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

Zach Brown
2007-02-22T21:25:10Z

> Plus there's the fundamental killer that KAIO is a /lot/ harder to
> implement (and to maintain) on the kernel side: it has to be
> implemented
> for every IO discipline, and even for the IO disciplines it
> supports at
> the moment, it is not truly asynchronous for things like metadata
> blocking or VFS blocking. To handle things like metadata blocking
> it has
> to resort to non-statemachine techniques like retries - which are bad
> for performance.

Yes, yes, yes.

As one of the poor suckers who has been fixing bugs in fs/aio.c and
fs/direct-io.c, I really want everyone to read Ingo's paragraph a few
times. Have it printed on a t-shirt.

Look at the number of man-years that have gone into fs/aio.c and fs/
direct-io.c. After all that effort it *barely* supports non-blocking
O_DIRECT IO.

The maintenance overhead of those two files, above all else, is what
pushed me to finally try that nutty fibril attempt.

> Syslets/threadlets on the other hand, once the core is implemented,
> have
> near zero ongoing maintainance cost (compared to KAIO pushed into
> every
> IO subsystem) and cover all IO disciplines and API variants
> immediately,
> and they are as perfectly asynchronous as it gets.

Amen.

As an experiment, I'm working on backing the sys_io_*() calls with
syslets. It's looking very promising so far.

> So all in one, i used to think that AIO state-machines have a long-
> term
> place within the kernel, but with syslets i think i've proven myself
> embarrasingly wrong =B-)

Welcome to the party :).

- z