Re: [PATCH 0 of 4] Generic AIO by scheduling stacks

[Nick Piggin]

Nick Piggin wrote:
> Linus Torvalds wrote:
>
> > On Wed, 31 Jan 2007, Benjamin Herrenschmidt wrote:
> >
> >
> > > > - We would now have some measure of task_struct concurrency.  Read 
> > > > that twice,
> > > > it's scary.  As two fibrils execute and block in turn they'll each be
> > > > referencing current->.  It means that we need to audit task_struct 
> > > > to make sure
> > > > that paths can handle racing as its scheduled away.  The current 
> > > > implementation
> > > > *does not* let preemption trigger a fibril switch.  So one only has 
> > > > to worry
> > > > about racing with voluntary scheduling of the fibril paths.  This 
> > > > can mean
> > > > moving some task_struct members under an accessor that hides them in 
> > > > a struct
> > > > in task_struct so they're switched along with the fibril.  I think 
> > > > this is a
> > > > manageable burden.
> > >
> > >
> > > That's the one scaring me in fact ... Maybe it will end up being an easy
> > > one but I don't feel too comfortable...
> >
> >
> >
> > We actually have almost zero "interesting" data in the task-struct.
> >
> > All the real meat of a task has long since been split up into 
> > structures that can be shared for threading anyway (ie 
> > signal/files/mm/etc).
> >
> > Which is why I'm personally very comfy with just re-using task_struct 
> > as-is.
> >
> > NOTE! This is with the understanding that we *never* do any 
> > preemption. The whole point of the microthreading as far as I'm 
> > concerned is exactly that it is cooperative. It's not preemptive, and 
> > it's emphatically *not* concurrent (ie you'd never have two fibrils 
> > running at the same time on separate CPU's).
>
>
> So using stacks to hold state is (IMO) the logical choice to do async
> syscalls, especially once you have a look at some of the other AIO
> stuff going around.
>
> I always thought that the AIO people didn't do this because they wanted
> to avoid context switch overhead.
>
> So now if we introduce the context switch overhead back, why do we need
> just another scheduling primitive? What's so bad about using threads? The
> upside is that almost everything is already there and working, and also
> they don't have any of these preemption or concurrency restrictions.

In other words, while I share the appreciation for this clever trick of
using cooperative switching between these little thriblets, I don't
actually feel it is very elegant to then have to change the kernel so
much in order to handle them.

I would be fascinated to see where such a big advantage comes from using
these rather than threads. Maybe we can then improve threads not to suck
so much and everybody wins.

--
SUSE Labs, Novell Inc.
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