From: Honghai Liu (liu789_at_hotmail.com)
Date: Sat Feb 28 2004 - 23:03:20 PST
Reviewer: Honghai Liu
Denali is a special operating system that provides scalable and
simple isolation virtual machine for untrusted Internet software.
Contemporary approaches of Virtual machine normally emulate the
guest OS as much as possible at the cost of performance and
complexity. Denali, targeting at simplicity and scalability,
redesign and modify the virtual machine architectures. First
of all, Denali exposes lower-level resources to user-level
(like ExoKernels) to increase the performance. This, of course,
requires a new guest operating system (Ilwaco in Denali). Second,
Isolation is enforced by exposing only private namespaces and the
only sharing among VMs is through the virtual network. Third, web
application's Zipf distribution suggests a (dynamic) scalable
solution. Lastly, Denali is focused on performance, simplicity
and scalabity requires a significant modification to existing OS.
The Denali virtual architecture consists of virtual instruction
set for performance and simplicity, a rigid separated memory
among VMs and straightforward I/O device and interrupt model.
Isolation Kernel Implementation in CPU virtualization requires
a gatekeeper policy for selecting a subset of active machines
to the system and scheduler policy for context switching. Memory
Management periodically redistributes physical memory from inactive
VMs to active VMs. I/O Device and Interrupt Model is simplified,
for example, sending and receiving package over the network
interface only requires one virtual operation each.
Performance evolution confirms that the scalability of Denali system meets the
goal (potentially support 10,000 VMs) with reasonable overhead. In a word, Denali
provides an interesting approach for Virtual Machine to support untrusted software
distribution in the era of Internet, although backward compatibility may be of a c
oncern.
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