2020-05-14: Site rewrite — disaster struck and I'm having to reconstruct the site from old databases and converting formats by hand. Still, it was time for an update!
I think Swapspace has shown itself to be robust, stable, and useful over time. It needed only minimal changes in many years.
Why am I no longer maintaining it myself? When I started it, I had to work out all the hosting myself and that became more work than I could handle: create a Subversion repository, write a project index, configure firewalls, set up and manage mail / web / svn servers, run Trac, organise authentication and authorisation, tweak fonts and images, fight spam, block network attacks, install OS updates, schedule backups and delete old backups. A lot of the work wasn't automated, so I even had to write my own scripts for much of it.
The development world has changed a lot since then, and it's easy to host a project like this on a free service. But I never thought I needed to. Swapspace remained pretty good just as it was. And so... one day I noticed that the svn repo had broken a long time ago, probably in some upgrade, and I no longer had backups from that far back.
So if you like Swapspace, go with Jacob's fork.
What Swapspace 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.
Where it comes from
This software was originally written for the Suriyan project, a Debian-based server system primarily aimed at small and medium-sized enterprises in Thailand.
Suriyan was created by the Open Source Department of the Software Industry Promotion Agency, an enterprise by the Thai Ministry of Information and Communication Technology. The goal is to stimulate effective use of information technology in Thailand.
The Suriyan server system was meant to run on a wide range of hardware from shiny new servers to hand-me-down PCs. Another design parameter was to avoid confronting the user with unnecessary installation choices. To help meet these requirements we developed this program which deals with the limitations of both memory and hard-disk sizes as they arise for any given system.
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. As long as 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:
the program doesn't ask for memory just when the least memory is available;
it eliminates one of the most important causes of 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.
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. Some statistics
for one test on a 32-bit PC with 1 gigabyte of memory:
|Number of swapfiles||25|
|Largest swapfile||5.6 GB|
|Total swap allocated||44 GB|
In other words, applications on this 1 GB desktop computer took up a whopping (by 2005 standards) 44 GB of memory space without falling over. They weren't very fast, of course, having to swap so much; but they did keep running.
After this point, when we tried to use up even more memory, the hard disk filled up with swap files so no more could be allocated. The program has provisions to deal with this case robustly and fairly gracefully. It does not like to leave you with a 100% full disk with no room to save your data. After we closed some of the running applications, Swapspace started steadily decommissioning swap files before settling in a more realistic state.
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 probably 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!