From: Honghai Liu (liu789_at_hotmail.com)
Date: Sat Feb 28 2004 - 00:39:33 PST
Reviewer: Honghai Liu
The paper presents Disco, a virtual machine system, which can support variety
of different OS and shows an alternative way of efficiently supporting
multiprocessors system as opposed to tediously rebuilding existing operating
system for them.
There is a new notion of address translation between physical and machine
address. Software-reloaded TLB was to facilitate the translation. Disco
keeps a VM pmap data structure that contains one entry for each physical
page of a virtual page to get corrected TLB entry. Even different nodes
could have point to the same physical page, however, they would be finally
mapped to the local machine pages by the translation. The policy of the
MUMA memory management of Disco is that page are read-shared are
replicated and write-shared are not migrated or moved around.
Because of the shared underlying layer, different nodes can easily shared
disk. In fact, Disco intercepts every disk request that DMAs data into
memory, so sharing disk is just sharing memory. Copy-on-write is heavily
used to minimize the cost of copying operation.
It seems there are few difference between sharing disk and memory. In fact,
same techniques can be applied for sending data over the network interface
on the same VM across nodes. Data sending and receiving is handled at
memory sharing level. The implementation is to use a global disk cache.
Virtual Machines were not a new idea and had been experimented as
early as 1970's. However, due to overheads, resource management
and communication & sharing limitations, it was not so successful.
With the development of modern technology such as those in distributed
environment, Disco redefines many areas on VM and justifies itself by
its competitive scalability, reliability and simplicity.
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