de-swap
-------

Bring back swapped out pages into memory.

The program works by performing a read operation (which forces a
"swap-in") on a region of memory that has been paged out to swap.

In most cases, it is unnecessary to run this, because the kernel
knows what it's doing. However, if there is a really good reason
to (quicky) force "swapped-out" pages of a specific process back
into memory, `de-swap' is faster compared to running `swapoff',
because it tries to "swap-in" everything.

Before running the program (requires superuser privileges), it is
probably a good idea verify that there is enough memory. If there
isn't enough "free" memory to swap into, `oom-kill' will likely be
triggered. Note that the "swapped-in" pages will not be cleared
from the swap space because after the pages have been brought back
into memory they are "SwapCached" [2], to avoid extra I/O (if) when
they have to be paged out again.

The regions of memory that have been swapped out can be fetched by
parsing "/proc/<PID>/smaps" for a process. The process's virtual
address space (its memory) [2] can then be accessed via by reading
its "/proc/<PID>/mem" file. By "seek()"-ing to an offset obtained
by parsing "/proc<PID>/smaps" (the "paged-out-to-swap" regions),
and making a "read()" operation will bring them back to memory.

For more information, the answers and comments in this [3] thread
are quite helpful. "Deactivating a Swap Area (11.9)" [4], describes
how the swap area is freed in good detail.


BUILD
    To generate the binary, run:
        make GOOS="linux" [GOARCH="<arch>"]


USAGE
    de-swap -p <pid> [-n -j <threads> -r <bytes> -v[v]]


ARGUMENTS
    -p     PID of the process to swap-in
    -n     no-op mode; turned off by default
    -j     number of parallel operations; runs as a
           single thread if unspecified
    -r     split memory regions with large swap areas
           into smaller regions for better throughput
	   during reads; disabled if set to 0, and if
	   enabled it must be > 0 and in exponents of
	   2; defaults to 64 kB if unspecified
    -v[v]  output verbosity; off by default


REFERENCE
    [1]: https://www.kernel.org/doc/Documentation/sysctl/vm.txt
    [2]: https://www.kernel.org/doc/Documentation/filesystems/proc.txt
    [3]: https://unix.stackexchange.com/questions/45673
    [4]: https://www.kernel.org/doc/gorman/html/understand/understand014.html