From: Muench, Joanna (jmuench_at_fhcrc.org)
Date: Mon Mar 01 2004 - 13:42:44 PST
Bugnion, et al. (1997) revive the concept of virtual machine monitors by
presenting them as a solution to the lag of operating system capabilities
behind hardware performance. While implementing a better OS is the ideal
solution, Disco provides a less expensive alternative with most of the
performance benefit. The target system is the FLASH shared memory
multiprocessor, which has the challenge of being a ccNUMA. The paper
specifically addresses how Disco provides a superior interface to this type
of memory system.
Disco is a virtual machine monitor that provides an abstraction layer
between commodity operating systems and the underlying system. The
individual components that make Disco possible aren't particularly new, but
the application is. Probably most important is the use of physical memory,
which Disco handles with use of dynamic page migration and replication. This
provides the ability for a non-NUMA aware OS to run efficiently on the
underlying NUMA architecture. Another important component is that the
virtual CPU executes on the real CPU, minimizing the performance hit of a
virtual machine.
The disappointment of the paper was discovering it was only run on a
simulation OS, rather than the real thing. The simulation did, however,
provide some insights into potential performance issues. One of the
significant overheads in running Disco was the high overhead of servicing
kernel TLB-faults. The authors thought that larger pages could reduce this
overhead. The good news was that with only 8 processors the pure IRIX system
bogged down due to a bottleneck at the spinlock protecting IRIX memory
management data structures. Partitioning the system into different virtual
machines reduced the execution time by 60%, a significant improvement.
The bit I liked most about the paper was the (brief) section on SPLASHOS, as
an example of being able to run a very lightweight OS efficiently. In
general this paper, important as it is, proved a bit of disappointment in
terms of clarity of writing.
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