Re: Syslets, Threadlets, generic AIO support, v6

[Davide Libenzi <davidel@xmailserver.org>]

On Wed, 30 May 2007, Ingo Molnar wrote:

> 
> * Linus Torvalds <torvalds@linux-foundation.org> wrote:
> 
> > > To echo Uli and paraphrase an ad, "it's the interface, silly."
> > 
> > THERE IS NO INTERFACE! You're just making that up, and glossing over 
> > the most important part of the whole thing!
> > 
> > If you could actually point to something specific that matches what 
> > everybody needs, and is architecture-neutral, it would be a different 
> > issue. As is, you're just saying "memory-mapped interfaces" without 
> > actually going into enough detail to show HOW MUCH IT SUCKS.
> > 
> > There really are very few programs that would use them. [...]
> 
> looking over the list of our new generic APIs (see further below) i 
> think there are three important things that are needed for an API to 
> become widely used:
> 
>  1) it should solve a real problem (ha ;-), it should be intuitive to 
>     humans and it should fit into existing things naturally.
> 
>  2) it should be ubiquitous. (if it's about IO it should cover block IO,
>     network IO, timers, signals and everything) Even if it might look
>     silly in some of the cases, having complete, utter, no compromises,
>     100% coverage for everything massively helps the uptake of an API, 
>     because it allows the user-space coder to pick just one paradigm 
>     that is closest to his application and stick to it and only to it.
> 
>  3) it should be end-to-end supported by glibc.
> 
> our failed API attempts so far were:
> 
>  - sendfile(). This API mainly failed on #2. It partly failed on #1 too.
>    (couldnt be used in certain types of scenarios so was unintuitive.)
>    splice() fixes this almost completely.
> 
>  - KAIO. It fails on #2 and #3.
> 
> our more successful new APIs:
> 
>  - futexes. After some hickups they form the base of all modern 
>    user-space locking.
> 
>  - splice. (a bit too early to tell but it's looking good so far. Would
>    be nice if someone did a brute-force memcpy() based vmsplice to user
>    memory, just to make usage fully symmetric.)
> 
> partially successful, not yet failed new APIs:
> 
>  - epoll. It currently fails at #2 (v2.6.22 mostly fills the gaps but
>    not completely). Despite the non-complete coverage of event domains a
>    good number of apps are using it, and in particular a couple really
>    'high end' apps with massive amounts of event sources - which apps 
>    would have no chance with poll, select or threads.
> 
>  - inotify. It's being used quite happily on the desktop, despite some
>    of its limitations. (Possibly integratable into epoll?)

I think, as Linus pointed out (as I did a few months ago), that there's 
confusion about the term "Unification" or "Single Interface".
Unification is not about fetching all the data coming from the more 
diverse sources, into a single interface. That is just broken, because 
each data source wants a different data structure to be reported. 
This is ABI-hell 101. Unification is the ability to uniformly wait for 
readiness, and then fetch data with source-dependent collectors (read(2), 
io_getvents(2), ...). That way you have ABI isolation on the single 
data source, and not monster structures trying to blob together the more 
diverse data formats.
AFAIK, inotify works with select/poll/epoll as is.



- Davide


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