Re: SMP/cc Cluster description

[Larry McVoy]

On Thu, Dec 06, 2001 at 12:15:54PM -0800, David S. Miller wrote:
>    From: Larry McVoy <[e-mail]>
>    Date: Thu, 6 Dec 2001 12:10:04 -0800
> 
>    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.
> 
> We're talking about two things.
> 
> Once the data is MMAP'd, sure things are coherent just like on any
> other SMP, for the user.
> 
> But HOW DID YOU GET THERE?  That is the question you are avoiding.
> How do I look up "/etc/passwd" in the filesystem on a ccCluster?
> How does OS image 1 see the same "/etc/passwd" as OS image 2?
> 
> If you aren't getting rid of this locking, what is the point?
> That is what we are trying to talk about.

The points are:

a) you have to thread the entire kernel, every data structure which is a
   problem.  Scheduler, networking, device drivers, everything.  That's
   thousands of locks and uncountable bugs, not to mention the impact on
   uniprocessor performance.

b) I have to thread a file system.

So I'm not saying that I'll thread less in the file system (actually I am,
but let's skip that for now and assume I have to do everything you have
to do).  All I'm saying is that I don't have to worry about the rest of
the kernel which is a huge savings.

You tell me - which is easier, multithreading the networking stack to 
64 way SMP or running 64 distinct networking stacks?