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    SubjectRe: SMP/cc Cluster description
    On Thu, Dec 06, 2001 at 12:15:54PM -0800, David S. Miller wrote:
    > From: Larry McVoy <>
    > 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?
    Larry McVoy lm at
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