GC leaks debugging

Erik Groeneveld erik@cq2.nl
Mon Apr 4 08:53:00 GMT 2011


On Mon, Apr 4, 2011 at 10:13 AM, Andrew Haley <aph@redhat.com> wrote:
> On 03/04/11 18:59, Erik Groeneveld wrote:
>> On Sun, Apr 3, 2011 at 7:14 PM, Erik Groeneveld <erik@cq2.nl> wrote:
>>> On Sat, Apr 2, 2011 at 11:38 AM, Erik Groeneveld <erik@cq2.nl> wrote:
>>>>
>>>>> Note that in the information you posted, the GC was scanning around 7.5MB of roots conservatively.  It might be worth checking what those regions are.
>> [...]
>>> So I am now off into JvCreateJavaVM,
>>
>> and I found that the 7.5 MB roots are the static data area of libgcj
>> itself.  The GC calls back -- the last arg being the size:
>>
>> _Jv_GC_has_static_roots(../gccinstall/lib/libgcj.so.12, 0xb704f000, 7544028)
>>
>> and since libgcj is in 'the store' (_Jv_print_gc_store() prints
>> "../gccinstall/lib/libgcj.so.12"), it tells the GC to scan its static
>> data area conservatively.
>>
>> As of yet I don't understand why this static area is so big, and what
>> could be on it, but when I lay myself to rest, the little gray cells
>> will sing to me (free after Hercules Poirot ;-).
>
> It'll mostly be introspection data.  Every class and every method has
> this, and it can get to be quite large.

I saw the (old) patch of yours that moves static Java objects onto the
heap, avoiding it to be scanned conservatively, so I couldn't think of
anything else to be on the static data area of libgcj than Java
pointers to the objects heap.  Now I see that there is still a lot
more data that must be scanned conservatively, so couldn't there be
similar problems as back then?  Couldn't it be an idea to try to move
this introspection data to the heap as well?

> I doubt it's the cause of
> your memory leak unless there's a bug elsewhere.

Probably there is no clear bug, or clear leak, perhaps just a matter
of pushing the GC to the limits?  Some code is running quite well for
long times, other isn't.  I all cases, the heap grows very fast, with
lots of black listing messages, and sometimes the GC just seems to
manage, sometimes it doesn't and things explode while issuing the
famous "need to allocate large block etc" repeatedly.  From what Hans
suggested and from what I see in the logs, the GC is under very heavy
stress, right from the beginning.  It doesn't get a fair chance so to
say.

My minimal program is now this:

int main(int argc, char *argv[]) {
    _Jv_InitGC();
}

It starts out with:

roots: 7,072 kB
heap: 64 kB
free: 64 kB
blacklisted: 15/16
blacklist messages: 991

Any real program produces so much blacklist messages that it hardly
runs.  I'd like to investigate this or am I on the wrong track
completely?

Erik

>
> Andrew.
>



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