Question on GCJ/Boehm Memory Utilization, Part II

Craig A. Vanderborgh
Tue Feb 21 16:11:00 GMT 2006

Hello Again!

First of all, thanks for the constructive replies to my previous 
"installment" from David and Rutger.

I have done a whole lot more testing and "dump diving" since then, and I 
have at least determined what is happening with our GCJ/Boehm 
"leaking".  There is no leaking - or at least no leaking that I am 
concerned about in what I can see.

I believe the correct word for what is happening is "fragmentation".  
Hans also calls it "unexpectedly large heap growth".  Maybe this latter 
term is the most appropriate.  What is happening is pretty clear:

1. For processing a large-ish DOM element containing text, the 
application requests and receives a (say) 12932 block from GC.
2. This large-ish block is used to do the job, and then dereferenced.
3. Garbage collection happens, and the 12932-byte block is put back on 
the free list
4. The application requests a small block for a short string, and the 
collector decides to allocate the unused 12932 byte block, even though 
it's way too big.
5. Goto (1)

I have done enough logging and Daney-dumping to know that this how the 
heap becomes large.  It does asymoptotically stabilize at a large value, 
but the value is too large for us to live with.

And so - what to do?  Is it possible to instruct Boehm not to allocate 
block sizes above a certain threshold for short string char[]'s?  Should 
the large-ish blocks be unmapped, so that this can't happen?  Should I 
write some special-purpose classes that make use of CNI to use malloc() 
allocations so that these large-ish (12932) blocks are not requested 
from Boehm GC?

I also want to point out that I do have a "control" test now.  On a test 
that does not have JS tags <script> with large texts in them, but is 
exactly the same otherwise, I get a steady-state heapsize of 1.5 MB 
instead of 13 MB, with all other things being equal.  This shows that if 
my application does not ever request the 12932 blocks, things really 
work quite well with GCJ/Boehm allocation.

Please, if possible, help me reach a clue about how to proceed in the 
most constructive direction.  This would be greatly appreciated.

Thanks in advance,
craig vanderborgh
voxware incorporated

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