4837564: (bf) Please make DirectByteBuffer performance enhancements

Alan Bateman Alan.Bateman at oracle.com
Mon Oct 11 07:50:46 PDT 2010

A long standing gripe for many is that direct buffers are page aligned 
with the result that the memory usage can be significantly higher than 
expected. We've never documented the alignment and I'm not aware of 
anyone that depends on it (if you do then speak up now!).  The patches 
here change the implementation so these buffers aren't page aligned by 
default. A new VM option is introduced to force page alignment if  
really needed. Mapped buffers will of course continue to be page aligned.
A related issue is that the addition of the management interface for 
buffer pools subtly changed the semantics of the MaxDirectMemorySize 
option. That option used to set a limit on the total capacity of all 
direct buffers, whereas now it limits the total memory size. This is bug 
6743526, and the webrev includes the changes to restore the original 
semantics (which is course is only interesting when these buffers are 
page aligned).

The webrev with the changes is here. I'm only looking for a reviewer for 
the changes to the jdk repo as I will be going to hotspot-runtime-dev 
for the hotspot change.


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