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

Davide Libenzi
2007-02-28T16:17:09Z

On Wed, 28 Feb 2007, Ingo Molnar wrote:

>
> * Davide Libenzi <davidel@xmailserver.org> wrote:
>
> > Why can't aio_* be implemented with *simple* (or parallel/unrelated)
> > syscall submit w/out the burden of a complex, limiting and heavy API
>
> there are so many variants of what people think 'asynchronous IO' should
> look like - i'd not like to limit them. I agree that once a particular
> syslet script becomes really popular, it might (and should) in fact be
> pushed into a separate system call.
>
> But i also agree that a one-shot-syscall sys_async() syscall could be
> done too - for those uses where only a single system call is needed and
> where the fetching of a single uatom would be small but nevertheless
> unnecessary overhead. A one-shot async syscall needs to get /8/
> parameters (the syscall nr is the seventh parameter and the return code
> of the nested syscall is the eighth). So at least two parameters will
> have to be passed in indirectly and validated, and 32/64-bit compat
> conversions added, etc. anyway!

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.
No compat code required (ok, besides the trivial async_wait).
My point is, the syslet infrastructure is expensive for the kernel in
terms of compat, and extra code added to handle the cond/jumps/etc. Is
also non-trivial to use from userspace. Are those big performance
advantages there to justify its existence? I doubt that the price of a
sysenter is a lot bigger than a atom decoding, but I'm looking forward in
being proven wrong by real life performance numbers ;)

- Davide