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    Subject[speculation] Partitioning the kernel
    I'm wondering whether we have or need a formalisation of how work might be
    divided in future kernels.

    The question I'm interested in is how the work gets split up among various
    components at different levels within a single box (not SMP with many at
    the same level, or various multi-box techniques), in particular how you
    separate computation and I/O given some intelligence in devices other than
    the main CPU (or SMP set).

    There are a bunch of examples to look at:

    IBM mainframe
    "channel processors" do all the I/O
    main CPU sets up a control block, does an EXCP instruction
    there's an interrupt when operation completes or fails

    VAX 782: basically two 780s with a big cable between busses
    one has disk controllers, most of the (VMS) kernel
    other has serial I/O, runs all user processes

    various smart network or disk controllers
    and really smart ones that do RAID or Crypto

    I2O stuff on newer PCs

    Larry McVoy's suggestion that the right way to run, say, a 32-CPU
    box is with something like 8 separate kernels, each using 4 CPUs
    If one of those runs the file system for everyone, this somewhat
    overlaps the techniques listed above.

    All of these demonstrably work, but each partitions the work between processors
    in a somewhat different way.

    What I'm wondering is whether, given that many drivers have a top-half
    vs. bottom-half split as a fairly basic part of their design, it would
    make sense to make it a design goal to have a clean partition at that
    boundary.

    On well-endowed systems, you then have the main CPUs running the top half
    of everything, while I2O processors handle all the bottom halves and the
    I/O interrupts. On lesser boxes, the CPU does both halves.

    It seems to me this might give a cleaner design than one where the work
    is partitioned between devices at some other boundary.

    If the locks you need between top and bottom halves of the driver are also
    controlling most or all CPU-to-I2O communication, it might go some way
    toward avoiding the "locking cliff" McVoy talks of.
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