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On Wed, 3 Jan 2001, Pat O'Rourke wrote:
> Sorry to keep this thread going, but my original question to Michael
> was about SMP performance on 2.2. In our experiments we've been seeing
> an SMP director perform significantly worse than a uni-processor one
> (using the same hardware - only difference was booting an SMP kernel
> or uni-processor).
My understanding is that SMP absolutely will not directly help with the IP
stack under 2.2 _at all_. From what I understand, the stack is has
basically no code that will run in parallel, so only one processor can be
working with it at any given time. This is supposedly why the Mindcraft
tests worked so much in favor of NT -- it's stack does take advantage of
extra processors.
I would hypothesize that you generally lose performance on an SMP system,
because of the extra semantics (semaphores, etc) that the SMP kernel must
go through that the non-SMP kernel does not. So I would fully expect the
loss of performance.
Mind you, I can see instances where you might gain perfomance -- for
example, you expect the director to be heavily laden with userspace
programs that can run in parellel on the second processor with the IP
stack on the first.
2.4 supposedly addresses this problem. Mind you, this is from analysis
I've read; I haven't actually read the source code in any detail, so I
could be parroting nonsense :) YMMV.
Kyle Sparger
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