USE option questions

wilburpan · l33t Joined: 21 Jan 2003 Posts: 977

I have some questions that aren't addressed in the Gentoo Guide to USE flags:

1. I understand the attractiveness of using USE flags to optimize at the time of compilation. What are the downsides of having a USE flag off if you emerge a package that would use it? For example, one of the first packages I emerged was KDE, so my USE flags were set to "kde qt -gnome -gtk", as suggested in the Gentoo Desktop installation guide. What really is the difference between emerging KDE with gnome and gtk turned off and with gnome and gtk turned on? Faster compile time? Quicker execution speed? More compact code?

2. What would happen if I then emerged Evolution without changing my USE flags? I am pretty sure would need the gnome and gtk options turned on.

3. Is there a list of which USE flags should be turned on/off for each packages before emerging?

4. If I don't specify a USE flag setting in make.conf, does gentoo default to the setting in make.defaults, or is it automatically set to off?

Sargonas · Posted: Tue Feb 11, 2003 2:41 am Post subject:

While I dont know the specifics of the use variables here is the docs and list of them:

http://www.gentoo.org/doc/en/use-howto.xml
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InvsbleChuck · n00b Joined: 17 Jul 2002 Posts: 28 Location: Brooklyn, NY

This is probably duped somewhere, but here is how I understand it.

1. The use flags AFAIK are only used when a package has the option to use something or not, IOW, not a requirement but a feature. For instance you emerge a program that uses graphic file formats. It supports GIF JPEG PNG and TIFF. If you don't have TIFF in you USE var, then that option is not compiled in.

So in your example it would not have any effect on KDE. But if a package in KDE used a gtk or gnome library as a feature (doubtful, but for the sake of argument) if you have it disabled in your USE variable it would still compile just without the GTK/GNOME specific features.

2. It would pick up gnome and gtk because it is a requirement and not an optional feature.

3. Since the use flags only affect optional features, the choices of which flags to keep and which to leave in are really entirely up to you.

4. The way I understand it is that whatever is in make.conf overrides what is in make.defaults. So if the package does not appear in either file it is automatically off. That is why there is the '-' setting in use so that you can turn off what is in make.defaults without editing the file directly.

Hope this helps

wilburpan · l33t Joined: 21 Jan 2003 Posts: 977

OddFox · Posted: Tue Feb 11, 2003 4:51 am Post subject:

David_Escott · l33t Joined: 12 Jan 2003 Posts: 952 Location: Boston, MA

it adds a little to bloat especially in multimedia/gui
for instance I have no interest in kde or qt or any of that my box is gtk only
but if I wanted to build some applications that have bindings/ui's for kde then they will want to build qt and kde
another example is that i dont use xmms but gstreamer has plugins for xmms so building gnome wants to build gstreamer wants to build xmms etc

basically unset a use flag if you really arent interested in what it offers thats about it

wilburpan · l33t Joined: 21 Jan 2003 Posts: 977

At the risk of sounding like a broken record, I haven't seen a really comprhensive answer to my questions.

I realize that some "people simply don't want some thing to be installed... period." Also that having excessive flags turned on may add to bloat.

What I guess I am asking is how much does this impact on actual performance? Given that it takes 24 hours for me to emerge KDE, I would like to know if having all possible flags turned on will hurt performance because of excessive bloat, or improve performance because of tight code in the final product. On the flip side, would turning off a flag result in suboptimal code due to the compiled program not being able to take advantage of a plugin?

This is not a trivial question, since obviously a lot of effort was put into implementing the USE flag function into Gentoo. Therefore, it would be nice to demonstrate that this function adds to performance in some way, as well as recommendations for each package. Maybe benchmarks would be useful.

mrgalihad · Tux's lil' helper Joined: 30 Jan 2003 Posts: 91

I may not be a 100% correct in this, as I am new to gentoo as well, but I don't really think anything in the USE space would either add to or take away from performance. It simply addes extensible features to the application you are trying to merge in.

Many things have little features that are designed to fit in with other applications. If you turn it on or off in your USE flags, then you simply won't have those features available to you when you use the application. If there is a feature you want, then you can add it in your USE variable. Its not more or less efficicient when the build is done, because in order to use that feature, it would have had to have been compiled against those libraries somewhere in the process anyway.

I can see an argument that removing things from the USE space could increase perforamance in given application, just bacuase there isn't as many libraries to load when the application loads, but the overhead of doing that is pretty minimal, and really shouldn't affect things much, as its pretty probable that the library its accessing has already been loaded and is in the file cache anyway (unless your system is stressed for CPU and or memory of course)

InvsbleChuck · n00b Joined: 17 Jul 2002 Posts: 28 Location: Brooklyn, NY

rac · Posted: Wed Feb 12, 2003 6:36 pm Post subject:

If you're talking about something as drastic as taking X support out, it can dramatically reduce the size of binaries and also increase the free hard disk space you have for other things. When I compile XEmacs without X and Mule support, it starts about 20-30% faster.

USE variables are not just about performance. Sometimes you may like a program, but its support for something optional is buggy or incompatible with other software you have. For example, I need Japanese support in Mozilla. If I compile it with XFt support, that breaks. So I need to use the "moznoxft" USE variable to get what I want.

I am personally of the opinion "when in doubt, leave it out". This simplifies debugging. I understand that many other people prefer to toss everything unknown on, because they figure they may need it later. That's certainly another reasonable position. The great thing about Gentoo is that the distribution itself doesn't have to make tradeoffs between these two schools of thought, because it doesn't distribute binaries.
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mr-simon · Posted: Thu Feb 13, 2003 12:50 am Post subject:

wilburpan · l33t Joined: 21 Jan 2003 Posts: 977

Delphiki · Guru Joined: 04 Oct 2002 Posts: 337 Location: A2

plate · Posted: Thu Feb 13, 2003 4:11 am Post subject:

Moved from Installing Gentoo.

David_Escott · l33t Joined: 12 Jan 2003 Posts: 952 Location: Boston, MA

mozilla doesnt draw with gtk or gtk2, it uses its own library but provides bindings to gtk and gtk2 for use in galeon, and can emulate the look and feel of gtk and gtk2

rac · Posted: Fri Feb 14, 2003 6:12 pm Post subject:

wilburpan · l33t Joined: 21 Jan 2003 Posts: 977

Perhaps this could be added to the portage documentation. I just looked, and the closest thing to a -v flag that is mentioned in either documentation file is the --verbose flag, which does not look to be the same at all.