RFR (M): 8207200: Committed > max memory usage when getting MemoryUsage

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that fixes some races when reading
from G1MonitoringSupport members, causing some assertion failing after
weeks of running some programs that poll the G1 MemoryPools.

It does so by adding a mutex that is held during reading and writing
them.

There is also a new explicit call in CollectedHeap to get a consistent
overall MemoryUsage information instead of the existing code looping
over the heap's memory pools. That one is problematic too otherwise.

There is still no change in the calculation of the values of the
MemoryUsage members.

CR:
https://bugs.openjdk.java.net/browse/JDK-8207200
Webrev:
http://cr.openjdk.java.net/~tschatzl/8207200/webrev
Testing:
hs-tier1-4,jdk-tier1-3

This change is based on the changes for JDK-8209062.

Thanks,
  Thomas

[1] http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2018-August/0
22766.html