RFR: Traversal: Don't traverse new objects

[Aleksey Shipilev]

On 02/05/2018 12:16 PM, Roman Kennke wrote:
> With Traversal GC I'm currently observing final-pauses of several dozen (e.g. ~70) ms. Turns out
> that almost all of it is just traversing just-allocated objects. It occurs to me that this is
> pointless work: we're not going to evacuate any new object during this cycle, and liveness
> information may not be that useful either into next cycle because it usually degenerates quickly
> (for new stuff). Let's not traverse new-alloc-regions, and don't count liveness there, which means
> it's going to end up in next cycle's cset (just like the alloc-regions between cycles). This cuts
> pauses down to 3-4ms. Yay!
> 
> Notice that this is safe because new objects don't point to anything, and any ref stored into new
> objects during the pause get picked up by the enqueing barrier for I-U.
> 
> Notice that we *do* need to traverse and account evac-regions, because they have references pointing
> to interesting stuff. Hence the disctinction for is_alloc_region() (better ideas welcome!).
> 
> Also notice that this opens up an interesting opportunity for optimizing the enqueue-barrier: we
> could now elide ENQBs on objects known to be new. (In addition to known-NULL, which we already
> check, and already-enqueued, which we don't do yet. We could also do pretty much what we do with WBs
> and RBs and hoist them out of loops, etc. Lots of work to do there, and opportunities for
> improvements).
> 
> http://cr.openjdk.java.net/~rkennke/traversal-no-traverse-new/webrev.00/

I need more time to understand the logic of this patch. It is seriously weird to see next-TAMS setup
in ShenandoahFreeSet::try_allocate_in, which is on generic allocation path which also should work
for non-Traversal GC modes.

The patch fails at least AllocHumongousFragment with:
 #  assert(!_heap->is_marked_next(oop(humongous_obj))) failed: must not be marked

Other comments:

*) So wait, this means alloc-regions are not the candidates for immediate reclamation now?

 546         if (r->is_humongous_start() && !r->has_live() && !r->is_alloc_region()) {

*) Same for regular trashing?

 556         } else if (!r->is_empty() && !r->has_live() && !r->is_alloc_region()) {

*) Excess parentheses:

shenandoahTraversalGC.cpp:
  73       if ((!_heap->is_marked_next(obj)) && _heap->mark_next(obj)) {

shenandoahTraversalGC.inline.hpp:
  54     if ((!_heap->is_marked_next(obj)) && _heap->mark_next(obj)) {

*) Comment is obsolete.

 981           _verify_liveness_disable,    // liveness data must be complete here


-Aleksey