[Sussex] LINUX takes on XServe

[Geoff Teale]

Morning,

THis is an interesting debate.  I had assumed that OS X was using a
microkernel because it was based on Mach.  Seems I was wrong.. :)

It does sound like this is definitely not a microkernel, although the
central architecure may be the same as one.  This kind of hybrid (suddenly
the Darwin platypus makes sense) seems a bit clunky really, but can have
certain advantages. I'd take a bet that the Apple engineers prefer the
internal architecture of Mach to that of BSD.

Of course performance wise you'll still take a lot of the hits of a normal
microkernel (because the mechanisms required will mostly be the same),
although you reduce the number of context switches.

All of this remind me of Windows NT.  NT famously started out as a pure
Microkernel and was actually a fairly stable product, however custromers
complained that it was slow (it really was very very slow) so for NT 4 they
re-engineered it to reduce the time taken for interface calls - the way they
achieved this was by moving almost all of the OS functionality back into the
kernel.  They got a significant speed increase but they resulted in a flaky
OS (hence the problems with all subsequent NT versions.. including 2000 and
XP).  

Theoretically all microkernels should be more reliable than Monolithic
kernels.  In reality a well written microkernel is easier to maintain
without introducing bugs (because of the seperation of functionality via
interfaces and the fact that most OS functionality runs in protected memory)
but most Unicies are living proof that Monolithic architecures can be made
to be very stable.  

Equally there are microkernels out there that disprove the rule that
Monolithic kernels are faster - but the generel case is true.

Tony wrote:
-----------
> However, micro or macro, it doesn't invalidate Geoff's point that a 
> monolithic kernel is normally faster than a microkernel, presumably 
> because of the clock cycles spent on passing the layers?

Yup, context switching, lack of caching in the kernel.. etc, etc..

> Apple's have never been renowned for blazing speed or stability or 
> low-cost, so no real surprise in finding a foreign OS works faster on 
> the same hardware.  Must admit I can't see any reason why 
> anyone would 
> buy an Apple X server over a Linux box, unless in a specialist field. 
> Anymore than anyone earning their living in the graphics world using 
> Linux rather than a Mac - horses for courses.

Mostly agree with that.  One thing however, Apple's streaming media software
is very good.  I know a number of people are running it on Darwin on x86.  

I have to say that, before Sarah got her iMac, I tried Darwin on x86 and I
found it slow, clunky and very weird in terms of it's filesystem layout.

I was expecting something BSDish - I have been a long term fan of FreeBSD
for it's cleanliness and sensible tools.  It's only since cleaner LINUX
distros came into my life (in particular Gentoo) that I've been truely happy
with LINUX.  It was always attractive because it advances so much quicker
than BSD - but most major Distros are just messy.  

-- 
GJT
geoff.teale at claybrook.co.uk




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