RFR(M) 8150721: Don't explicitly manage G1 young regions in YoungList

[Thomas Schatzl]

Hi Mikael,

On Thu, 2016-04-28 at 14:37 +0200, Mikael Gerdin wrote:
> Hi,
> 
> Please review this change to change G1 to stop explicit tracking of
> all 
> young regions in the YoungList class.
> With JDK-8150393 the survivor regions are tracked in a
> GrowableArray. 
> This removes the final dependency on keeping a linked list of young 
> regions in YoungList and this change is a cleanup to get rid of a lot
> of 
> code related to that.
> 
> There are a few notable changes due to this cleanup:
> * During full GCs, calls to uninstall the surv rate group for young 
> regions were moved to TearDownRegionSetsClosure
> * The young RSet sampling thread can no longer iterate over the young
> list to sample RSets, it must instead walk the incremental collection
> set. I would argue that this is more correct anyway since eden
> regions which had not yet been added to the collection set would not
> have any interesting RSets anyway.
> 
> I decided to make eden and survivors two different classes for now, 
> since it's more clear that they are actually distinct that way.
> The reason for keeping G1EdenRegions around when it's practically
> empty is that I plan to move the eden SurvRateGroup object these
> YoungList replacements to get rid of set_region_{eden,survivor}
> 
> Webrevs:
> Webrev for entire change
> http://cr.openjdk.java.net/~mgerdin/8150721/webrev.0.full/

  - there are significant merge errors due to recent changes.

  - pre-existing: in G1CollectedHeap::check_young_list_empty() there is
no need to iterate over the regions if young_regions_count() is already
zero. The method could also be made ASSERT only (or do the usual
PRODUCT_ONLY decoration trick).

  - the comment in line 5144 of g1CollectedHeap.cpp is out of date now.

  - g1ConcurrentMark.cpp:313: s/comparision/comparison

  - I am not sure about the gain of the assert in
line g1ConcurrentMark.cpp:313, but I won't object to have it.

  - g1DefaultPolicy.cpp:399-401: I think the CR is not needed in 399,
and the comment should be removed as it seems obsolete.

  - in a follow-up CR, maybe change the name of youngList.* to
something fitting the naming scheme.

  - the includes in youngList.cpp should be trimmed down.

Thanks,
  Thomas