Re: [patch 05/11] syslets: core code

From: Evgeniy Polyakov
Date: Sat Feb 17 2007 - 05:22:39 EST

On Fri, Feb 16, 2007 at 08:54:11PM -0800, Ray Lee (ray-lk@xxxxxxxxxxxxx) wrote:
> (This is probably why, by the way, most people are staying silent on
> your excellent kevent work. The kernel side is, in some ways, the easy
> part. It's getting an interface that will handle all users [ users ==
> producers and consumers of kevents ], that is the hard bit.)

Kevent interface was completely changed 4 (!) times for the last year
after kernel developers request without any damage to its kernel part.

> Or, let me put it yet another way: How do you prove to the rest of us
> that you, or Ingo, or whomever, are not building another dnotify? (Maybe
> you're smart enough in this problem space that you know you're not --
> that's actually the most likely possibility. But you still have to prove
> it to the rest of us. Sucks, I know.)

I only want to say that when system is designed correctly there is no
problem to change interface (yes, I again said 'to change' just because
I hope everyone understand that I'm talking about time when system is
not yet committed to the tree).

Btw, dnotify had problems in its design highlighted at inotify statrt -
mainly that watchers were not attached to inode.

It is right now the time to ask users what interface they expect from
AIO - so I asked Linus and proposed three different ones, two of them
were designed in a way that user would not even know that some
allocation/freeing was done - and as a result I got 'you suck' response
exactly the same as was returned on the first syslet release - just
_anly_ fscking _just_ because it had ugly interface.

> > Situations when system is designed from interface down to system ends up
> > with one thread per IO and huge limitations on how system is going to be
> > used at all.
> The other side is you start from the goal in mind and get Ingo's state
> machines with loops and conditionals and marmalade in syslets which
> appear a bit baroque and overkill for the majority of us userspace folk.

Well, I designed kevent AIO in the similar way, but it has even more
complex one which is built on top of internal page population functions.

It is complex a bit, but it works fast. And it works with any type (if I
would not be lazy and implement bindings) of AIO.

Interface of syslets is not perfect, but it can be changed (I said it
again? I think we all understand what I mean by that already) trivially
right now (before it is included) - it is not the way to throw thing
just because it has bad interface which can be extended in a moment.

> (No offense intended to Ingo, he's obviously quite a bit more conversant
> on the needs of high speed interfaces than I am. However, I suspect I
> have a bit more clarity on what us normal folk would actually use, and
> kernel driven FSMs ain't it. Userspace often makes a lot of contextual
> decisions that I would absolutely *hate* to write and debug as a state
> machine that gets handed off to the kernel. I'll happily take a 10% hit
> in efficiency that Moore's law will get me back in a few months, instead
> of spending a bunch of time debugging difficult heisenbugs due to the
> syslet FSM reading a userspace variable at a slightly different time
> once in a blue moon. OTOH, I'm also not Oracle, so what do I know?)
> The truth of this lies somewhere in the middle. It isn't kernel driven,
> or userspace interface driven, but a tradeoff between the two.
> So:
> > Userspace_API_is_the_ever_possible_last_thing_to_ever_think_about.
> > Period
> Please listen to those of us who are saying that this might not be the
> case. Maybe we're idiots, but then again maybe we're not, okay?
> Sometimes the API really *DOES* change the underlying implementation.

It is exactly the time to say what interface sould be good.
System is almost ready - it is time to make it looks cool for users.

> Ray

Evgeniy Polyakov
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