RFR: 8240556: Abort concurrent mark after effective eager reclamation of humongous objects [v5]

[Thomas Schatzl]

> Hi all,
> 
>   please review this change that implements concurrent mark abort as proposed by Liang Mao from Alibaba.
> 
> Basically, if at the end of the concurrent start pause we notice that we went below the IHOP threshold (and it has been
> a concurrent start pause caused by humongous allocation), instead of scheduling a mark operation, we schedule a
> "concurrent undo" operation that undoes the changes.  Regarding the removal of the
> test/hotspot/jtreg/gc/stress/jfr/TestStressBigAllocationGCEventsWithG1.java test: it only ever accidentally worked in
> G1. G1 never sent the AllocationRequiringGC events for GCs caused by going over the IHOP threshold for humongous
> allocations that the test actually expects.  That test previously only worked because G1 could not reclaim the
> humongous objects fast enough so crossing the IHOP threshold causes a full concurrent mark. Allocations during that
> concurrent mark do not cause a GC that can reclaim these objects, so ultimately some young GC that sends the
> AllocationRequiringGC event will be sent.  With concurrent undo this is not guaranteed any more, i.e. only in
> environments where concurrent undo is slow (and we'll improve it soon) this test works.  The test is too timing
> dependent, and checking for the wrong thing in this case, so removing it.  Testing: tbd.

Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:

  ayang comments

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/177/files
  - new: https://git.openjdk.java.net/jdk/pull/177/files/d61a43d6..0824a091

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=177&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=177&range=03-04

  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/177.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/177/head:pull/177

PR: https://git.openjdk.java.net/jdk/pull/177