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LightDM, or: an examination of a misunderstanding of the problem
LightDM's a from-scratch implementation of an X display manager, ie the piece of software that handles remote X connections, starts any local X servers, provides a login screen and kicks off the initial user session. It's split into a nominally desktop-agnostic core (built directly on xcb and glib) and greeters, the idea being that it's straightforward to implement an environment-specific greeter that integrates nicely with your desktop session. It's about 6500 lines of code in the core, 3500 lines of code in the gtk bindings to the core and about 1000 in the sample gtk greeter, for a total of about 11,000 lines of code for a full implementation. This compares to getting on for 60,000 in gdm. Ubuntu plan to switch to LightDM in their next release (11.10).
This is a ridiculous idea.
To a first approximation, when someone says "Lightweight" what they mean is "I don't understand the problems that the alternative solves". People see gtk and think "Gosh, that's kind of big for a toolkit library. I'll write my own". And at some point they have to write a file dialog. And then they need to implement support for handling remote filesystems. And then they need to know whether the system has a functioning network connection or not, and so they end up querying state from Network Manager. And then they suddenly have a library that's getting on for the size of gtk, has about the same level of complexity and has had much less testing because why would you want to use a lightweight toolkit that either does nothing or is 90% of the size of the alternative and crashes all the time.
Adding functionality means that code gets larger. Given two codebases that are of significantly different sizes, the two possible conclusions are either that (a) the smaller one is massively more competently written than the larger one, or (b) the smaller one does less. The gdm authors have never struck me as incompetent, even if some people may disagree with some of their design decisions, and the LightDM authors don't seem to have argued on that basis either. So the obvious conclusion is that LightDM does less.
And, indeed, LightDM does less. Part of this is by design - as the proposal to the Gnome development list shows, one of the key advantages of LightDM is claimed as it not starting a Gnome session. And from that statement alone, we can already see that there's been a massive failure of understanding the complexity of the problem.
Let's go back to the comparisons of code size. LightDM's simple GTK greeter is about 1000 lines of code. gdm's greeter is almost 20,000. Some of this is arbitrary shiny stuff like the slidy effects that occur, but a lot of it is additional functionality. For example, some of it is devoted to handling the interface with AccountsService so gdm can automatically update when users are created or deleted. Some of it is providing UI for accessibility functionality. Some of it is drawing a clock, which I'll admit may be a touch gratuitous.
But if your argument is that your software is better because it's doing less, you should be able to ensure that you can demonstrate that the differences aren't important. And the differences here are important. For example, one of the reasons gdm starts a local gnome session is that it wants gnome-power-manager to be there to handle power policy. Closing the lid of my laptop should suspend the system regardless of whether it's logged in or not. LightDM takes a different approach. Because there's no session, it has to take care of this kind of thing itself. So the backend daemon code speaks to upower directly, and the greeters ask the daemon to implement their policy decisions.
This is pretty obviously miserable. Now you've got two sets of policy - one at the login screen, and one in your session. How do I ensure they're consistent? The only sane solution is to ignore the functionality the backend provides and have my greeter run gnome-power-manager. And now how about accessibility preferences? Again, if I want to have the same selection of policy, I need to run the same code. So you end up with a greeter that's about as complex and large as the gdm one, and unused functionality in the backend. Lighter weight through code duplication. We have always been at war with Eurasia.
The entirety of LightDM's design is based on a false premise - that you can move a large set of common greeter functionality into a daemon and just leave UI presentation up to the greeter code. And if you believe that, then yes, you can absolutely implement a greeter in 1000 lines of code. It'll behave differently to your desktop - the range of policy you can implement will be limited to what the daemon provides, even if your desktop environment has a different range of features. It'll have worse accessibility for much the same reason. And eventually you'll end up with a daemon that's absolutely huge in order to attempt to provide the superset of functionality that each different desktop makes use of.
The only real problem LightDM solves is that it makes it easier to write custom greeters, and if you're really seeking to differentiate your project based on your login screen then maybe your priorities are a little out of line. I'm sure that Ubuntu will release with a beautiful 3D greeter that has a wonderful transition to the desktop. It's just a shame that it won't do any of the useful things that the existing implementations already do.
And if you think that when LightDM finally has the full feature set of gdm, kdm and lxdm it'll still be fewer lines of code and take less memory - I hear the BSD kernel is lighter weight than Linux. Have fun with it.
This is a ridiculous idea.
To a first approximation, when someone says "Lightweight" what they mean is "I don't understand the problems that the alternative solves". People see gtk and think "Gosh, that's kind of big for a toolkit library. I'll write my own". And at some point they have to write a file dialog. And then they need to implement support for handling remote filesystems. And then they need to know whether the system has a functioning network connection or not, and so they end up querying state from Network Manager. And then they suddenly have a library that's getting on for the size of gtk, has about the same level of complexity and has had much less testing because why would you want to use a lightweight toolkit that either does nothing or is 90% of the size of the alternative and crashes all the time.
Adding functionality means that code gets larger. Given two codebases that are of significantly different sizes, the two possible conclusions are either that (a) the smaller one is massively more competently written than the larger one, or (b) the smaller one does less. The gdm authors have never struck me as incompetent, even if some people may disagree with some of their design decisions, and the LightDM authors don't seem to have argued on that basis either. So the obvious conclusion is that LightDM does less.
And, indeed, LightDM does less. Part of this is by design - as the proposal to the Gnome development list shows, one of the key advantages of LightDM is claimed as it not starting a Gnome session. And from that statement alone, we can already see that there's been a massive failure of understanding the complexity of the problem.
Let's go back to the comparisons of code size. LightDM's simple GTK greeter is about 1000 lines of code. gdm's greeter is almost 20,000. Some of this is arbitrary shiny stuff like the slidy effects that occur, but a lot of it is additional functionality. For example, some of it is devoted to handling the interface with AccountsService so gdm can automatically update when users are created or deleted. Some of it is providing UI for accessibility functionality. Some of it is drawing a clock, which I'll admit may be a touch gratuitous.
But if your argument is that your software is better because it's doing less, you should be able to ensure that you can demonstrate that the differences aren't important. And the differences here are important. For example, one of the reasons gdm starts a local gnome session is that it wants gnome-power-manager to be there to handle power policy. Closing the lid of my laptop should suspend the system regardless of whether it's logged in or not. LightDM takes a different approach. Because there's no session, it has to take care of this kind of thing itself. So the backend daemon code speaks to upower directly, and the greeters ask the daemon to implement their policy decisions.
This is pretty obviously miserable. Now you've got two sets of policy - one at the login screen, and one in your session. How do I ensure they're consistent? The only sane solution is to ignore the functionality the backend provides and have my greeter run gnome-power-manager. And now how about accessibility preferences? Again, if I want to have the same selection of policy, I need to run the same code. So you end up with a greeter that's about as complex and large as the gdm one, and unused functionality in the backend. Lighter weight through code duplication. We have always been at war with Eurasia.
The entirety of LightDM's design is based on a false premise - that you can move a large set of common greeter functionality into a daemon and just leave UI presentation up to the greeter code. And if you believe that, then yes, you can absolutely implement a greeter in 1000 lines of code. It'll behave differently to your desktop - the range of policy you can implement will be limited to what the daemon provides, even if your desktop environment has a different range of features. It'll have worse accessibility for much the same reason. And eventually you'll end up with a daemon that's absolutely huge in order to attempt to provide the superset of functionality that each different desktop makes use of.
The only real problem LightDM solves is that it makes it easier to write custom greeters, and if you're really seeking to differentiate your project based on your login screen then maybe your priorities are a little out of line. I'm sure that Ubuntu will release with a beautiful 3D greeter that has a wonderful transition to the desktop. It's just a shame that it won't do any of the useful things that the existing implementations already do.
And if you think that when LightDM finally has the full feature set of gdm, kdm and lxdm it'll still be fewer lines of code and take less memory - I hear the BSD kernel is lighter weight than Linux. Have fun with it.
And now, for an examination of YOUR misunderstanding.
Now, LightDM is fundamentally flawed because it doesn't give you full integration, bells and all, with GNOME. Of course. No one uses KDE. No one uses XFCE, Enlightenment or LXDE. No one takes Openbox, tint2, their own handwritten daemons and what-not and builds essentially their own desktop and try to find a desktop-agnostic DM and write their own integration. No one.
The GDM devs are competent? Of course. Everyone has some sort of competency. But some people are more competent than others. BitKeeper, the revision control system Linus Torvalds used before writing Git is good software, no? I'm quite sure it is, since Torvalds used it. But then he wrote Git. It is better, at least in my opinion. By what black magic is it that it beats a competently written piece of software? It is MORE competently written.
Besides, LightDM doesn't do less, it assumes less. Its devs see that not everyone uses GNOME. But the capability is there. Or will be. Its developing. Rule of Modularity: Write simple parts connected by clean interfaces. Rule of Simplicity: Design for simplicity; add complexity only where you must. Rule of Diversity: Distrust all claims for "one true way".
"Those who don't understand UNIX are condemned to reinvent it, poorly." – Henry Spencer
You think lightweight software is flawed? Go use Windows. Install 4GB applications that can only do one thing while looking ugly at it. Linux is for people who know how to customize or at least be open about others' customization.
Re: And now, for an examination of YOUR misunderstanding.
Citation needed.
I've no objection to LightDM existing. I've no objection to people choosing to run it if they find that it fits their needs better in some way. But replacing gdm with it by default on a Gnome-based distribution is a demonstrable reduction in functionality, for what benefit? git took a completely different design philosophy to bitkeeper, and its benefits are more in that than in the implementation. LightDM is effectively just a rewrite of gdm with a different API between the greeter and the daemon.
Re: And now, for an examination of YOUR misunderstanding.
(Anonymous) 2011-05-13 02:11 pm (UTC)(link)Pardon my ignorance (I am a Gentoo user), but isn't Ubuntu just moving away from GNOME, as demonstrated by their recent adoption of unity?
Re: And now, for an examination of YOUR misunderstanding.
(Anonymous) 2011-05-15 10:05 am (UTC)(link)I'm not a particularly large fan of Unity because it looks kinda cheesy. It's also kinda ironic that it forks from GNOME and is called Unity. But then it can also refer to its internal unity, so I can't contest that.
Re: And now, for an examination of YOUR misunderstanding.
(Anonymous) 2011-05-15 09:59 am (UTC)(link)It's true that the LightDM may have been a bit overenthusiastic when they suggested it as a replacement for GDM in GNOME, but it doesn't seem as though your post is primarily targeted at that.
Re: And now, for an examination of YOUR misunderstanding.