this post was submitted on 07 Jul 2024
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    No Mercy (files.catbox.moe)
    submitted 3 months ago by [email protected] to c/[email protected]
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    [–] [email protected] 2 points 3 months ago* (last edited 3 months ago) (1 children)

    This particular issue could be solved in most cases in a monolithic kernel. That it isn't, is by design.

    It was(see CLONE_DETATCHED here) and is(source)

    Create a program that is guaranteed to become a zombie. Run it within a filesystem mounted by an in-kernel module, like a remote nfs mount. You now have a permanently mounted NFS mount point.

    Ok, this is not really good implementation. I'm not sure that standard requires zombie processes to keep mountpoints(unless its executable is located in that fs) untill return value is read. Unless there is call to get CWD of another process. Oh, wait. Can't ptrace issue syscall on behalf of zombie process or like that? Or use vfs of that process? If so, then it makes sense to keep mountpoint.

    Every module is killable, crashable, upgradable - all without forcing a reboot or affecting any processes not using the module.

    except without the benefits of actually being a microkernel.

    Except Linux does it too. If graphics module crashes, I still can SSH into system. And when I developed driver for RK3328 TRNG, it crashed a lot. Replaced it without reboot.

    Microkernels are better. Popularity does not prove superiority, except in the metric of popularity.

    As I said, we live in post-meltdown world. Microkernels are MUCH slower.

    [–] [email protected] 2 points 3 months ago* (last edited 3 months ago) (1 children)

    As I said, we live in post-meltdown world. Microkernels are MUCH slower.

    I've heard this from several people, but you're the lucky number by which I'd heard it enough that I bothered to gather some references to refute this.

    First, this is an argument that derived from first generation microkernels, and in particular, MINIX, which - as a teaching aid OS, never tried to play the benchmark game. It's been repeated, like dogma, through several iterations of microkernels which have, in the interim, largely erased most of those performance leads of monolithic kernels. One paper notes that, once the working code exceeds the L2 cache size, there is marginal advantage to the monolithic structure. A second paper running benchmarks on L^4^Linux vs Linux concluded that the microkernel penalty was only about 5%-10% slower for applications than the Linux monolithic kernel.

    This is not MUCH slower, and - indeed - unless you're doing HPC applications, is close enough to be unnoticeable.

    Edit: I was originally going to omit this, as it's propaganda from a vested interest, and includes no concrete numbers, but this blog entry from a product manager at QNX specifically mentions using microkernels in HPC problem spaces, which I thought was interesting, so I'm post-facto including it.

    [–] [email protected] 1 points 3 months ago* (last edited 3 months ago)

    First, this is an argument that derived from first generation microkernels, and in particular, MINIX, which - as a teaching aid OS, never tried to play the benchmark game.

    Indeed, first generation microkernels were so bad, that Jochen Liedtke in rage created L3 "to show how it's done". While it was faster than existing microkernels, it was still slow.

    One paper notes that, once the working code exceeds the L2 cache size, there is marginal advantage to the monolithic structure.

    1. The paper was written in pre-meltdown era.
    2. The paper is about hybrid kernels. And gutted Mach(XNU) is used as example.
    3. Nowdays(after meltdown) all cache levels are usually invalidated during context switch. Processors try to add mechanisms to avoid this, but they create new vulnreabilities.

    A second paper running benchmarks on L^4^Linux vs Linux concluded that the microkernel penalty was only about 5%-10% slower for applications than the Linux monolithic kernel.

      1. Waaaaay before meltdown era.

    I'll mark quotes from paper as doublequotes.

    a Linux version that executes on top of a first- generation Mach-derived µ-kernel.

    1. So, hybrid kernel. Not as bad as microkernel.

    The corresponding penalty is 5 times higher for a co-located in-kernel version of MkLinux, and 7 times higher for a user- level version of MkLinux.

    Wait, what? Co-located in-kernel? So, loadable module?

    In particular, we show (1) how performance can be improved by implementing some Unix services and variants of them directly above the L4 µ-kernel

    1. No surprise here. Hybrids are faster than microkernels. Kinda proves my point, that moving close to monolithic improves performance.

    Right now I stopped at the end of second page of this paper. Maybe will continue later.

    this blog entry

    Will read.