How much of microkernel?
dillon at apollo.backplane.com
Tue Aug 22 11:08:07 PDT 2006
:I think L4 and Mungi have proven that doesn't have to be the case these
: Thomas E. Spanjaard
: tgen at xxxxxxxxxxxxx
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.
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.
Now, I am not against microkernel architectures... I've written several
embedded OSs that use such architectures. But the plain fact of the
matter is that there is a performance/protection trade-off and you have
to be willing to pay the price to get the protection.
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