Heap fragmentation (Was: Debugging "Leaks" With Boehm-GC)

[Martin Egholm Nielsen]

Hi,

>> We are currently experiencing some dire difficulties with one of GCJ's 
>> most opaque aspects - garbage collection.
>>
>> The application exhibiting "leaks" is an XML browser of our own 
>> creation.  The use case that "leaks" is one where our browser software 
>> works its way through interpreting some page elements, and then throws 
>> an "event", which is implemented by throwing a Java exception, which 
>> something up the stack elements then catches.  The software works 
>> beautifully, except it "leaks".
> The boehm GC in certain circumstances suffers from memory fragmentation. 
>  When this happens, even though there may be 'free' memory, it cannot be 
> used so the GC must try to obtain more memory from the OS.
> 
> The solution we found was to increase the GC_free_space_divisor (to a 
> value of 20).  Using this in conjunction with calling 
> _Jv_SetMaximumHeapSize() to set the maximum heap size gives us very good 
> results (A fixed heap size that never fills up).  There is a trade off 
> in increasing the GC_free_space_divisor, in that it increases the 
> frequency of GC events with results in reduced performance.

Just stumpled across this old thread - seems that I'm beginning to run 
into fragmentation problems - bot being able to allocate memory for some 
things.
I have set GC_MAXIMUM_HEAP_SIZE to 14000000 (14mb), but I wonder where I 
can configure GC_free_space_divisor? In "alloc.c" it seems - but there 
it's set to 3, whereas "gc.h" says it's initially 4?!

Moreover, is 20 a super value? Or is this trial-and-error?

Regards,
  Martin Egholm