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    SubjectRe: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3
    On Wed, 28 Feb 2007, Linus Torvalds wrote:

    > On Wed, 28 Feb 2007, Davide Libenzi wrote:
    > >
    > > At this point, given how threadlets can be easily/effectively dispatched
    > > from userspace, I'd argue the presence of either single/parallel or syslet
    > > submission altogether. Threadlets allows you to code chains *way* more
    > > naturally than syslets, and since they basically are like functions calls
    > > in the fast path, they can be used even for single/parallel submissions.
    >
    > Well, I agree, except for one thing:
    > - user space execution is *inherently* more expensive.
    >
    > Why? Stack. Stack. Stack.
    >
    > If you support threadlets with user space code, it means that you need a
    > separate user-space stack for each threadlet. That's a potentially *big*
    > cost to bear, both from a setup standpoint and from simply a memory
    > allocation standpoint.

    Right, point taken.



    > In short - the only thing I *don't* think is a great idea are those linked
    > lists of atoms. I still think it's a pretty horrible interface, and I
    > still don't think it really buys us very much. The only way it would buy
    > us a lot is to change the linked lists dynamically (ie add new events at
    > the end while old events are still executing), but quite frankly, that
    > just makes the whole interface *even*worse* and just makes me have
    > debugging nightmares (I'm also not even convinced it really would help
    > us: we might avoid some costs of adding new events, but it would only
    > avoid them for serial execution, and if the whole point of this is to
    > execute things in parallel, that's a stupid thing to do).
    >
    > So I would repeat my call for getting rid of the atoms, and instead just
    > do a "single submission" at a time. Do the linking by running a threadlet
    > that has user space code (and the stack overhead), which is MUCH more
    > flexible. And do nonlinked single system calls without *either* atoms *or*
    > a user-space stack footprint.

    Here we very much agree. The way I'd like it:

    struct async_syscall {
    unsigned long nr_sysc;
    unsigned long params[8];
    long result;
    };

    int async_exec(struct async_syscall *a, int n);

    or:

    int async_exec(struct async_syscall **a, int n);

    At this point I'm ok even with the userspace ring buffer, returning
    back pointers to "struct async_syscall".




    - Davide


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