Scuttlebutt: Torvalds Thinks OS X Is Crap
Subject:   Micro-kernel vs. Monolithic-kernel?
Date:   2001-04-09 17:38:10
From:   joelreed
The impression I took away from this article and from what I had already read elsewhere is that Linus's "design issue" statements were about Apple's decision to use a Mach based microkernel. Linus has traditionally been opposed to microkernel's but I am not sure that he is fully aware of how Mach and Mac OS X are put together. What traditionally differentiated microkernel's from monolithic kernels was what components were in kernel space and what were in user space. Monolithic kernels traditional being implemented with more components in kernel space, while microkernels moved those components from kernel space to user space. Linus of course being on the monolithic design side of the fence (Linux being a monolithic kernel.)

However in a quote from Apple's "Inside Mac OS X: Kernel Environment" document:

"... in Mac OS X, Mach is linked with other kernel components into a single kernel address space. This is primarily for performance; it is much faster to make a direct call between linked components than it is to send messages or do RPCs between separate tasks. This modular structure results in a more robust and extensible system than a monolithic kernel would allow, without the performance penalty of a pure microkernel."

Wether any of that would change the mind or opinion of Linus who knows? Probably not, most geeks are deeply religious and there's nothing wrong with that. However I think it is not fair to assume that Apples just blindly walked down the microkernel path without considering any potential "design issues". From the above quote and the rest of the document it sounds like Apple is aware of the "design issues" and has their own solutions. Wether Apples design choices for its Mach implementation and to a larger extent the Mac OS X kernel are right or wrong who knows.

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