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

[Davide Libenzi]

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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