On Fri, Feb 02, 2007 at 04:56:22PM -0800, Linus Torvalds wrote:
>
> On Sat, 3 Feb 2007, Ingo Molnar wrote:
> >
> > Well, in my picture, 'only if you block' is a pure thread utilization
> > decision: bounce a piece of work to another thread if this thread cannot
> > complete it. (if the kernel is lucky enough that the user context told
> > it "it's fine to do that".)
>
> Sure, you can do it that way too. But at that point, your argument that we
> shouldn't do it with fibrils is wrong: you'd still need basically the
> exact same setup that Zach does in his fibril stuff, and the exact same
> hook in the scheduler, testing the exact same value ("do we have a pending
> queue of work").
>
> So at that point, you really are arguing about a rather small detail in
> the implementation, I think.
>
> Which is fair enough.
>
> But I actually think the *bigger* argument and problems are elsewhere,
> namely in the interface details. Notably, I think the *real* issues end up
> how we handle synchronization, and how we handle signalling. Those are in
> many ways (I think) more important than whether we actually can schedule
> these trivial things on multiple CPU's concurrently or not.
>
> For example, I think serialization is potentially a much more expensive
> issue. Could we, for example, allow users to serialize with these things
> *without* having to go through the expense of doing a system call? Again,
> I'm thinking of the case of no IO happening, in which case there also
> won't be any actual threading taking place, in which case it's a total
> waste of time to do a system call at all.
>
> And trying to do that actually has implications for the interfaces (like
> possibly returning a zero cookie for the async() system call if it was
> doable totally synchronously?)
This would be useful - the application wouldn't have to set up state
to remember for handling completions for operations that complete synchronously
I know Samba folks would like that.
The laio_syscall implementation (Lazy asynchronous IO) seems to have
experimented with such an interface
http://www.usenix.org/events/usenix04/tech/general/elmeleegy.html
Regards
Suparna
>
> Signal handling is similar: I actually think that a "async()" system call
> should be interruptible within the context of the caller, since we would
> want to *try* to execute it synchronously. That automatically means that
> we have semantic meaning for fibrils and signal handling.
>
> Finally, can we actually get POSIX aio semantics with this? Can we
> implement the current aio_xyzzy() system calls using this same feature?
> And most importantly - does it perform well enough that we really can do
> that?
>
> THOSE are to me bigger questions than what happens inside the kernel, and
> whether we actually end up using another thread if we end up doing it
> non-synchronously.
>
> Linus
>
> --
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--
Suparna Bhattacharya ([email protected])
Linux Technology Center
IBM Software Lab, India
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