Pre-Virtualization: Slashing the cost of virtualization

Despite its current popularity, para-virtualization has an enormous cost. Its diversion from the platform architecture abandons many of the benefits that come with pure virtualization (the faithful emulation of the platform API): stable and well-defined platform interfaces, single binaries for kernel and device drivers (and thus lower testing, maintenance, and support cost), and vendor independence. These limitations are accepted as inevitable for significantly better performance and the ability to provide virtualization-like behavior on non-virtualizable hardware, such as x86. We argue that the above limitations are not inevitable, and present pre- virtualization, which preserves the benefits of full virtualization without sacrificing the performance benefits of para-virtualization. In a semi-automatic step an OS is prepared for virtualization. The required modifications are orders of magnitudes smaller than for para-virtualization. A virtualization module, that is collocated with the guest OS, transforms the standard platform API into the respective hypervisor API. The guest OS is still programmed against a common architecture, and the binary remains fully functional on bare hardware. The support of a new hypervisor or updated interface only requires the implementation of a single interface mapping. We validated our approach for a variety of hypervisors, on two very different hardware platforms (x86 and Itanium), with multiple generations of Linux as guests. We found that pre-virtualization achieves essentially the same performance as para-virtualization, at a fraction of the engineering cost

Concurrent Search Data Structures Can Be Blocking and Practically Wait-Free

We argue that there is virtually no practical situation in which one should seek a "theoretically wait-free" algorithm at the expense of a state-of-the-art blocking algorithm in the case of search data structures: blocking algorithms are simple, fast, and can be made "practically wait-free". We draw this conclusion based on the most exhaustive study of blocking search data structures to date. We consider (a) different search data structures of different sizes, (b) numerous uniform and non-uniform workloads, representative of a wide range of practical scenarios, with different percentages of update operations, (c) with and without delayed threads, (d) on different hardware technologies, including processors providing HTM instructions. We explain our claim that blocking search data structures are practically wait-free through an analogy with the birthday paradox, revealing that, in state-of-the-art algorithms implementing such data structures, the probability of conflicts is extremely small. When conflicts occur as a result of context switches and interrupts, we show that HTM-based locks enable blocking algorithms to cope with the

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Abstract The L4 micro kernel [Lie98] provides extremely fast inter-process communication (IPC) [LE