RFR(XS): 8214235: assertion in collectedHeap.cpp: attempt to clean empty remainder

[Thomas Schatzl]

Hi all,

On Mon, 2018-12-31 at 15:26 +0000, Hohensee, Paul wrote:
> Yes, please proceed with the fix to tlab_alloc_reserve(). The
> original assert was doing its job of detecting a bug.
> 
> You're correct, I was looking at 
> http://cr.openjdk.java.net/~bulasevich/8214235/webrev.01/. In that,
> chunks less than min_fill_size() could be left unfilled. Unlikely,
> but possible given a bug somewhere else.
> 
> In http://cr.openjdk.java.net/~bulasevich/8214235/webrev.00/, if the
> space available between top() and hard_end() is less than
> min_fill_size(), fill_with_dummy_object() should assert.
> 

The .00 patch is the correct and most space-efficient one.

As described earlier, if the VM can always fit a minimum sized object
in any remainder, we do not need a tlab alloc reserve (i.e. end ==
hard_end).

The suggested change in CollectedHeap::tlab_alloc_reserve() would just
waste some memory in these cases.

This is e.g. the case on 32 bit systems with ObjectAlignmentInBytes>=8
(the default) where the minimum instance size is 8 bytes, or on 64 bit
systems with ObjectAlignmentInBytes>=16 where minimum object instance
is 16 bytes.

A better check instead of "top() < hard_end()" would be "top() !=
hard_end()" - as in this case, if Universe::fill_with_dummy_object() is
still not able to fill in an object (it should if everything has been
done correctly), it would assert for us. Basically in case objects are
not aligned correctly.

(I already tried to suggest something like that in 
http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2018-November/024092.html
 , but re-reading my answer it may not have been clear enough).

Thanks,
  Thomas