RFR(S): 8132715: Add tests which check that no allocations allowed in any of humongous regions​

[Thomas Schatzl]

Hi Kirill,

On Tue, 2016-06-21 at 18:14 -0400, Derek White wrote:
> On 6/21/16 1:45 PM, Kirill Zhaldybin wrote:
> > 
> > Dear all,
> > 
> > Could please review this test for 8132715?
> > 
> > The test fills heap with humongous objects with a lot of wasted
> > space, 
> > then starts to allocate non-humongous objects and checks that no 
> > allocations happened in humongous regions.
> > There are 3 runs of the test: with filled 10%, 50% and 80% of heap.
> > 
> > CR: https://bugs.openjdk.java.net/browse/JDK-8132715
> > WebRev: 
> > http://cr.openjdk.java.net/~kzhaldyb/webrevs/JDK-8132715/webrev.01/
> > 
> > Thank you.
> > 
> > Regards, Kirill

  I would kind of prefer if the test took a more systematic approach,
i.e. increase the "opportunity" that allocations at the end of
humongous objects occur, and increase the kind of allocations that may
occur.

So the test first allocates one of three different kinds of humongous
objects (half a region, "just above a region" and "just above two
regions").

Then it allocates random object arrays, trying to make G1 allocate at
the end of an existing humongous object (apparently at least).

So, while the setup seems fairly okay, I am not sure whether
randomizing the initial humongous object sizes makes a lot of
difference (it does not to the allocator). Maybe, just maybe having all
of these in the test at the same time might make the test more
reproducable. It's not a big deal though.

My main concern is with the second part, the allocation of the "poison"
objects by the Allocator thread.
Since they are smaller than a region size, they will always be
allocated in young gen first. Which means, that it is very unlikely
(since they are fairly short living) that they may be allocated in old
gen.

So the amount of opportunities when such a bad allocation happens seems
too small.

Thanks,
  Thomas