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

Ingo Molnar
2007-02-26T20:07:32Z

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> > Reading from the disk is _exactly_ the same - the same waiting for
> > buffer_heads/pages, and (since it is bigger) it can be easily
> > transferred to event driven model. Ugh, wait, it not only _can_ be
> > transferred, it is already done in kevent AIO, and it shows faster
> > speeds (though I only tested sending them over the net).
>
> It would be absolutely horrible to program for. Try anything more
> complex than read/write (which is the simplest case, but even that is
> nasty).

note that even for something as 'simple and straightforward' as TCP
sockets, the 25-50 lines of evserver code i worked on today had 3
separate bugs, is known to be fundamentally incorrect and one of the
bugs (the lost event problem) showed up as a subtle epoll performance
problem and it took me more than an hour to track down. And that matches
my Tux experience as well: event based models are horribly hard to debug
BECAUSE there is /no procedural state associated with requests/. Hence
there is no real /proof of progress/. Not much to use for debugging -
except huge logs of execution, which, if one is unlucky (which i often
was with Tux) would just make the problem go away.

Furthermore, with a 'retry' model, who guarantees that the retry wont be
an infinite retry where none of the retries ever progresses the state of
the system enough to return the data we are interested in? The moment we
have to /retry/, depending on the 'depth' of how deep the retry kicked
in, we've got to reach that 'depth' of code again and execute it.

plus, 'there is not much state' is not even completely true to begin
with, even in the most simple, TCP socket case! There /is/ quite a bit
of state constructed on the kernel stack: user parameters have been
evaluated/converted, the socket has been looked up, its state has been
validated, etc. With a 'retry' model - but even with a pure 'event
queueing' model we redo all those things /both/ at request submission
and at event generation time, again and again - while with a synchronous
syscall you do it just once and upon event completion a piece of that
data is already on the kernel stack. I'd much rather spend time and
effort on simplifying the scheduler and reducing the cache footprint of
the kernel thread context switch path, etc., to make it more useful even
in more extreme, highly prallel '100% context-switching' case, because i
have first-hand experience about how fragile and inflexible event based
servers are. I do think that event interfaces for raw, external physical
events make sense in some circumstances, but for any more complex
'derived' event type it's less and less clear whether we want a direct
interface to it. For something like the VFS it's outright horrible to
even think about.

Ingo