RFR(S): 8138681: Runtime.getFreeMemory() reports wrong value after humongous allocation.

[Thomas Schatzl]

Hi David,

On Thu, 2015-11-19 at 11:08 +0100, David Lindholm wrote:
> Hi,
> 
> Please review this patch that changes how the usage of humongous regions 
> are calculated. Since we (currently) cannot use the remainder of the 
> last humongous region, we say that the complete region is used for the 
> humongous object.
> 
> This is done by setting top=end and adding dummy/filler objects to the 
> end of the last region.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8138681
> Webrev: http://cr.openjdk.java.net/~david/JDK-8138681/webrev.00/
> 
> Testing: Passed JPRT, vm.gc testlist and the attached test from the bug.

  two initial comments (I have not looked through the change completely
in detail):

- in g1CollecteHeap.cpp, line 343

 342   size_t fill_size = word_size_sum - word_size;
 343   if (fill_size > 0) {

the condition is wrong. It should be >= min_fill_size().

- I would somewhat prefer if the BOT would not be updated as if the
humongous object were a single object (I think). I won't object to that
if others do not object either. I mean, I do not really see an issue
with that for various reasons, but it would be nice if we tried to
improve that. Maybe it is not worth the effort.

It's nice that this change actually removes some special handling in a
few places.

Thanks,
  Thomas