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    SubjectRe: Linux Kernel constraints!
    On Thu, 21 Jan 1999, Yogesh Bansal wrote:

    > Recently(dec.) in WindowsNT magazine comparisons/similarities between
    > various flavours of unix and nt had come. In the same article Linux was
    > ignored as enterprise os on account of following kernel 'limitations' :
    >
    > 1. kernel is not preemptive. ie even a higher priority user thread cant
    > cause another thread to be swapped if the other thread is presently running
    > in privileged/kernel context.

    Not true. All things that are not running with interrupts disabled can be
    preempted.

    > 2. kernel is not reentrant. ie.only one thread in kernel context at a time.

    Pfft. This is absurd. Every process that is blocked on I/O is _in kernel
    context_. This is the way UNIX works.

    > 3. kernel is not multi processing in the sense that on multiprocessor
    > systems it will run on only one cpu at a time.

    And this is even more absurd. SMP is _symmetric_. Kernel runs on all
    processors _by definition_. There may be some confusion here with respect
    to lock granularity though. The very first SMP kernels had a single lock
    that protected most data structures, which drastically limited kernel
    concurrency. Current kernels have more fine-grained locking that allow
    much better concurrency and therefore better scaling.

    --
    "Love the dolphins," she advised him. "Write by W.A.S.T.E.."


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