- lower the threshold for creation of mmap-backed heaps to 64MB
- lower the threshold for creation of mmap-backed heaps to 64MB

  this gives the advantage that if those are made persistent later,
  saving to disk is much cheaper. It makes the system also better
  resistent against swapping.

  For non-persistent bats, the performance difference should be small.
  That does not hold for Windows, as it will effectively materialize
  a full file on disk before the mmap can be done (Linux and other Unixes
  just use a file with a hole whose creation is very cheap).

  Therefore, we retain the 1/3 of RAM limit for Windows.

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