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From Oleg Kalnichevski <ol...@apache.org>
Subject Re: Issues with DirectByteBufferAllocator and ExpandableBuffer.expand()
Date Fri, 20 May 2011 16:38:48 GMT
On Fri, 2011-05-20 at 05:03 +0530, Asankha C. Perera wrote:
> Hi All
> 
> Using DirectByteBufferAllocator could cause non-heap memory to be 
> allocated which the OS may find difficult to reclaim. On some JVMs this 
> could even lead to a OOM and/or crash (Seen on Debian esp with 32 bit 
> JDK). Using the Heap variant seems to be a solution, although it 
> increases the GC overhead.
> 
> I was also looking at the ExpandableBuffer.expand() method, and was not 
> able to understand why the size is increased as:
> int newcapacity = (this.buffer.capacity() + 1) << 1; The default 8192 
> causes the buffer to become 16386.. what is the reason behind adding 1 
> here..
> 

There is no particular reason but this algorithm appears to be used by
many JRE classes such as StringBuilder. 

> I am thinking of extending the ByteBufferAllocator interface so that one 
> could write an implementation that could cache and reuse buffers. e.g. a 
> pre-allocated number of direct buffers and/or some heap buffers may make 
> this much more optimal. Once a buffer becomes free, the same could be 
> handed back to the allocator, who could then cache or discard that. Will 
> this be a good idea to implement?
> 

There appears to be a general consensus that memory pooling in Java does
not really make much sense. The overhead of maintaining a pool of
objects on the heap is believed to be greater than garbage collecting
unused ones and allocating new ones when needed. Direct buffers may be
more expensive to allocate / deallocate so pooling those may prove
worthwhile on some platforms.

Oleg



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