From: Ankur Rawat \(Excell Data Corporation\) (a-arawat_at_microsoft.com)
Date: Wed Feb 18 2004 - 16:00:09 PST
This paper is an interesting read from the perspective of solving a
challenging problem - of providing coherency in shared memory system in
a memory and processor efficient manner.
I skimmed through the other paper from Kai Li and see the parallels: in
leveraging the existing Paging hardware and mechanisms in providing
solutions to the problem.
To provide coherency in a shared virtual memory system, some sort of
synchronization is necessary. Using a synchronization mechanism on top
of the existing Virtual memory mechanisms will definitely be an overhead
and lead to inefficient use of memory and Processor time. Leveraging the
existing mechanism of page fault handling is a smart choice.
But, synchronization is not enough to solve this problem: another level
of control is needed on top of synchronization because multiple
processors could generate write faults concurrently and invalidate the
same page concurrently, leading to total chaos.
Hence - the ownership mechanism. It controls who owns the page at any
instant to bring order to the read and writes.
There are 2 approaches to Page synchronization
* Invalidation
* Writeback
Writeback approach due to its high overhead is not discussed.
Similarly, page ownership can be done 2 ways
* Static
* Dynamic
The static approach like writeback is very expensive and hence not part
of the discussion.
The Dynamic page ownership strategy is discussed and algorithms for the
centralized and distributed management are described.
The number of messages required to locate a page is the factor
determining the efficiency of the approach and the algorithm. In
general, distributed algorithms after experimental results were found to
be more effective for typical usage scenarios.
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