RFR: 8062063: Usage of UseHugeTLBFS, UseLargePagesInMetaspace and huge SurvivorAlignmentInBytes cause crashes in CMBitMapClosure::do_bit

[Thomas Schatzl]

Hi Jon,

On Wed, 2015-01-07 at 11:44 -0800, Jon Masamitsu wrote:
> Stefan,
> 
> The CR indicates to me that the clearing would be done at the uncommit() but
> this fix has the clearing done at the commit().  Am I reading the CR and 
> webrev
> correctly?  If so, the arguments for doing the clearing at the 
> uncommit() seem
> good ones.

  Stefan and me talked about the fix again, and somewhat agreed to
implement the "cleaner" way of clearing at commit.

Clearing at uncommit would be less code, we do not need to keep track in
the _needs_clear_at_commit bitmap to avoid the problems with startup
performance, but then people might notice that heap shrinking (after
full gc, i.e. full gc time is slower) is slower than other collectors
because of that ;)

They would probably rather expect this to be done at commit time I guess
(regardless of whether the OS or the VM will do that).

This is of course guesswork, we did not measure the difference, just
talked about options how to fix the issue.

Either way seemed fine to us, this solution is not that much worse in
complexity (even Kim's suggestion probably would not).

Thanks,
  Thomas