tor:

    This is the change in compactibleFreeListSpace.cpp

    79  #if 0  // Investigating large parnew pauses for large perm gen
    80      _bt.alloc_block(addr, addr + fc->size());
    81  #endif



###@###.### 2004-12-21 19:46:54 GMT
                                     

WORK AROUND

For applications that have a large dynamic range of occupancy of
the perm gen there appears to be no real workaround.

For applications that do not have a large dynamic range in the
perm gen, a solution is to keep as little free space (head room)
above the perm gen as possible. This will avoid card-scanning/chunking
penalties associated with the large unused linear allocation block.
###@###.### 2004-12-21 18:55:15 GMT