[TUHS] Re: Fwd: [simh] Announcing the Open SIMH project

On Fri, Jun 03, 2022 at 06:05:43PM -0700, Larry McVoy wrote: So in essence what you're saying is that OpenBSD and FreeBSD weren't ABI compatible, and that's your definition of when two OS's are different. And so when Warner says that there are hundreds of forks of the Linux kernel, to the extent that the ABI's are compatible, they really aren't "different". Part of this comes from the the fact that the Linux kernel, C library, and core utilities are all shipped separately. The BSDs have often criticized this, claiming that shipping all of the OS in a single source control system makes it easier to rollout new features. There is no doubt upsides from having a single source tree; but one of the advantages of keeping things separate is that definition of the kernel <-> userspace interface is much more explicit. That being said, I will note that this always hasn't been true. There was a brief period where an early Red Hat Enterprise Linux version suffered from the "legacy Unix value-add disease", where Red Hat had added some kernel changes that impacted kernel interfaces, which didn't make it upstream, or made it upstream with a changed interface, such that when users wanted to use a newer upstream kernel, which had newer features, and newer device driver support, it wouldn't work with that version RHEL. Red Hat has criticized *heavily* for that, both by the upstream development community and by its users, and since then it has stuck to a "usptream first" policy, especially where new system calls, or some other kernel interface is concerned. One of the reasons why that early RHEL experience kept Red Hat in line was because none of the other Linux distributions had that property --- and because the core development in upstream hadn't slacked off, so there was a strong desire to upgrade to newer kernels on RHEL, and when that didn't worked, not only did that make customers and developers upset, but it also made life difficult for Red Hat engineers, since they now need to figure out how to forward port their "value add" changes onto the latest and greatest kernel release. An interesting question is if CSRG had been actively pushing the state of the art foreward, would that have provided sufficient centripetal force to keep the HP/UX, SunOS, DG/UX, etc., from spintering? After all, it's natural to want to get a competitive advantage over your competition by adding new features --- this is what I call the "Legacy Unix value-add disease". But if you can't keep up with the upstream developments, that provides a strong disincentive from making permanent forks. For that matter, why was it that successive new releases of AT&T System V wasn't able to play a similar role? Was it because the rate of change was too slow? Was it because applications weren't compatible anyway due to ISA differences? I don't know.... One other dynamic might be the whole worse is better is worse debate. As an example of this, Linux had PCMCIA support at least a year or two before NetBSD did, and in particular Linux had hot-add support where you could insert an ethernet PCMCIA into your laptop after the OS had booted, and the ethernet card would work. However, if you ejected the ethernet card, there was a roughly 1 in 4 chance that your system would crash. NetBSD took a lot longer to get PCMCIA support --- but when it did, it had hot-add and hot-remove working perfectly, while Linux took a year or two more after that point before hot-remove was solidly reliable. So from a computer science point of view, one could argue that NetBSD was "better", and that Linux had a whole bunch of hacks, and some might even argue was written by a bunch of hacks. :-) However, from the user's perspective, who Just Wanted Their Laptop To Work, the fact that Linux had some kind of rough PCMCIA support first mattered a lot more than a "we will ship no code before its time" attitude. And some of those users would become developers, which would cause a positive feedback loop. - Ted