On Microkernels and Virtual Machines: the Legacy of Mach 2. Simplify and Shrink

D uring the early years of Mach, anyone who talked to Rick Rashid soon encountered two ideas that excited him. One was the idea that operating systems had grown too large and complex, and that the time had come to simplify and shrink. The second was the idea that different operating systems offered the same core functionality XQGHUQHDWK D ORW RI VXSHUÀFLDO GLYHUVLW\ DQG LW VKRXOG WKHUHIRUH EH SRVVLEOH WR G\QDPLFDOO\ choose which guise to interact with. Although these two powerful ideas are interrelated in the Mach implementation, it is useful to think of them as distinct concepts from a historical perspective. In the roughly two decades since active research on Mach ended, how have these concepts fared in the marketplace of ideas? What is their intellectual legacy for OS research? Does anyone still care about them? We examine these issues in this brief note.