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ukernel Anonymous >>0b7a688ecce958cde1 [Reply]
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It's sad. Neither Hurd nor Mach reached the performance level of QNX. L4 finally did, although L4 now uses shared memory too much. (While it simplifies the kernel to do message passing via shared memory, it makes it easier for one process to mess up another. Queues in shared memory have to be managed very carefully.) 



The real issues in microkernel performance are subtle. Message passing and CPU dispatching have to be well-integrated. Otherwise, when you pass a message from one process to another in a subroutine/service like way and get a response, you go to the end of the line for CPU time twice. A good test of a message passing system is to write a client/server which does some trivial computation on the server and then returns a reply. Benchmark it. Then repeat the benchmark with the system CPU-bound. If performance drops by orders of magnitude under load, the message passing / CPU dispatch relation has been botched. 



On the security front, the "critique" says "programs are assumed to represent the interests of the user and, as such, are run with the user’s authority." This, of course, comes from the UNIX/Linux model and still gives us trouble. We still have trouble running something like Flash or a browser on a desktop without giving it far too much authority. 
(Of course, if you do have a more granular authority model, apps ask for too much, as the mobile world has learned. Grumble of the week: Firefox on Android now demands access to my phone book. Not because the browser needs it, but because their ancillary products, Sync or Pocket, might.)