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DragonFly kernel List (threaded) for 2006-08
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Re: How much of microkernel?

From: "Thomas E. Spanjaard" <tgen@xxxxxxxxxxxxx>
Date: Tue, 22 Aug 2006 19:13:51 +0000

Matthew Dillon wrote:
:I think L4 and Mungi have proven that doesn't have to be the case these :days.
Well, I am not an expert on L4 or Mungi, but I can count cpu cycles, and having to do a context switch eats a *lot* of computer cycles,
and having to do a context switch which involves a change in the
protection map eats even *more* computer cycles. So many, in fact,
that the overhead often exceeds the overhead of the operation one is
trying to execute.

A relevant benchmark here is the lat_ctx benchmark of lmbench, of which a comparison between Linux and L4 is given on [1]. Pretty impressive results I'd say, even when the rate of context switches is higher.

    One then winds up in a situation where one must hack the code to pieces
    to make it efficient... to reduce the number of context switches that
    occur.  For example, a number of people have advocated that the TCP stack
    be moved to userland.  To my mind this is *NOT* micro-kernelish, as one
    then has no protection between the userland application and the networking
    stack.  Shifting the work around without introducing new protection
    realms is NOT a microkernel architecture.  It offers no additional
    reliability or debuggability to the system, and makes the code such a
    huge mess that it becomes unmaintainable.

I don't agree with that either, but I do like a network stack as a server in userland, but ofcourse in its own protection domain; Given efficient sharing of memory between protection domains so you don't have to copy data and a fast context switch, I think the little loss in performance is outweighed by the advantages of live subsystem upgrade/replacement, more defined protection between subsystems and the other usual advantages of microkernel designs.

        Thomas E. Spanjaard

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