ghc runtime by default (otherwise depending on a "configure" option)
does memory allocation on their own by first mmapping a 1 TB range of
memory into the process and then parceling out chunks from it.
If one of the chunks is not needed, the kernel needs to be informed -
otherwise the system would quickly run out of available RAM.
ghc does that via madvise(2).
There are two options when doing this informing:
MADV_FREE - Means "I don't need this range or the data in it any more".
Kernel promises to fail later accesses to it.
MADV_DONTNEED - Means "I don't need this range right now - and I don't
need the data in it anymore". Kernel promises to make later accesses to
it succeed (if necessary by providing a new page initialized with zeroes).
MADV_FREE was introduced in Linux 4.5.
glibc 2.25 and later always define MADV_FREE.
Unpatched ghc 8.0.2 will use either MADV_FREE or MADV_DONTNEED, determined
at ghc compile time. Which of them will actually succeed is determined
by the Linux kernel at run time.
This patch makes ghc try MADV_FREE. If it doesn't work, it falls back to
MADV_DONTNEED.
The end result is that ghc programs free their memory with Linux <