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Diffstat (limited to 'open_issues/anatomy_of_a_hurd_system.mdwn')
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1 files changed, 518 insertions, 2 deletions
diff --git a/open_issues/anatomy_of_a_hurd_system.mdwn b/open_issues/anatomy_of_a_hurd_system.mdwn index a3c55063..33635b80 100644 --- a/open_issues/anatomy_of_a_hurd_system.mdwn +++ b/open_issues/anatomy_of_a_hurd_system.mdwn @@ -1,4 +1,4 @@ -[[!meta copyright="Copyright © 2011, 2012, 2013 Free Software Foundation, +[[!meta copyright="Copyright © 2011, 2012, 2013, 2014 Free Software Foundation, Inc."]] [[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable @@ -43,7 +43,11 @@ like Bushnell's Hurd paper. All this should be unfied and streamlined. <antrik> servers often depend on other servers for certain functionality -# IRC, freenode, #hurd, 2011-03-12 +# Bootstrap + +## [[hurd_init]] + +## IRC, freenode, #hurd, 2011-03-12 <dEhiN> when mach first starts up, does it have some basic i/o or fs functionality built into it to start up the initial hurd translators? @@ -76,6 +80,112 @@ like Bushnell's Hurd paper. All this should be unfied and streamlined. rest of the system up +## IRC, freenode, #hurd, 2014-01-03 + + <teythoon> hmpf, the hurd bootstrapping process is complicated and fragile, + maybe to the point that it is to be considered broken + <teythoon> aiui the hurd uses the filesystem for service lookup + <teythoon> older mach documentation suggests that there once existed a name + server instead for this purpose + <teythoon> the hurd approach is elegant and plan9ish + <teythoon> the problem is in the early bootstrapping + <teythoon> what if the root filesystem is r/o and there is no /servers or + /servers/exec ? + <teythoon> e. g. rm /servers/exec && reboot -> the rootfs dies early in the + hurd server bootstrap :/ + <braunr> well yes + <braunr> it's normal to have such constraints + <teythoon> uh no + <braunr> at the same time, the boot protocol must be improved, if only to + support userspace disk drivers + <teythoon> totally unacceptable + <braunr> why not ? + <teythoon> b/c my box just died and lost it's exec node + <braunr> so ? + <braunr> loosing the exec node is unacceptable + <youpi> well, linux dies too if you don't have /dev populated at least a + bit + <braunr> not being able to boot without the "exec" service is pretty normal + <braunr> the hurd turns the vfs into a service directory + <teythoon> the exec service is there, only the lookup mechanism is broken + <braunr> replacing the name server you mentioned earlier + <teythoon> yes + <braunr> if you don't have services, you don't have them + <braunr> i don't see the problem + <braunr> the problem is the lookup mechanism getting broken + <teythoon> ... that easily + <braunr> imagine a boot protocol based on a ramfs filled from a cpio + <teythoon> i do actually ;) + <braunr> there would be no reason at all the lookup mechanism would break + <teythoon> yes + <teythoon> but the current situation is not acceptable + <braunr> i agree + <teythoon> ^^ + <braunr> ext2fs is too unreliable for that + <braunr> but using the VFS as a directory is more than acceptable + <braunr> it's probably the main hurd feature + <teythoon> yes + <braunr> i see it rather as a circular dependency problem + <braunr> and if you have good ideas, i'm all ear for propel ... :> + <braunr> antrik already talked about some of them for the bootstrap + protocol + <braunr> we should sum them up somewhere if not done already + <teythoon> i've been pondering how to install a tmpfs translator as root + translator + <teythoon> braunr: we could create a special translator for /servers + <braunr> maybe + <teythoon> very much like fakeroot, it just proxies messages to a real + translator + <teythoon> but if operations like settrans fail, we handle them + transparently, like an overlay + <braunr> i consider /servers to be very close to /dev + <teythoon> yes + <braunr> so something like devfs seems obvious yes + <braunr> i don't even think there needs to be an overlay + <teythoon> y not ? + <braunr> why does /servers need real nodes ? + <teythoon> for persistence + <braunr> what for ? + <teythoon> e.g. crash server selection + <braunr> hm ok + <teythoon> network configuration + <braunr> i personally wouldn't make that persistent + <braunr> it can be configured in files and installed at boot time + <teythoon> me neither, but that's how it's currently done + <braunr> are you planning to actually work on that soon ? + <teythoon> if we need no persistence, we can just use tmpfs + <braunr> it wouldn't be a mere tmpfs + <teythoon> it could + <braunr> it's a tmpfs that performs automatic discovery and registration of + system services + <teythoon> with some special wrapper that preserves e.g. /servers/exec + <teythoon> oh + <braunr> so rather, devtmpfs + <teythoon> it is o_O :p + <braunr> ? + <braunr> what is what ? + <teythoon> well, it could be a tmpfs and some utility creating the nodes + <braunr> whether the node management is merged in or separate doesn't + matter that much i guess + <braunr> i'd personally imagine it merged, and tmpfs available as a + library, so that stuff like sysfs or netstatfs can easily be written + + +## IRC, freenode, #hurd, 2014-02-12 + + <teythoon> braunr: i fixed all fsys-related receiver lookups in libdiskfs + and surely enough the bootstrap hangs with no indication whats wrong + <braunr> teythoon: use mach_print :/ + <teythoon> braunr: the hurd bootstrap is both fragile and hard to tweak in + interesting ways :/ + <braunr> teythoon: i agree with that + <braunr> teythoon: maybe this will help : + http://wiki.hurdfr.org/upload/graphviz/dot9b65733655309d059dca236f940ef37a.png + <braunr> although i guess you probably already know that + <teythoon> heh, unicode for the win >,< + <braunr> :/ + + # Source Code Documentation Provide a cross-linked sources documentation, including generated files, like @@ -311,6 +421,9 @@ Actually, the Hurd has never used an M:N model. Both libthreads (cthreads) and l <Tekk_> spiderweb: well, there's 1 advantage of minix for you :P <braunr> the main idea of mach is to make it easy to extend unix <braunr> without having hundreds of system calls + +[[/system_call]]. + <braunr> the hurd keeps that and extends it by making many operations unprivileged <braunr> you don't need special code for kernel modules any more @@ -539,6 +652,9 @@ Actually, the Hurd has never used an M:N model. Both libthreads (cthreads) and l <damo22> it must translate these system calls into ipc or something <damo22> then mach handles it? <braunr> exactly + +[[/system_call]]. + <braunr> that's why i say it's not the exokernel way of doing things <damo22> ok <damo22> so does every low level hardware access go through mach?' @@ -811,3 +927,403 @@ Actually, the Hurd has never used an M:N model. Both libthreads (cthreads) and l <braunr> ahungry: ctrl-c does work, you just missed something somewhere and are running a shell directly on a console, without a terminal to handle signals + + +# IRC, freenode, #hurd, 2013-11-04 + + <braunr> nalaginrut: you can't use the hurd for real embedded stuff without + a lot of work on it + <braunr> but the hurd design applies very well to embedded environments + <braunr> the fact that we're able to dynamically link practically all hurd + servers against the c library can visibly reduce the system code size + <braunr> it also reduces the TCB + <nalaginrut> what about the memory occupation? + <braunr> code size is about memory occupation + <teythoon> also, the system is composable like lego, don't need tcp - don't + include pfinet then + <braunr> the memory overheald of a capability based system like the hurd + are, well, capabilities + <braunr> teythoon: that's not an argument compared to modular kernels like + linux + <teythoon> yes it is + <braunr> why ? + <braunr> if you don't need tcp in linux, you just don't load it + <braunr> same thing + <teythoon> ok, right + <braunr> on the other hand, a traditional unix kernel can never be linked + against the c library + <braunr> much less dynamically + <teythoon> right + <nalaginrut> I think the point is that it's easy to cut, since it has + better modularity than monolithic, and could be done in userland relative + easier + <braunr> modularity isn't better + <braunr> that's a big misconception + <teythoon> also, restarting components is easier on a distributed system + <braunr> on the hurd, this is a side effect + <braunr> and it doesn't apply well + <nalaginrut> braunr: oops, misconception + <braunr> many core servers such as proc, auth, exec, the root fs server + can't be restarted at all + <teythoon> not yet + <braunr> and servers like pfinet can be restarted, but at the cost of posix + servers not expecting that + <braunr> looping on errors such as EBADF because the target socket doesn't + exist any more + <teythoon> I've been working on a restartable exec server during some of my + gsoc weekends + <braunr> ah right + <braunr> linux has kexec + <braunr> and can be patched at run time + <nalaginrut> sounds like Hurd needs something similar to generalizable + continuation + <braunr> so again, it's not a real advantage + <braunr> no + <nalaginrut> sorry serilizable + <braunr> that would persistence + <braunr> personally, i don't want it at all + <teythoon> yes it is a real advantage, b/c the means of communication + (ports) is common to every IPC method on Hurd, and ports are first class + objects + <teythoon> so preserving the state is much easier on Hurd + <braunr> if a monolithic kernel can do it too, it's not a real advantage + <teythoon> yes, but it is more work + <braunr> that is one true advantage of the hurd + <braunr> but don't reuse it each time + <nalaginrut> oh, that's nice for the ports + <teythoon> why not? + <braunr> what we're talking about here is resilience + <braunr> the fact that it's easier to implement doesn't mean the hurd is + better because it has resilience + <braunr> it simply means the hurd is better because it's easier to + implement things on it + <braunr> same for development in general + <braunr> debugging + <braunr> virtualization + <braunr> etc.. + <nalaginrut> yes, but why we stick to compare it to monolithic + <braunr> but it's still *one* property + <teythoon> well, minix advertises this feature a lot, even if minix can + only restart very simple things like printer servers + <braunr> minix sucks + <braunr> let them advertise what they can + <teythoon> ^^ + <nalaginrut> it has cool features, that's enough, no need to find a feature + that monolithic can never done + <braunr> no it's not enough + <braunr> minix isn't a general purpose system + <braunr> let's just not compare it to general purpose systems + + +# IRC, freenode, #hurd, 2013-11-08 + + <teythoon> and, provided you have suitable language bindings, you can + replace almost any hurd server with your own implementation in any + language + <crocket> teythoon: language bindings? + <crocket> Do you mean language bindings against C libraries? + <teythoon> either that or for the low level mach primitives + <crocket> For your information, IPC is independent of languages. + <teythoon> sure, that's the beauty + <crocket> Why is hurd best for replacing parts written in C with other + languages? + <teythoon> because Hurd consists of many servers, each server managing one + kind of resource + <teythoon> so you have /hurd/proc managing posix processes + <teythoon> you could reimplement /hurd/proc in say python or go, and + replace just that component of the Hurd system + <teythoon> you cannot do this with any other (general purpose) operating + system that I know of + <teythoon> you could incrementally replace the Hurd with your own + Hurd-compatible set of servers written in X + <teythoon> use a language that you can verify, i.e. prove that a certain + specification is fulfilled, and you end up with an awesome stable and + secure operating system + <crocket> Any microkernel OS fits the description. + <crocket> teythoon, Does hurd have formal protocols for IPC communications? + <teythoon> sure, name some other general purpose and somewhat + posix-compatible microkernel based operating system please + <teythoon> what do you mean by formal protocols ? + <crocket> IPC communications need to be defined in documents. + <teythoon> the "wire" format is specified of course, the semantic not so + much + <crocket> network protocols exist. + <crocket> HTTP is a transport protocol. + <crocket> Without formal protocols, IPC communications suffer from + debugging difficulties. + <crocket> Formal protocols make it possible to develop and test each module + independently. + <teythoon> as I said, the wire format is specified, the semantics only in + written form in the source + <teythoon> this is an example of the ipc specification for the proc server + http://git.savannah.gnu.org/cgit/hurd/hurd.git/tree/hurd/process.defs + <crocket> teythoon, how file server interacts with file clients should be + defined as a formal protocol, too. + <teythoon> do you consider the ipc description a kind of formal protocol ? + <crocket> + http://git.savannah.gnu.org/cgit/hurd/hurd.git/tree/hurd/process.defs can + be considered as a formal protocol. + <crocket> However, the file server protocol should be defined on top of IPC + protocol. + <teythoon> the file server protocol is in fs.defs + <teythoon> every protocol spoken is defined in that ipc description + language + <teythoon> it is used to derive code from + <braunr> crocket: not any system can be used to implement system services + in any language + <braunr> in theory, they do, but in theory only + <braunr> the main reason they don't is because most aren't posix compliant + from the ground up + <braunr> posix compliance is achieved through virtualization + <braunr> which isolates services too much for them to get useful, + notwithstanding the impacts on performance, memory, etc.. + <crocket> braunr, Do you mean it's difficult to achieve POSIX compliance + with haskell? + <braunr> crocket: i mean most l4 based systems aren't posix + <braunr> genode isn't posix + <braunr> helenos is by design not posix + <braunr> the hurd is the only multi server system providing such a good + level of posix conformance + <braunr> and with tls on the way, we'll support even more non-posix + applications that are nonetheless very common on unices because of + historical interfaces still present, such as mcontext + <braunr> and modern ones + <braunr> e.g. ruby is now working, go should be there after tls + * teythoon drools over the perspective of having go on the Hurd... + <crocket> braunr, Is posix relevant now? + <braunr> it's hugely relevant + <braunr> conforming to posix and some native unix interfaces is the only + way to reuse a lot of existing production applications + <braunr> and for the matter at hand (system services not written in c), it + means almost readily getting runtimes for other languages than c + <braunr> something other microkernel based system will not have + <braunr> imagine this + <braunr> one day, one of us could create a company for a hurd-like system, + presenting this idea as the killer feature + <braunr> by supporting posix, customers could port their software with very + little effort + <braunr> *very little effort* is what makes software attractive + <crocket> + http://stackoverflow.com/questions/1806585/why-is-linux-called-a-monolithic-kernel/1806597#1806597 + says "The disadvantage to a microkernel is that asynchronous IPC + messaging can become very difficult to debug, especially if fibrils are + implemented." + <crocket> " GNU Hurd suffers from these debugging problems (reference)." + <braunr> stackoverflow is usually a nice place + <braunr> but concerning microkernel stuff, you'll read a lot of crap + anywhere + <braunr> whether it's sync or async, tracking references is a hard task + <braunr> it's a bit more difficult in distributed systems, but not that + much if the proper debugging features are provided + <braunr> we actually don't suffer from that too much + <braunr> many of us have been able to debug reference leaks in the past, + without too much trouble + <braunr> we lack some tools that would give us a better view of the system + state + <crocket> braunr, But is it more difficult with microkernel? + <braunr> crocket: it's more difficult with distributed systems + <crocket> How much more difficult? + <braunr> i don't know + <crocket> distributed systems + <braunr> not much + <crocket> braunr, How do you define distributed systems? + <braunr> crocket: not monolithic + <crocket> braunr, Hurd is distributed, then. + <braunr> multiserver if you prefer + <braunr> yes it is + <crocket> braunr, So it is more difficult with hurd. + <crocket> How much more difficult? How do you debug? + <braunr> just keep in mind that a monolithic system can run on a + microkenrel + <braunr> we use tools that show us references + <crocket> braunr, like? + <braunr> like portinfo + <crocket> braunr, Does hurd use unix-socket to implement IPC? + <braunr> no + <braunr> unix-socket use mach ipc + <crocket> I'm confused + <braunr> ipc is provided by the microkernel, gnumach (a variant of mach) + <braunr> unix sockets are provided by one of the hurd servers (pflocal) + <braunr> servers and clients communicate through mach ipc + <crocket> braunr, Do you think it's feasible to build servers in haskell? + <braunr> why not ? + <crocket> ok + <teythoon> I've been thinking about that + <teythoon> in go, with cgo, you can call go functions from c code + <teythoon> so it should be possible to create bindings for say libtrivfs + <crocket> I'd like to write an OS in clojure or haskell. + <braunr> crocket: what for ? + <crocket> braunr, I want to see a better system programming language than + C. + <braunr> i don't see how clojure or haskell would be "better system + programming languages" than c + <braunr> and even assuming that, what for ? + <crocket> braunr, It's better for programmers. + <crocket> haskell + <crocket> haskell is expressive. + <braunr> personally i disagree + <braunr> it's better for some things + <braunr> not for system programming + <gnufreex> For system programming, Google Go is trying to replace C. But I + doubt it will. + <braunr> we may not be referring to the same thing here when we say "system + programming" + <crocket> braunr, What do you think is a better one? + <braunr> crocket: i don't think there is a better one currently + <crocket> braunr, Even Rust and D? + <braunr> i don't know them well enough + <braunr> certainly not D if it's what i think it is + <crocket> C is too slow. + <crocket> C is too slow to develop. + <braunr> depends + <braunr> again, i disagree + <braunr> rust looks good but i don't know it well to comment + <crocket> C is a tank, and clojure is an airplane. + <crocket> A tank is reliable but slow. + <crocket> Clojure is fast but lacks some accuracy. + <braunr> c is as reliable as the developer is skilled with it + <braunr> it's clearly not a tank + <braunr> there are many traps + <gnufreex> crocket: are you suggesting to rewrite Hurd in Clojure? + <crocket> no + <crocket> Why rewrite hud? + <crocket> hurd + <crocket> I'd rather start from scratch. + <braunr> which is what a rewrite is + <gnufreex> I am not expert on Clojure, but I don't think it is made for + system programming. + <gnufreex> If you want alternate language, I thing Go is only serious + candidate other than C + <crocket> Or Rust + <crocket> However, some people wrote OSes in haskell. + <braunr> again, why ? + <braunr> if it's only for the sake of using another language, i think it's + bad reason + <crocket> Because haskell provides a high level of abstraction that helps + programmers. + <crocket> It is more secure with monads. + <gnufreex> If you want your OS to become successful Free Software project, + you have to use popular language. Haskell is not. + <gnufreex> Most Haskell programmers are not into kernels + <gnufreex> They do high level stuff. + <gnufreex> So little contributors. + <braunr> crocket: so you aim at security ? + <gnufreex> I mean, candidats for contribution + <crocket> braunr, security and higher abstraction. + <braunr> i don't understand higher abstraction + <crocket> braunr, FP can be useful to systems. + <braunr> FP ? + <neal> functional programming + <braunr> right + <braunr> but you can abstract a lot with c too, with more efforts + <crocket> braunr, like that's easy. + <braunr> it's not that hard + <braunr> i'm just questioning the goals and the solution of using a + particular language + <braunr> the reason c is still the preferred language for system + programming is because it provides control over how the hardware does + stuff + <braunr> which is very important for performance + <braunr> the hurd never took off because of bad performance + <braunr> performance doesn't mean doing things faster, it means being able + to do things or not, or doing things a new way + <braunr> so ok, great, you have your amazing file system written in + haskell, and you find out it doesn't scale at all beyond some threshold + of processors or memory + <crocket> braunr, L4 is fast. + <braunr> l4 is merely an architecture abstraction + <braunr> and it's not written in haskell :p + <braunr> don't assume anything running on top of something fast will be + fast + <crocket> Hurd is slow and written in C. + <braunr> yes + <braunr> not because of c though + <crocket> Becuase it's microkernel? + <braunr> because c wasn't used well enough to make the most of the hardware + in many places + <braunr> far too many places + <crocket> A microkernel can be as fast as a monolithic kernel according to + L4. + <braunr> no + <braunr> it can't + <braunr> it can for very specific cases + <braunr> almost none of which are real world + <braunr> but that's not the problem + <braunr> again, i'm questioning your choice of another language in relation + to your goals, that's all + <braunr> c can do things you really can't do easily in other languages + <braunr> be aware of that + <crocket> braunr, "Monolithic kernel are faster than microkernel . while + The first microkernel Mach is 50% slower than Monolithic kernel while + later version like L4 only 2% or 4% slower than the Monolithic kernel ." + <braunr> 14:05 < braunr> but concerning microkernel stuff, you'll read a + lot of crap anywhere + <braunr> simple counterexample : + <braunr> the measurements you're giving consider a bare l4 kernel with + nothing on top of it + <braunr> doing thread-to-thread ipc + <braunr> this model of communication is hardly used in any real world + application + <braunr> one of the huge features people look for with microkernels are + capabilities + <braunr> and that alone will bump your 4% up + <braunr> since capabilities will be used for practically every ipc + <crocket> ok + + +# Hurd From Scratch + +## IRC, freenode, #hurd, 2013-11-30 + + <hurdmaster> because I think there is no way to understand the whole pile, + you need to go step by step + <hurdmaster> for example, I'm starting with mach only, then adding one + server, then another and on each step I have working system + <hurdmaster> that's how I want to understand it + <teythoon> you are interested in the early bootstrapping of the hurd system + ? + <hurdmaster> now I'm starting debian gnu/mach, it hungs, show me black + screen and I have no idea how to fix it + <teythoon> if you are unable to fix this, why do you think you can build a + hurd system from scratch ? + <hurdmaster> not gnu/mach, gnu/hurd I mean + <teythoon> or, you could describe your problem in more detail and one of + the nice people around here might help you ;) + <hurdmaster> as I said, it will be easier to understand and fix bugs, if I + will go step by step, and I will be able to see where bugs appears + <hurdmaster> so you should help me with that + <teythoon> and I tend to disagree + <teythoon> but you could always read my blog. you'll learn lots of things + about bootstrapping a hurd system + <teythoon> but it's complicated + <hurdmaster> http://www.linuxfromscratch.org/ + <teythoon> also, you'll need at least four hurd servers before you'll + actually see much + <teythoon> five + <teythoon> yeah, i know lfs + <hurdmaster> if somebody is interested in creating such a project, let me + know + <teythoon> you seem to be interested + <hurdmaster> yes, but I need the a real hurd master to help me + <teythoon> become one. fix your system and get to know it + <hurdmaster> I need knowledge, somebody built the system but didn't write + documentation about it, I have to extract it from your heads + <teythoon> hurdmaster: extract something from here + http://teythoon.cryptobitch.de + <teythoon> I need my head ;) + <hurdmaster> thanks + <hurdmaster> okay, what's the smallest thing I can run? + <teythoon> life of a Hurd system starts with the root filesystem, and the + exec server is loaded but not started + <teythoon> you could get rid of the exec server and replace the root + filesystem with your own program + <teythoon> statically linked, uses no unix stuff, only mach stuff + <hurdmaster> can I get 'hello world' on pure mach? + <teythoon> you could + <teythoon> hurdmaster: actually, here it is: + http://darnassus.sceen.net/gitweb/rbraun/mach_print.git/ + <teythoon> compile it statically, put it somewhere in /boot + <teythoon> make sure you're running a debug kernel + <teythoon> load it from grub instead of /hurd/ext2fs.static + <teythoon> look at the grub config for how this is done + <teythoon> let me know if it worked ;) |