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Proper microkernelĭesigns avoid needless context switches. Only a naive implementation switches eagerly.
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Though Mach is not used as a true microkernel in Mac OS X, Darwin's internal discipline with respect to Mach also helps Darbat's cause. L4 is specifically designed to be as cache friendly as Spend a lot of time just fetching things from further down the memory When you do a Mach IPC,īasically you end up scrubbing your L1 cache and vast amounts of L2.
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With all these pieces now in user space, won't this just add even more, costly transitions between kernel space and user space? How in the world can this ever be faster than the current arrangement where Mach and BSD are in the same address space and device drivers run in kernel mode? Charles Gray answers. This arrangement begs us to circle back to performance, however.
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In Darbat, the entire Mac OS X kernel ( XNU) runs as a fully de-privileged application on top of L4. In Mac OS X today, the Mach and the BSD portions of the kernel run in the same address space for performance reasons. Increased stability is on the menu as well. Performance isn't the only goal of Darbat. This means in complex systems with webservers, databases, applications and what not should be clear winners. We expect to find lower CPU usage (and hence better throughput) in heavily multi-tasked or multi-threaded systems since L4 can do much more light-weight scheduling, switching, synchronisation and message passing. Of course, L4 also does less for you, but that's because you don't need all the features all the time.ĭon't so much think of it as replacing Mach, but lifting Mach up and making short-cuts in and around the edges. L4 has far better performance on raw IPC cost, thread operations and VM operations.
Is the Darbat team confident that it can actually outperform the stock Mac OS X kernel? If so, why? Here's Charles Gray's answer. The overall strategy for is to basically optimise out the bits of Mach where it counts.ĭarbat is still in the "get it to work" stage, however. Mach provides a lot of complex services which work and don't appear to be a performance bottle-neck. Also, there's no really good reason to do so.
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Vast amounts of user-land code rely on Mach - getting rid of it completely would be a huge task. To start, here's what he had to say about the decision to put Mach on top of L4 instead of entirely replacing it. I recently exchanged email with Charles Gray, project manager for the Darbat project, about the feasibility and wisdom of using L4 in Darwin, and about the possibility that Apple might use L4 in Leopard. But doesn't this setup add even more overhead? If there are doubts that an L4-based Mac OS X kernel would be any faster than a Mach-based one, surely a kernel that layers Mach on top of L4 would be slower still. Keeping Mach around solves the feature and API issues cited above. As some readers noted, Darbat doesn't actually replace Mach with L4. Context switching and IPC overhead would be just as bad or worse in L4.Īt the end of my last post, I linked to the " Darbat" project, an actual port of the core of Mac OS X to the L4 microkernel. Mach's weaknesses would not be improved upon by L4. Any replacement kernel would not only have to do everything that Mach currently does, it'd also have to provide the same APIs. Some Mach APIs are exposed and supported by Apple in Mac OS X. All of Mach's features that are missing from L4 would have to be re-implemented. Dubious readers immediately chimed in by email and in the comments section, some rejecting the idea outright. The suggestion that Apple could use the L4 microkernel in Leopard had a very brief honeymooon period, it seems.
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