EVFILT_VNODE is not complicated to use for the cases for which it was actually designed — monitoring a file descriptor. For more general file system monitoring, a new kqueue filter type would be ideal, but adding that does not require breaking or deprecating kqueue API; the whole point of kqueue is to provide a generic extensible event mechanism.
Adding a recursive path monitoring filter is as simple as:
- Defining a new `EVFILT_DIR` filter type.
- Accepting a file path or st_dev/ino_t pair via the generic `uinptr_t ident` kevent identifier.
- Providing extended data via the existing filter-controlled `intptr_t data` value.
Boom. Done. Using the nice, generic, well-designed kqueue() API that can also monitor file descriptors, processes, AIO events, signals, timers, and user-defined events.
I wish Linux — like Mac OS X, and all the BSDs — had adopted kqueue, or at least participated in the conversation. Instead, Linux went through 2-3 different mechanisms before finally settling on the odd-duck single-purpose inotify interface, despite kqueue's design having been published (http://people.freebsd.org/~jlemon/papers/kqueue.pdf) and released as part of FreeBSD 5 years earlier.
While Jon Lemon published a detailed paper covering kqueue's design, implementation, and performance benchmarking, the inotify developer published a 30 line README with erudite gems such as "Rumor is that the "d" in "dnotify" does not stand for "directory" but for "suck.": https://www.kernel.org/pub/linux/kernel/people/rml/inotify/R...
If you begin your comment with "Bah. This is complete nonsense." then you should at least address all of my points...
Especially when you confirm one thing I've said in your first sentence. EVFILT_VNODE's design it too limited. It can watch directories (which after being opened are just a file descriptor). But it doesn't give you the information you need although they are available when the event is generated. The only explanation for this I can think of is because the API designers wanted to cover every event even if they didn't understand the particular use case. Which is exactly what's going to happen when you try to design an API in such a way.
Yes, you could change kqueue and add another event type. But then why keep EVFILT_VNODE, except for legacy reasons?
You complain about nonsense and then you compare a paper covering kqueue, which again is an API trying to do everything, to the documentation of inotify, which only covers a single class of events. And then you are disingenuous enough to only look at the README and not at the provided manpages and other material. inotify actually has pretty good documentation. But if you want to compare the 30 lines README to the kqueue paper ... well the kqueue paper covers EVFILT_VNODE with 19 lines, 7 of which simply list the possible actions.
inotify is single purpose because it follows the "do one thing and do it right"-principle. The Linux folks had the freedom and time to come up with a good enough API because inotify wasn't forced into epoll, which itself follows the "do one thing and do it right"-principle.
Meanwhile the kqueue designers came up with an fs notification API that is even inferior to what w32 offers...
Why do you keep ascribing EVFILT_VNODE's limitations to the the fact that the kqueue API is well designed and extensible? The inotify designers had 5 years to figure out the constraints and implications of file system notifications -- of course that gave them the opportunity to explore a different approach. Imagine that, instead of ignoring the hard problem of generic event APIs and NIH'ing a bad one-off hack, they'd adopted kqueue and extended it with an EVFILT_DIR? The whole industry would have benefited.
Instead, they did the standard Linux thing of hacking together an incoherent bolt-on API because to do otherwise would require careful consideration and thought.
Blaming the BSDs for not adopting Linux's hack is nonsense.
I have explained it several times now: The Linux devs had 5 years to come up with the inotify design because their API is more flexible. If you design things along the lines of "do one thing and do it well" instead of "let's cram everything into it" then you gain that flexibility. That's where the kqueue API fails and that's why it is not as good as the Linux approach. You talk about "extensible". But extensible really means touching the kqueue API because that's the only way to extend it. On Linux epoll is far more extensible because it just means adding a new descriptor type.
I don't know why you claim that Linux ignored the hard problem of generic event APIs. Unlike kqueue, Linux has a truly generic event API. That's what I've explaining since the beginning.
Even if Linux had added kqueue and added an EVFILT_DIR then there would have been no guarantee that the BSD's would adopt the same API. There is no committee in charge of kqueue. The licenses are not portable and even if they'd agree on being compatible for kqueue then this would touch parts beyond it. And then there is the huge questions, why a directory and a file should be treated differently, when the only difference is the provided meta data.
Apple apparently has added a completely different approach to fs notifications. Based on a device and a deamon. Why didn't they just add an EVFILT_DIR, if that's as easy as you claim? Why didn't the BSDs do it?
Claiming that inotify is a hack or incoherent is just beyond ridiculous. Inotify is far better than anything kqueue offers. So if anything then kqueue is a hack and badly thought out...
If the BSD folks would simply adopt inotify then it would be one less painful thing to deal with when porting applications to BSD. All I hear nowadays is BSD folks complaining that the Linux folks are doing things differently and BSD has a hard time getting all the software to run they want. So adopting inotify would make it easier for the BSDs. In the end I don't care. But at least they should come up with a somewhat decent API. Doesn't matter if it's inotify or some NIH'ing.
You're absolutely 100% full of crap. I already explained how kqueue's flexibility means that an EVFILT_DIR can be added easily, and work with all other event mechanisms on the system.
> The Linux devs had 5 years to come up with the inotify design because their API is more flexible.
That doesn't even begin to make sense. There's nothing inherent in the kqueue API that mandates that all filter types be defined upfront and never be extended or modified.
> On Linux epoll is far more extensible because it just means adding a new descriptor type.
How is that more extensible? The whole point of kqueue is that there exists a class of event types for which a read-based FD API is not the most efficient mechanism for event dispatch.
> Apple apparently has added a completely different approach to fs notifications. Based on a device and a deamon. Why didn't they just add an EVFILT_DIR, if that's as easy as you claim?
Corporate developer politics. The people working on kqueue and the people working on Spotlight (the primary FSEvent consumer) were in two separate universes.
> Why didn't the BSDs do it?
It clearly hasn't been a priority for anyone; kqueue meets most needs just fine.
You are not helping your point by insulting me. I know that EVFILT_DIR can be added. I never disputed that. But it needs changes to the kqueue API. And apparently it is not so easy because nobody has done it and even operating systems using kqueue have opted to do it differently. You can talk all day about how things could be done theoretically. But that doesn't matter if reality looks so much different. Maybe you should ask yourself why EVFILT_VNODE was so poorly designed.
If by "changes to the kqueue" API you mean "adding a new filter type, as kqueue was designed to support", then how is the kqueue API poorly designed? Why should the BSDs adopt a completely separate inotify hack?
"100% full of crap" is an accurate assessment when you keep trotting out completely nonsense arguments as fact instead of actually engaging on the technical content; consider it my adopting of Linus' discussion style. Is there some specific problem that can't be solved within the confines of the existing kevent types and data structures?
This seems pretty simple:
- EVFILT_VNODE isn't poorly designed for the use-case of monitoring a file.
- EVFILT_VNODE is poorly designed for the use-case of monitoring a directory hierarchy, but that isn't the intended use case.
Nowhere have you demonstrated why the BSDs should drop kqueue in favor of Linux's hack; as for why it hasn't been implemented: because apparently no other kernel developers have needed it badly enough.
Mac OS X FSEvents as a counter-example doesn't hold technical water; the reasons for FSEvents being separate from kqueue are solely to do with Apple's internal political machinations, and you've provided no technical analysis that demonstrate otherwise.
EVFILT_VNODE is not complicated to use for the cases for which it was actually designed — monitoring a file descriptor. For more general file system monitoring, a new kqueue filter type would be ideal, but adding that does not require breaking or deprecating kqueue API; the whole point of kqueue is to provide a generic extensible event mechanism.
Adding a recursive path monitoring filter is as simple as:
- Defining a new `EVFILT_DIR` filter type.
- Accepting a file path or st_dev/ino_t pair via the generic `uinptr_t ident` kevent identifier.
- Providing extended data via the existing filter-controlled `intptr_t data` value.
Boom. Done. Using the nice, generic, well-designed kqueue() API that can also monitor file descriptors, processes, AIO events, signals, timers, and user-defined events.
I wish Linux — like Mac OS X, and all the BSDs — had adopted kqueue, or at least participated in the conversation. Instead, Linux went through 2-3 different mechanisms before finally settling on the odd-duck single-purpose inotify interface, despite kqueue's design having been published (http://people.freebsd.org/~jlemon/papers/kqueue.pdf) and released as part of FreeBSD 5 years earlier.
While Jon Lemon published a detailed paper covering kqueue's design, implementation, and performance benchmarking, the inotify developer published a 30 line README with erudite gems such as "Rumor is that the "d" in "dnotify" does not stand for "directory" but for "suck.": https://www.kernel.org/pub/linux/kernel/people/rml/inotify/R...