Re: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3

Davide Libenzi
2007-02-28T18:23:00Z

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