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

Evgeniy Polyakov
2007-02-27T12:13:39Z

On Tue, Feb 27, 2007 at 06:52:22AM -0500, Theodore Tso (tytso@mit.edu) wrote:
> On Tue, Feb 27, 2007 at 01:28:32PM +0300, Evgeniy Polyakov wrote:
> > Obviously there are bugs, it is simply how things work.
> > And debugging state machine code has exactly the same complexity as
> > debugging multi-threading code - if not less...
>
> Evgeniy,

Hi Ted.

> I think what you are not hearing, and what everyone else is saying
> (INCLUDING Linus), is that for most programmers, state machines are
> much, much harder to program, understand, and debug compared to
> multi-threaded code. You may disagree (were you a MacOS 9 programmer
> in another life?), and it may not even be true for you if you happen
> to be one of those folks more at home with Scheme continuations, for
> example. But it is true that for most kernel programmers, threaded
> programming is much easier to understand, and we need to engineer the
> kernel for what will be maintainable for the majority of the kernel
> development community.

I understand that - and I totally agree.
But when more complex, more bug-prone code results in higher performance
- that must be used. We have linked lists and binary trees - the latter
are quite complex structures, but they allow to have higher performance
in searching operatins, so we use them.

The same applies to state machines - yes, in some cases it is hard to
program, but when things are already implemented and are wrapped into
nice (no posix) aio_read(), there is absolutely no usage complexity.

Even if it is up to programmer to programm state machine based on
generated events, that higher-layer state machines are not complex.

Let's get simple case of (aio_)read() from file descriptor - if page is in the
cache, no readpage() method will be called, so we do not need to create
some kind of events - just copy data, if there is no page or page is not
uptodate, we allocate a bio and do not wait until buffers are read - we
return to userspace and start another reading, when bio is completed and
its end_io callback is called, we mark pages as uptodate, copy data to userspace,
and mark event bound to above (aio_)read() as completed.
(that is how kevent aio works, btw).
Userspace programmer just calls
cookie = aio_read();
aio_wait(cookie);
or something like that.

It is simple, it is straightforward, especially if data read must then
be used somewhere else - in that case processing thread will need to operate
with main one, which is simple in event model, since there is a place,
where events of _all_ types are gathered.

> Regards,
>
> - Ted

--
Evgeniy Polyakov