RFR (M): 8027959: Investigate early reclamation of large objects in G1

Thomas Schatzl thomas.schatzl at oracle.com
Mon Jul 21 13:20:57 UTC 2014


Hi Mikael,

  thanks for the review, and finding these issues!

On Thu, 2014-07-17 at 16:52 +0200, Mikael Gerdin wrote:
> Hi Thomas,
> 
> On Tuesday 15 July 2014 11.10.53 Thomas Schatzl wrote:
> > Hi all,
> > 
> >   could I have reviews for the following change that allows G1 to
> > eagerly/early reclaim humongous objects on every GC?
> > 
> > Problem:
> > 
> > In G1 large objects are always allocated in the old generation,
> > currently requiring a complete heap liveness analysis (full gc, marking)
> > to reclaim them.
> > [...]
> > The CR contains a graph showing large improvements on average humongous
> > object reclamation delay. In total we have seen some benchmarks
> > reclaiming GBs of heap space over time using this functionality (instead
> > of waiting for the marking/full GC). This improves throughput
> > significantly as there is more space available for the young gen on
> > average now.
> > 
> > Also it might avoid users to manually increase heap region sizes just to
> > avoid humongous object troubles.
> > 
> > CR:
> >  https://bugs.openjdk.java.net/browse/JDK-8027959
> > 
> > Webrev:
> >  http://cr.openjdk.java.net/~tschatzl/8027959/webrev/
> 
> g1CollectedHeap.hpp:
> 
> Most of the new functions you add for humongous regions operate on region 
> indices, except for
> +  bool humongous_region_is_always_live(HeapRegion* region);
> Is there any reason for it not operating on a region index as well? I think 
> that would make the API nice and consistent.

Fixed, although internally the code will immediately convert to
HeapRegion* anyway.

> g1CollectedHeap.cpp:
> 
> This comment is pretty confusing to me, can you try to rephrase it?
> 3798     if (is_candidate) {
> 3799       // Do not even try to reclaim a humongous object that we already 
> know will
> 3800       // not be treated as live later. A young collection will not 
> decrease the
> 3801       // amount of remembered set entries for that region.

Hopefully fixed (changed to):

+// Is_candidate already filters out humongous regions with some
remembered set.
+// This will not lead to humongous object that we mistakenly keep alive because
+// during young collection the remembered sets will only be added to.


> 
> forwardee can only be null here for humongous objects, correct?
> Would it make sense to add an else-clause asserting that to keep some of the 
> old assert functionality?
> 4674     if (forwardee != NULL) {
> 4675       oopDesc::encode_store_heap_oop(p, forwardee);
> 4676       if (do_mark_object != G1MarkNone && forwardee != obj) {
> 4677         // If the object is self-forwarded we don't need to explicitly
> 4678         // mark it, the evacuation failure protocol will do so.
> 4679         mark_forwarded_object(obj, forwardee);
> 4680       }
> 4681 
> 4682       if (barrier == G1BarrierKlass) {
> 4683         do_klass_barrier(p, forwardee);
> 4684       }
> 4685       needs_marking = false;
> 4686     }

I refactored the change so that the closure do_oop_work() methods
directly use the value from the in_cset table, so this is obsolete now.

> 
> g1OopClosures.inline.hpp
> 
> Are you sure that the change to is_in_cset_or_humongous is correct here?
> I think that when FilterIntoCSClosure is used for card refinement we are not 
> actually interested in references to humongous objects since they don't move. 
> So this could cause us to needlessly add cards to the into_cset_dcq.
> 
> However in the case of scanning remembered sets it's the new check is probably 
> exactly what you are looking for.
>   43 template <class T>
>   44 inline void FilterIntoCSClosure::do_oop_nv(T* p) {
>   45   T heap_oop = oopDesc::load_heap_oop(p);
>   46   if (!oopDesc::is_null(heap_oop) &&
>   47       _g1-
> >is_in_cset_or_humongous(oopDesc::decode_heap_oop_not_null(heap_oop))) {
>   48     _oc->do_oop(p);
>   49   }
>   50 }

I think it does not hurt too much.

> 
> Perhaps this should be left for a further cleanup, one approach could be to 
> fold the logic using is_in_cset_or_humongous into HeapRegion_DCTOC since 
> scanCard seems to be the only place where that closure is used.

I agree.
> 
> Previsously the else-clause here would cause a remembered set entry of the 
> region containing "obj" to be added, to remember the reference "p"
> Now that you're removing humongous objects from this consideration, can we 
> miss remembered set entries for humongous regions?
>   65 inline void G1ParScanClosure::do_oop_nv(T* p) {
>   66   T heap_oop = oopDesc::load_heap_oop(p);
>   67 
>   68   if (!oopDesc::is_null(heap_oop)) {
>   69     oop obj = oopDesc::decode_heap_oop_not_null(heap_oop);
>   70     if (_g1->is_in_cset_or_humongous(obj)) {
>   71       // We're not going to even bother checking whether the object is
>   72       // already forwarded or not, as this usually causes an immediate
>   73       // stall. We'll try to prefetch the object (for write, given that
>   74       // we might need to install the forwarding reference) and we'll
>   75       // get back to it when pop it from the queue
>   76       Prefetch::write(obj->mark_addr(), 0);
>   77       Prefetch::read(obj->mark_addr(), (HeapWordSize*2));
>   78 
>   79       // slightly paranoid test; I'm trying to catch potential
>   80       // problems before we go into push_on_queue to know where the
>   81       // problem is coming from
>   82       assert((obj == oopDesc::load_decode_heap_oop(p)) ||
>   83              (obj->is_forwarded() &&
>   84                  obj->forwardee() == oopDesc::load_decode_heap_oop(p)),
>   85              "p should still be pointing to obj or to its forwardee");
>   86 
>   87       _par_scan_state->push_on_queue(p);
>   88     } else {
>   89       _par_scan_state->update_rs(_from, p, _worker_id);
>   90     }
> 

Fixed.

> I also somewhat agree with Bengt's opinion about explicitly checking for 
> humongous objects.

Done.

There were some more changes that are related to Bengt's suggestions. I
will answer there.

Diff to last:
http://cr.openjdk.java.net/~tschatzl/8027959/webrev.0_to_1
Complete:
http://cr.openjdk.java.net/~tschatzl/8027959/webrev.1

Thanks,
  Thomas




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