Question on GCJ/Boehm Memory Utilization

Craig A. Vanderborgh craigv@voxware.com
Sun Feb 19 06:30:00 GMT 2006


Hello!

We're running GCJ 3.3 w/Boehm GC 6.2 on the arm-wince-pe platform 
(please don't laugh!)

Our Windows CE embedded devices are pretty memory-challenged, as they 
have only 128 MB of RAM, and about 35 MB of this is consumed by the CE 
"operating system" itself.

Our application (on the CE target device) is regularly running out of 
memory, and we're trying to figure out what we can do to improve the 
situation.

Using the Avtrex GC dumping tool, I have observed that GCJ and Boehm are 
making extremely inefficient use of the available memory in our most 
important use case.  The use case involves parsing and tokenizing XML, 
and then storing those results in a DOM tree.  The implementation makes 
use of the Xerces DOM classes and it's all pretty straight ahead stuff 
and it all seems to work correctly.

The character arrays created by GCJ/Boehm for storing the 
java.lang.String tokens that result from XML processing are where I 
believe there is a problem.  These are short strings, typically around 
10 or so characters in length.  GCJ/Boehm seems to be allocating 
2048-byte heap blocks, and in many cases - even 12932-byte heap blocks, 
and then putting only one or two of the tokens resulting from XML 
parsing into them.  Just a few characters.  In some cases, 2 of these 
short tokens share a 2048-byte or 12932-byte block, but very often they 
don't.

The end result is that the limited amount of memory that we do have on 
our target device is largely squandered.  Is there a good reason why the 
character arrays for java.lang.String's are always stored within large 
(2048/12932 byte) PTRFREE Boehm heap allocations?  Are there things we 
could do to coax Boehm GC into using smaller allocations in these 
situations, say 128 bytes instead of 2048?

And finally, is there anything that was changed in later versions (than 
3.3/6.2) of GCJ/Boehm to mitigate this problem?

Please advise.  We are trying hard to find a little bit more breathing 
room...

Thanks in advance,
craig vanderborgh
voxware incorporated



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