Matthew Pickering authored Feb 26, 2021 and Marge Bot committed Mar 08, 2021

See #19357

The event reports the
  * Current number of megablocks allocated
  * The number that the RTS thinks it needs
  * The number is managed to return to the OS

When current > need then the difference is returned to the OS, the
successful number of returned mblocks is reported by 'returned'.

In a fragmented heap current > need but returned < current - need.