Re: SMP/cc Cluster description

[Larry McVoy]

On Thu, Dec 06, 2001 at 09:10:51PM +0100, Daniel Phillips wrote:
> On December 6, 2001 08:53 pm, Larry McVoy wrote:
> > On Thu, Dec 06, 2001 at 08:42:05PM +0100, Daniel Phillips wrote:
> > > On December 6, 2001 09:02 am, Larry McVoy wrote:
> > > > On Wed, Dec 05, 2001 at 11:56:17PM -0800, David S. Miller wrote:
> > > > > These lockless algorithms, instructions like CAS, DCAS, "infinite
> > > > > consensus number", it's all crap.  You have to seperate out the access
> > > > > areas amongst different cpus so they don't collide, and none of these
> > > > > mechanisms do that.
> > > > 
> > > > Err, Dave, that's *exactly* the point of the ccCluster stuff.  You get
> > > > all that seperation for every data structure for free.  Think about
> > > > it a bit.  Aren't you going to feel a little bit stupid if you do all
> > > > this work, one object at a time, and someone can come along and do the
> > > > whole OS in one swoop?  Yeah, I'm spouting crap, it isn't that easy,
> > > > but it is much easier than the route you are taking.  
> > > 
> > > What I don't get after looking at your material, is how you intend to do the 
> > > locking.  Sharing a mmap across OS instances is fine, but how do processes on 
> > > the two different OS's avoid stepping on each other when they access the same 
> > > file?
> > 
> > Exactly the same way they would if they were two processes on a traditional
> > SMP OS.
> 
> They'd use locks internal to the VFS and fs, plus Posix-style locks and
> assorted other userland serializers, which don't come for free.  As davem 
> said, you'll have to present a coherent namespace, that's just one of the 
> annoying details.  So far you haven't said much about how such things are 
> going to be handled.

Huh?  Of course not, they'd use mutexes in a mmap-ed file, which uses
the hardware's coherency.  No locks in the vfs or fs, that's all done
in the mmap/page fault path for sure, but once the data is mapped you
aren't dealing with the file system at all.