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When all you can tell about a microkernel is that it is NT-like, I would refrain from questioning other people's technical knowledge.

In any case, Linux, BSDs, and OpenSolaris... all use different kernels, threading approaches, and heck even io subsystems. Simply saying that "it is different" doesn't quite answer the question of what makes Darwin interesting. Don't you think?

Exept... XNU isn't a microkernel.

It's a hybrid kernel - a loaded term among Linux enthusiasts, but a valid term, nonetheless. Linux, the BSDs, and Solaris all use monolithic kernels, meaning there certainly is a difference between the two. If you don't know the differences between micro, monolithic, and hybrid kernels, then feel free to Google around.



From the top of my head, these first two came to my mind. If you think that employing a totally different kernel type, as well as a radically different device driver kit unlike any of the others mentioned in this thread doesn't justify calling Darwin unique... Well, then you might as well call Linux, Solaris, BSD, Windows NT, Mac OS X, AmigaOS, etc. etc. the same.

Me thinks people are simply too lazy to study the more technical differences, instead focussing on the stuff they can grasp, which rarely - if ever - goes any further than the colour scheme and 3D effects used.

Edited 2008-12-26 18:03 UTC

It's not very NT-like. It's a Mach kernel, "inherited" by Apple from its acquisition of NeXT. The Mach kernel has some interesting architectural features that make an open source Darwin-based OS an interesting idea indeed, and a worthy project to pursue.

Microsoft's NT kernel pretended to offer similar architectural features with all its dithering on about how it uses a "hybrid" kernel, but the truth is that the NT kernel just made use of some superficial characteristics of a modularized monolithic kernel without actually leveraging any of the advantages thereof. PureDarwin may actually prove to offer the kind of benefits NT only pretended (and failed) to offer, because of its choice of core architecture.

but also some very bad performance drawbacks ( that other microkernels like L4 or KeyKos have successfully tackled in the last decades, btw)

what some people don't remember or deliberately ignore, is that:
1) Darwin is NOT NT, and its architecture is now totally different;
2) NT too was based on a pure microkernel (actually a Mach derivative) for some time, in the early days
3) NT being hybrid doesn't have to do with filesystem and networking stacks being merged into the kernel, but with the latter being a layered (and "ringed") structure made of elements interfaced by versioned APIs and accessed with LPC
4) NT4+ applications could be written, link, and work the same way (and with the same runtime libraries) as application for previous (3.x) versions of NT, and these could continue to run on the newer (integrated hybrid) kernel - this was because (thanks to architectural provisioning for messaging, transactions and local RPC, and the use of a dispatch system) they were independent from whether function providers were in- kernel or external;
5) "inner core" facilities (those that would be placed in a microkernel too) like VM, scheduling, messaging, are logically separated from less vital ones (whose memory may also be *paged out* to disk, btw - few other monolit... ehm, macrokernels can swap out parts of their address space, afaik...);
then, NT can be said to have a *microkernel centered*, structured, hybrid kernel

given the above, you see that NT has not "failed" in delivering a structured hybrid kernel
at least less of a failure than some other OS's one at leveraging the monolithic kernel design...
if the monolithic approach is chosen to reduce context switch overhead;
but, if user space daemons have to be present anyway to supply functionality required for the base platform to *work* at all (functionality such as event notifications or messaged ipc, that even other *microkernels* have implemented internally for ages);
and moreover, if they have to communicate in the most inefficient way possible (plain marshaling over sockets, even for local interaction - when things like protected procedure calls and Lightweight RPC, have existed for decades on other systems);
then i'm inclined to believe that in that case, there's something wrong in the way the monolithic kernel has been designed and deployed ...

after all, everything boils down to NIH syndrome, doesnt it?

Edited 2008-12-31 20:23 UTC