Re: Setting memory-chunk-size claimed when free-memory approacheszero?

[Bryce McKinlay <mckinlay at redhat dot com>]

Martin Egholm Nielsen wrote:

> Hi again,
>
> Now I'm up for a new question:
>
> If I continue allocating chunks of memory - say, like "new 
> byte[10000]"    - and of course keep references to the allocated 
> arrays - I will at some time run out of "free memory" in the sense of 
> "Runtime#freeMemory()". Hence, some mechanism in GCJ allocates some 
> more memory so that "freeMemory()" increases, and same goes for 
> "Runtime#totalMemory()".
>
> That's fine...
>
> However, the amount of memory claimed every time this 
> "low-memory"-boundary is near, seems to be proportional with the 
> previous total-memory-incrementation.
> This may in many cases be the most simple/plausible algorithm. But 
> when I hit the roof while allocating the next chunk - say going from 
> 20megs total to what would have been 26megs of total (according to 
> Runtime#totalMemory()) - my application terminates because there 
> simply isn't that amount available from the system:
>
> *** MEM CHUNK TAKEN: 98304
> *** MEM CHUNK TAKEN: 131072
> *** MEM CHUNK TAKEN: 176128
> *** MEM CHUNK TAKEN: 233472
> *** MEM CHUNK TAKEN: 311296
> *** MEM CHUNK TAKEN: 417792
> *** MEM CHUNK TAKEN: 557056
> *** MEM CHUNK TAKEN: 741376
> *** MEM CHUNK TAKEN: 987136
> *** MEM CHUNK TAKEN: 1318912
> *** MEM CHUNK TAKEN: 1757184
> *** MEM CHUNK TAKEN: 2342912
> *** MEM CHUNK TAKEN: 3125248
> *** MEM CHUNK TAKEN: 4165632
> Terminated
>
> Then I'd expected one of two situation - but neither occur:
>
> 1) The memory allocated is adjusted to the largest possible - so that 
> the increase in the above example may be from 20mb to 24mb.
>
> 2) I'm thrown an OutOfMemoryError error.
>
> But neither happens - my application gets terminated by the Linux 
> kernel...
>
> Now, the real question: Is there a way to circumvent this? Can I 
> configure the chunk-eating-mechanism so that it does not allocate 
> chunks larger than, say, 100kb at a time...


GCJ's garbage collector does not, by default, set a maximum heap size. 
This means that it will keep requesting more memory from the system, if 
required, until VM is exhausted. On Linux systems this might mean that 
the kernel kills your application.

The way to avoid this is to set the maximum heap size to the maximum 
amount of memory you want your application to use. You can do this by 
setting the environment variable GC_MAXIMUM_HEAP_SIZE=<bytes>

Regards

Bryce