Re: [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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