On Thu, 15 Feb 2007, Evgeniy Polyakov wrote:
> On Thu, Feb 15, 2007 at 09:05:13AM -0800, Davide Libenzi ([email protected]) wrote:
> >
> > I actually think that building chains of syscalls bring you back to a
> > multithreaded solution. Why? Because suddendly the service thread become
> > from servicing a syscall (with possible cachehit optimization), to
> > servicing a whole session. So the number of service threads needed (locked
> > down by a chain) becomes big because requests goes from being short-lived
> > syscalls to long-lived chains of them. Think about the trivial web server,
> > and think about a chain that does open->fstat->sendhdrs->sendfile after an
> > accept. What's the difference with a multithreaded solution that does
> > accept->clone and execute the above code in the new thread? Nada, NIL.
>
> That is more ideological question about micro-thread design at all.
> If syslet will be able to perform only one syscall, one will have 4
> threads for above case, not one, so it is even more broken.
Nope, just one thread. Well, two, if you consider the "main" dispatch
thread, and the syscall service thread.
> So, if Linux moves that way of doing AIO (IMO incorrect, I think that
> the correct state machine made not of syscalls, but specially crafted
> entries - like populate pages into VFS, send chunk, recv chunk without
> blocking and continue on completion and the like), syslets with attached
> state machines are the (smallest evil) best choice.
But at that point you don't need to have complex atom interfaces, with
chains, whips and leather pants :) Just code it in C and submit that to
the async engine. The longer is the chain though, the closer you get to a
fully multithreaded solution, in terms of service thread consuption. And
what do you save WRT a multithreaded solution? Not thread
creation/destroy, because that cost is fully amortized inside the chain
execution cost (plus a pool would even save that).
IMO the plus of a generic async engine is mostly from a kernel code
maintainance POV. You don't need anymore to have AIO-aware code paths,
that automatically transalte to smaller and more maintainable code.
- Davide
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