[Gc]: Patch resubmission: Default stop_func

Boehm, Hans hans.boehm at hp.com
Tue Sep 22 14:40:10 PDT 2009


> From:  Ivan Maidanski
> Sent: Tuesday, September 22, 2009 12:07 AM
> To: gc at napali.hpl.hp.com
> Subject: Re[2]: [Gc]: Patch resubmission: Default stop_func
> 
> Hi!
> 
> "Boehm, Hans" <hans.boehm at hp.com> wrote:
> > Ivan -
> >
> > I just got a chance to look at this, and I don't quite 
> understand the logic.
> 
> The logic in brief is described in gc.h for GC_set_stop_func:
> /* Set and get the default stop_func.  The default stop_func 
> is used by */
> /* GC_gcollect() and by implicitly trigged collections 
> (except for the  */
> /* case when handling out of memory).  Must not be 0.         
>           */
> 
> That is, whenever we are collecting (for reasons other than 
> in case of out of memory), a user-supplied stop_func is used 
> instead of never_stop_func. Initially, this user-supplied 
> (default) stop_func is never_stop_func (so, the logic remains 
> exactly as before unless the client uses GC_set_stop_func).
Just to be clear, if I read the code correctly, it does seem to be used when an allocation request cannot be satisfied with memory already in the heap because the heap is full of garbage.  In that case GC_should_collect() would be true, and thus it would try to collect.  It would use the timeout unless GC_bytes_allocd == 0, i.e. unless the last collection was unsuccessful.

Thus for a steady state application that's running in its proper heap size, you may still get a timeout here, the GC may fail, and you may increase the heap size further.
> 
> The purpose of a stop_func itself is described in the gc.h 
> comment for GC_try_to_collect(). (It might be good, also, to 
> note there that stop_func is called with the lock held and 
> that it must not manipulate pointers to GC heap.)
Agreed.

> 
> >
> > GC_collect_or_expand() is called in cases in which we need 
> more memory, but none is available in the existing heap.  I 
> think your patch changes the nonincremental case to collect 
> with a timeout here.
> 
> Yes. (I've also changed some simple cases of incremental mode 
> - the rest I've marked with FIXME).
> 
> >  If it actually times out, you will end up growing the heap.
> 
> That's upon the client - he is warned by GC_set_stop_func and 
> GC_try_to_collect comments (I think warned enough). The 
> client might use a sophisticated logic (e.g, use 
> GC_get_heap_size_inner()), while the GC logic for it remains 
> simple - call stop_func regularly.
> 
> >  The problem is that for a long running application with 
> fixed space requirements, this will probably happen 
> regularly, and I suspect the heap will grow without bounds, 
> since every time it runs out, there is some probability it 
> will grow the heap even if there would have been plenty of 
> space after collection.
> 
> This shouldn't be a problem due to:
> - there is a heap limit (which, again, could be preset by the client);
Yes, but I also don't think it helps here.  You''l just collect and fail before you do the real collection.
> - in case of unmapping enabled, the heap might shrink down 
> after several successful collections;
Virtual memory doesn't currently shrink.  Thus we're still exhausting a resource, though one that'as usually more plentiful.  It may not be as plentiful as we would like on 32-bit platforms.
> - the client could do GC_gcollect() (which uses client 
> stop_func too) in yield loops (regularly or base on some heap 
> size condition).
I think I would rather have the client do explicit GC_try_to_collect calls occasionally (hopefull after checking that enough allocation has occurred), and have the collector run to completion when triggered internally.
> 
> >
> > This isn't critical, since none of this applies if you 
> don't use the new functionality.  But I wonder what the 
> intent was, and if this is really useful as is.  At least we 
> should document the hazards.
> 
> The intent is for time-critical applications: suppose you 
> need a time-guaranteed reaction for a external/async signal 
> (eg., a user presses a button). With a client "default 
> stop_func", it's possible to make the world resume nearly 
> immediately (I mean in a some guaranteed max delay) after the 
> event, and process the event within a guaranteed time period 
> (unless, of course, the case when GC is unavoidable to 
> prevent returning NULL).
> 
> You can't use GC_disable/enable_gc() instead of the proposed 
> solution (because GC_dont_gc doesn't interrupt the 
> collection, and because it prevents a collection even 
> initiated by an out-of-memory event).
> 
> More over, with the "default stop_func" concept, it's 
> possible to make an existing application have a 
> time-guaranteed reaction (for external signals) without much 
> code rewrote - just place GC_set_stop_func call at start-up 
> (after GC_INIT).
> 
> Of course, I agree that playing with the "default stop_func" 
> in an application needs its careful profiling.
> 
> Could anybody suggest a better solution (other than "default 
> stop_func" concept)?
> 
> >
> > I really don't think there's a safe way to use 
> GC_try_to_collect that doesn't involve collecting before 
> you're out of memory.
> 
> Why not, even if we set default stop to the corner-case 
> "never collect" (returns 1), then the collections would 
> happen only if the heap is full and the heap reaches it's 
> limit - this should be safe (in real world this could be safe 
> only if GC_MAXIMUM_HEAP_SIZE is set) but the performance 
> would be really poor...
I think it would require a maximum heap size to be set, as you say, and then I think you may still not collect soon enough that you could avoid running a full collection even when told to stop.

A better alternative may be to try to start a collection premptively (maybe in GC_allocobj and GC_alloc_large) when you notice that you may be running low on memory, but you still have blocks left to satisfy the allocation request.  That way if you "time out", you have a chance to try again the next time, without running out of heap or growing it.  Since that test probably isn't free, and this seems fairly special purpose, I would probably put it under yet another compile-time flag.

The alternative is to require the client to do this itself, based GC_get_bytes_since_gc() and GC_get_heap_size() or the like.  That's essentially the current solution.

Hans
> 
> >
> > Hans
> > > From: gc-bounces at napali.hpl.hp.com
> > > [mailto:gc-bounces at napali.hpl.hp.com] On Behalf Of Ivan Maidanski 
> > > Hi!
> > >
> > > This suggested patch (ivmai129.diff), superseding diff47 
> [Nov 20], 
> > > introduces the default stop_func used on implicitly trigged 
> > > collections (unless handling OOM, of course). For more info, see 
> > > (the first 2 paragraphs):
> > > http://permalink.gmane.org/gmane.comp.programming.garbage-coll
> > ection.boehmgc/2513
> > >
> > > Note: the default stop_func is also used for 
> GC_gcollect() - this is 
> > > a bit arguable but I think it's more logical (explicit fn is used 
> > > for GC_try_to_collect(), implicit default stop_func is used for 
> > > collections trigged by other
> > > means) and more useful (eg., a timeout policy could be 
> introduce to 
> > > an existing application by just adding
> > > GC_set_stop_func(fn) instead of replacing all GC_gcollect() calls 
> > > with something like GC_try_to_collect(GC_get_stop_func())).
> > > ...
> 
> Bye.
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