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L4-hurd discuss

From: Fortes Marcelo
Subject: L4-hurd discuss
Date: Wed, 22 Jun 2005 14:19:46 -0300 (ART)

--- Marcus Brinkmann
<address@hidden> escreveu:

> Hi,

 Hi Marcus!
> I am happy to discuss multi-server system design
> with you.  I am not
> interested in OpenAnything.

Ok but put in the mind that all i wrote was only a
sugestion to an hipothethic possibility.

> At Mon, 20 Jun 2005 21:21:32 -0300 (ART),
> Fortes Marcelo <address@hidden> wrote:
> > Mach have several performance pennalitys in his
> design that
> > compromises a performance for an multi-server
> based operating
> > system. Beind more indicated to a Single-server OS
> Design as it was
> > in NeXT and MacOS - X. However in this way it does
> not take the real
> > advantages of a micro-kernel porpose becouse it
> reflects another way
> > of a Monolithic system becouse almost system core
> are exported to a
> > unic bloated server... And device drivers that
> still being into the
> > kernel.
> Mach was the first attempt to make it work, and it
> does support
> multi-server operating systems in a functional way. 
> IE, it has all
> the required functions to make it work.  It was not
> meant to be used
> only for single servers.  Your observation that this
> does not reap
> much benefits from the design is right on.

Well first i meant that Mach works in better perfornce
 in comparision if you have a single monolith server
on top of it. I dont meant that it is not designed do
work with a multi server architecture. And as you
wrote it was the first tentative to this intention. So
its just normal it have not the better design to do
some things with the same performance 

> There was an attempt to put device drivers out of
> Mach---Mach in
> principle supports it.  But it was a performance
> desaster, and never
> made it anywhere.

Yes i knew that.

> > By other Hand L4 trys to solve some of this
> problems with a radical
> > design; exports device control to outside from
> kernel, Synchronous
> > message passing a minimum of system calls to Micro
> Kernel, and maybe
> > the most imprtant for performance a faster IPC.
> Again, you are right on.  Which makes me wonder why
> you mentioned
> OpenSolaris, as this gets us nowhere near to this
> goal.

Becouse the lack of a usable GNU system Man!
Yes i know you could say use GNU/Linux intead but
Linux itself dont honours GNU and is not related with
FSF spirit. GNU based on HURD/Mach is very slow (and
yet with same problem inside as Device drivers).
But unfortunatelly as i right now get aknowledgements 

Solaris cant be released under GPL even if it was
released with changes inside.

It could put a final dot here. 

> > But it does not solve everything becouse AFAIK
> thre is yet the
> > overhead in TLB, Context changig in regioster to
> each
> > thread/server/object as i dont know as L4 solve.

> Well, it's an open question.  It's true that there
> is some overhead,

So lets talk about this question.

> but there are also benefits.  It's extremely hard to
> predict
> performance of even simple software systems, and it
> is also extremely
> hardware dependent.  For example, the cost of
> context switches depends
> on the question if you have tagged TLBs and the
> number of registers.

And how can it be solved? By L4? i think not.

> L4 is heavily optimized.  The question is how much
> we need to add on
> top of it to make it do the things we want it to do,
> and how this
> affects performance in a negative way.

Now you touched in a point i was wondering for a time,
does have you on mind that some servers can not run on
top of L4?

or even try to imagine the follow situation.

In a teoric vision way a multi server system have more
flexibility as you can debug a server in user space,
shutdown it and restart without affect the rest of
sytem but only if servers have not a particular
interdependence between they and particular processes
being executed at the same time.
as maybe for exemple if maybe HURD Proc or Exec or
File System or Memory Mananger servers was shutdowned.

So to avoid IPC Overhead for exemple could crytical
servers as Auth, Proc and Exec run as a single server
just as objects inside it? 

it means that you have less servers running on top of
L4. Have you plans about of it?

>  It remains
> to be seen.  We can
> only find out by trying, and then analyzing, and
> trying again.
> Furthermore, there are non-performance related
> benefits that may very
> well outweigh the performance cost, if it is
> acceptably small.

your could be right in this way.

> Thanks,
> Marcus

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