get_usable_physical_ram()
returns the total physical RAM, usable by a
process, in bytes. It will limit to 2 GB for 32 bit processes. Starting
with GDAL 2.4.0, it will also take into account resource limits (virtual
memory) on Posix systems. Starting with GDAL 3.6.1, it will also take into
account RLIMIT_RSS on Linux. Wrapper of CPLGetUsablePhysicalRAM()
in the
GDAL Common Portability Library.