Memory not being released?

Gordon McMillan gmcm at hypernet.com
Mon Nov 15 19:18:46 CET 1999


Steve Tregidgo writes:

> I have a process (goes up to 250+ MB, cue lots of virtual memory
> usage and hence slowdown), which busies the server so much that
> it just can't bring itself to do other things -- in other words
> it's severely impractical.
> 
> The actual data that results from the process is relatively small
> (the useful stuff is less than 1MB, and other data -- such as
> objects that have been dealt with, to prevent following circular
> references -- is no more than a few MBs), and certainly well
> below the size of the actual footprint.
> 
> I've gone through the loops and recursion with a fine-tooth comb,
> and found that by eliminating certain function calls the
> footprint is reduced -- the process is thus rendered useless
> (they were the important functions, of course), but at least I
> get an idea of where the memory is going.
> 
> On the other hand, commenting out some other expressions (such as
> those that remember data -- in other words, the things that I
> would expect to take up memory and not release it again) doesn't
> make a blind bit of difference.

That's a pretty good sign that you have circular references that 
are keeping objects alive. Check out Cyclops from 
ftp://ftp.python.org/pub/python/contrib/System/.

Also, be aware that even if your python objects are getting 
collected, that doesn't mean your c runtime is giving the 
memory back to the OS. Although 250M would indicate a 
pretty horrid c runtime.
 
> So what's going on?  My guesses so far have been along the lines
> of refcounts not going down -- how else do I explain why memory
> that was allocated was seemingly not deallocated?  I've tried
> del'ing everything in sight, storing ids instead of objects,
> inserting the odd sys.exc_traceback=None to clear those pesky
> traceback reference holders ... none of it has helped so far.
> 
> Are there other hidden things that might hold references to my
> objects?  And if so, what can I do about them?  After using an
> object once, I don't want (don't need) to keep it in memory any
> more, so for those objects I'll be revisiting at a later date, I
> just remember their id and delete them.

In your code, dict1 is holding a reference to every object 
you've visited, and thus keeping them all alive. OTOH, judging 
by "for item in obj.subs:", it looks like the whole graph is alive 
all the time anyway. Essentially, you've got a huge pointer-
based data structure. You need to hoist most of the smarts up 
to a Graph object, and use some kind of persistent key to 
identify objects (which are really on disk, or recreated on the 
fly...). So that snippet might be:
  for itemname in obj.subs:
    obj = graph.getobject(itemname)
 
> If it's any help, the process does something like this:
> 
> def recurse(obj, list, dict1, dict2):
>   # Prevent circular recursion...
>   if dict1.has_key(obj.__id__):
>     return
>   else:
>     dict1[obj.__id__] = None
> 
>   a = do_thing(obj, dict2)
> 
>   # Recurse into obj's children, or do something else
>   for item in obj.subs:
>     if item.spam:
>       recurse(item, list, dict1, dict2)
>     else:
>       b = do_thing(item, dict2)
> 
>   c = do_thing(obj, dict2)
> 
>   # Remember some things -- commenting
>   # out produces no memory saving.
>   list.append(a,b,c)
> 
> The function do_thing implements a loop; in total (over the whole
> process) the loop's body is executed tens of thousands of times
> -- it causes the biggest memory saving when commented out (and of
> course it's the most important bit), but does no obvious
> "remembering" of things.
> 
> Does anybody have any ideas?  Or can you maybe point me to some
> documentation that deals with this -- the FAQ looked promising
> but ultimately didn't help.
> 
> Cheers,
> Steve
> 
> --
>        -- Steve Tregidgo --
> Developer for Business Collaborator
>         www.enviros.com/bc
> 
> 
> Sent via Deja.com http://www.deja.com/
> Before you buy.
> 
> -- 
> http://www.python.org/mailman/listinfo/python-list



- Gordon




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