swapspace - dynamic swap manager for Linux

http://thaiopensource.org/development/swapspace/

What it does for you

This system daemon for the Linux kernel aims to do away with the need for large,
fixed swap partitions or swap files.

When installing a Linux-based system (invariably GNU/Linux) with swapspace, the
usual swap partition can be omitted, or it can be kept quite small.  Whenever
swapspace finds, during normal system usage, that more virtual memory is needed,
it will automatically claim space from the hard disk.  Conversely, swap space
that is no longer needed is freed up again for regular use by the filesystem.

This means that with swapspace installed, sizing the system's available swap
space during installation is no longer a life-or-death choice.  It now becomes
practical to run GNU/Linux off just a single, big partition--with no disk space
lost to regrettable installation choices.  The system should also be able to
handle the occasional memory-intensive task that takes much more swap space than
was originally foreseen, without leaving the same swap space unused and unusable
during normal operation as is normally the case.

Swapspace is made available for use under the GNU General Public License (GPL).
See the file COPYING for an open copyright license.

Copyright (C) 2005 Software Industry Promotion Agency (SIPA), Thailand.  Written
by Jeroen T. Vermeulen <jtv@xs4all.nl>


How it compares

Unlike similar programs such as dynswapd and the older (and more portable)
swapd, swapspace also adapts the sizes of the swap files it creates to meet
demand.  This means it is less dependent on limits that the kernel may impose on
the total number of swapfiles, while reducing the need for manual configuration.
If the daemon finds that more and more swap files are needed, it will start
creating larger ones to anticipate demand.  While demand for swap files is
modest, it will stick to smaller ones that can be initialized more quickly and
so respond more fluently to present requirements.

Robustness and user-friendliness are the first priorities in developing this
program.  For example, all alternatives we looked at perversely needed to
allocate multiple chunks of memory in dealing with low-memory situations;
allocation failure would typically crash these programs.  It turned out that
none of these allocations were really necessary, and swapspace manages to avoid
them categorically.  This kills two birds with one stone when it comes to
reliability: (i) the program doesn't ask for memory just when the least memory
is available and (ii) it eliminates one of the most important causes of
programming bugs as a risk factor.

User-friendliness primarily means that no silly questions are asked of the user.
The daemon tries to be sensible and figure out what is needed at runtime, by
itself, and without user intervention.  You should not have any need to learn
about the configuration parameters, or tweak them.  They exist mostly for
development purposes.

The swapspace daemon has been in production use for several months on various
32-bit architectures before it was first released to the public, and has been
tested with swapfiles larger than can be addressed in 32 bits.

Swapspace itself is a small program: about 50 kilobytes on my system--or even
less in a special version that only accepts the most basic configuration options
and ignores its configuration file.  On top of that it allocates no memory at
runtime (although the system will allocate some, of course) and does not use
much stack space.


When not to use it

In its current form, swapspace is probably not a good choice for systems that
need to remain responsive at all times; depending on the system and the
cicrumstances, the creation a large new swapfile can take as long as half a
minute and occupy quite a lot of the system's attention.  The program minimizes
the number of times swapfiles are created, but it wouldn't be very useful if it
never created any swapfiles at all!

We are hoping to bring further improvements in the future.  Since the problem
appears to be caused mostly by system code, however, it's hard to be sure that
this is really possible.  It may turn out to be possible for the kernel, with
some modifications, to extend an existing swapfile while it is already in use.
That would probably help a great deal, but we don't know at the moment how much
work it would take.


Where to start

The program is available both as a source archive and as a Debian package built
from that same source archive.

To build and install from source, enter the main swapspace source directory and
run "make".  Some editing of the Makefile may, but generally shouldn't, be
required as long as gcc is used as the C compiler.  The program code is written
in standard C99 (the 1999 edition of the C standard), plus one POSIX extension.
The easiest mode of installation is by running "make install" with root
privileges.

See the provided manpage for details on how to run swapspace for troubleshooting
or debugging purposes.  A sample configuration file is also provided, but the
average user should not need to take an interest.  An init script is provided to
start and stop swapspace as a regular system service.

The "make install" procedure installs the init script in /etc/init.d, but does
not currently ensure that swapspace is run on system startup.


Technical details: Installation

The installation procedure creates a directory /var/lib/swapspace, which must be
accessible to the system's superuser (root) only; granting any kind of access
for this directory to an untrusted user is likely to constitute a serious
security hole.

According to the Filesystem Hierarchy Standard, other appropriate places for
this kind of file might be /var/tmp or /var/run.  The former was not deemed
appropriate because it is accessible to all users, and the latter because most
system administrators would probably expect it to occupy very little space and
may confine it to a partition that isn't large enough to hold useful swap space.

Also, files in /var/lib survive reboots whereas those in /var/tmp and /var/run
need not.  Obviously any data in swap files can be safely erased on reboot (it's
even tempting to think that that would be safer, though I don't think it really
is).  But consider a system consistently short on physical memory: during boot,
swapspace will see the swapfiles left by the previous session, and reinstate
them immediately at very little cost, ready for use when they are needed.  If
they had been erased during reboot, swapspace would have to allocate new ones on
disk when it recognized the need for more virtual memory, which takes much more
time and resources.

Technical details: Algorithm

Choices of allocation and deallocation are driven by a "finite state machine"
consisting of four states: steady, hungry, overfed, and diet.  The program will
alternate through these states as circumstances dictate, applying different
policies depending on current state.  This was done to achieve a good tradeoff
between willingness to free up unused swap space on the one hand, and avoidance
of "thrashing" between deallocation and re-allocation of swapfiles on the other.

For those interested, here is a quick description of these states and their
associated policies:

Steady - normal operation; no additional swap space is needed, nor do we have
more than is needed.  However, the program can go into the hungry or overfed
states at the drop of a hat.

Hungry - more swap space was recently allocated.  The program is willing to
allocate even more if needed, but it will not consider dropping unneeded swap
files.  After a certain timeout period, the program reverts to the steady state.

Overfed - significantly more virtual memory is available than is needed.  If
this situation persists for a certain timeout period, a swapfile is deallocated
and the program returns to the steady state.

Diet - a recent allocation attempt has run into resource limits, e.g. because
the filesystem used for swap files was full.  No more swapspace can be allocated
but excess files can be freed up very rapidly.  Afer timeout, reverts to steady.

State information can be queried by sending the program the SIGUSR1 signal (see
"man 7 signal" to get the number for your architecture) which will cause it to
log debug information to the system's daemon log and/or standard output, as
appropriate.  The program can also be made to log state transitions by starting
it with the -v or --verbose option.

