Multiseat

Multiseat is a name of a PC configuration, when more users can work on a single PC locally. In practice, it means, that you connect into the PC another monitor, keyboard and a mouse a configure the computer so that to each user it seems that (s)he works on the computer alone. In the words of an advertisment: two computers in one :-).

If I omit commercial solution, when you connect a special device, that enables multiseat under Windows (but only with a special licence), then the only solution (for a typical PC) is use a Unix-based system – for example Linux. In these systems, graphical interface is controled by a normal program (not by a privileged one or even by kernel itself) called X server, so the only thing to do is to launch X server for every seat and it is done. Well, theoretically it is, but the situation is actually a bit more complicated and the configuration itself is only for users with strong nerves.

Aproaches to multiseat configuration

To set up multiseat (I will keep with this naming convention although in most cases I will actually mean just dualseat) you can use basically two approaches (however, this is my division and maybe there are other ones). Both are derivated from the way how X server works with the graphics card.

Current versions of X server occupies the whole graphics card no matter how many outputs it has (exceptions are some Maxtor cards I guess). So, even if we have dual-output card (a typical situation with cards used today), we are not able to run two independent X servers on it (better say: it is not possible to run them simultaneously: it is possible to launch more X servers but active is only one – others are waiting on inactive consoles). There are two possible solutions:

  • either add another graphics card
  • or above the X server – that is spread across both outputs – start some nested server and in this one start the session itself

Both aproaches have their pros and cons. In my case, I built two card solution first, but when one of the cards bereft of life (after all, it was a second-hand one and it was working for longer time than I have expected due to its price), I was forced to (because I was not able to obtain the right card) move to the single-carded solution that I use now.

About both solutions I wrote a short how-to – if you like the multiseat idea, have a look (if you know almost nothing about it, before experimenting have a look at general notes below):

Several notes

Here are put several notes that might be useful for people who hear about multiseat for the first time. I hope they will serve their purpose.

Why to built multiseat

The price of today's computers is pretty low – so is it worth set up computer the way that is not widely used? Wouldn't it be better to stick with verified solution of sharing data throught network? It depends on the circumstances. When taking as an opponent pretty widespread (especially at homes) configuration server (better machine with bigger HDD) and client (slower machine, usually older OS), I came up with following conclusions (their order does not correspond with their importance or anything else).

  • with multiseat you definitely save for hardware, because you need only additional monitor, keyboard and a mouse (thus you probably save for energy as weel – especially if we want to have both seats equivalent)
  • with multiseat you do not have to set up data sharing over network and also the necessity to mantain more independent machines
  • multiseat ties seats together, because you are limited by the length of cabels between card and monitor (I don't know the maximal radius but it would definetely be shorter than the one of network cable)
  • all the seats at multiseat are equivalent (power, system, rights)
  • multiseat itself is pretty new solution (compared to network sharing) so there are not that much of experience with it and for some programs it might be problematic and the configuration is not one of the easiest (on the other hand, multiseat de facto correspond to classical Unix terminals (the ones from 70.), so in fact with it we are returning to the very origins of UNIX systems)
  • on multiseat you are enforced to use the same operation system on all seats (if you do not virtualize, of course)
  • multiseat can surprise some users – I wrote this because it reminded me of the last-ditcher who was supposed to configure Internet and, somehow, refused to understand that it is one computer and there is no Start menu :-)

Setting up sound

Once you let 2 users access the computer at once, problem arises who has the privilege to access this-or-that device. At some of them, exclusive right is required (keyboard), with others sharing is acceptable. Sometimes, both approaches might be handy – both users should have the option to play music (each to his own headphones) but do we have to buy another card if we need just stereo output and we already have 5.1 card in the computer? No, one card is enough but the description of this is a bit longer so I created a standalone page.

Page was last changed on April 15, 2008 at 6:41 pm

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