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Date:      Wed, 27 Aug 1997 09:14:08 -0500 (CDT)
From:      Kyle Mestery <mestery@winternet.com>
To:        Christopher Petrilli <petrilli@amber.org>
Cc:        Peter Stubbs <peters@gil.com.au>, smp@FreeBSD.ORG
Subject:   Re: A how does it work question.
Message-ID:  <Pine.GSO.3.96.970827091116.7668F-100000@tundra.winternet.com>
In-Reply-To: <Pine.BSF.3.95q.970827100635.20292A-100000@chaos.amber.org>

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On Wed, 27 Aug 1997, Christopher Petrilli wrote:

> Because of this, wouldn't it be appropriate to say that FreeBSD is an
> Assymetric MP, not Symmetric?  Symmetric means that the kernel runs on
> each processor, and there is no "one processor" which controls exclusivity
> to the hardware.
> 
The symmetric/assymetric refers to the hardware.  This is from Curt
Schimmel's book on Cacheing and MP systems:

"To be considered an SMP system, all of the CPUs in the system must be
connected to a single bus and share a common pool of memory and share
access to all I/O devices.  Each CPU must also have equal access to the
memory and bus, and some form of bus arbitration must be considered.  The
bus arbitration is usually entirely up to the hardware."

In other words, an assymetric-MP system would be a system where each CPU
had it's own memory, own bus, etc.

Kyle Mestery
StorageTek's Network Systems Group
7600 Boone Ave. N., Brooklyn Park, MN 55428
mesteka@anubis.network.com, mestery@winternet.com
 

> Chris
> 
> On Wed, 27 Aug 1997, Kyle Mestery wrote:
> 
> > On Wed, 27 Aug 1997, Peter Stubbs wrote:
> > 
> > > I've been forced to sell my soul lately by doing a couple of MS Win
> > > NT courses ( mouths to feed etc.. ). It seems that NT runs a
> > > seperate instance of the kernel on each CPU present to provide it's
> > > SMP support.
> > > 
> > > Is this the way FBSD smp does it?
> > 
> > As far as I know, no.  There is only one copy of the kernel running.  At
> > present, access to the kernel is only allowed for one CPU (except for a
> > few areas), Steve has been working on making it reentrant.
> > 
> > > Is this the only way to do it?
> > 
> > No.  Having the kernel be reentrant is another way.  This requires the
> > correct lock "pushdown" into the kernel.  Instead of one giant lock,
> > subsystems can each have their own lock, allowing multiple processors to
> > be in different sections of the kernel.  This allows for increased
> > parallelism.
> > 
> > > Doesn't this mean that lots more memory would be used keeping data 
> > > for 2 kernels?
> > > 
> > I would assume so, but I dont know for sure.
> > 
> > Kyle Mestery
> > StorageTek's Network Systems Group
> > 7600 Boone Ave. N., Brooklyn Park, MN 55428
> > mesteka@anubis.network.com, mestery@winternet.com
> > 
> > 
> > 
> 




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