Re: SMP/cc Cluster description

Larry McVoy
2001-12-06T20:21:51Z

On Thu, Dec 06, 2001 at 12:15:54PM -0800, David S. Miller wrote:
> From: Larry McVoy <lm@bitmover.com>
> 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?
--
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Larry McVoy lm at bitmover.com http://www.bitmover.com/lm