Architecture of the oasis mobile shared virtual memory system

Architecture of the Oasis Mobile Shared
Virtual Memory System
Speaker ：呂宗螢
Adviser ：梁文耀　老師
Date ： 2008/3/4

Embedded and Parallel Systems Lab2
Paper
1. William H. Schroeder, Brett D. Fleisch “ Architecture of the Oasis
Mobile Shared Virtual Memory System“, The Department of
Computer Science and Engineering at the University of California,
Riverside (UCR)
2. Ballette, M. Liotta, A. Ramzy, S.M. ” Execution time prediction in
DSM-based mobile grids” : on IEEE International Symposium
Cluster Computing and the Grid, 2005. CCGrid 2005.

Outline (for paper 1)
 Introduction
 Oasis Collaborative Computing Application
 DSM vs. SVM
 Design
 System Architecture
 Site Architecture
 Protocol 1 and Protocol 2
 Disconnection
 Reconnection Algorithm
 Evaluation

Outline (for paper 2)
 Related Work
 Mobile Grid System Architecture
 This Paper’s Idea
 DSM vs. Grid

Introduction
 Support collaborative applications that operate
with mobile laptop computers and personal
digital assistants (PDAs).
 Require peer-to-peer and client-server
interactions in conditions of less than ideal
network connectivity.
 Has a highly consistent backbone that typically
execute either tightly or loosely coupled
distributed applications. (can disconnect
operation)

Introduction
 A time-based coherency approach is used which
uses a mechanism called a lease.
 Distribute systems software remains difficult and
expensive to develop.
 DSM (Distribution Share Memory).
 Transparency

Oasis Collaborative Computing Application
 POSIX DSM !?
Hospital Management system
Battlefield Tracking Application
Grocery Shopping Assistant
Distributed Appointment Scheduler
Distributed Grading system

DSM vs. SVM
 Tight DSM
 Loose SVM (Shared Virtual Memory)
CPU
Memory
Mapping
CPU
Memory
Mapping
CPU
Memory
Mapping
Node 0 Node 1 Node N
...
Shared memory
The picture source by 張志標 “ Design and Implementation of a User-Level Multi-threaded Distributed Shared Memory System”

DSM vs. SVM
 Cache for persistent store object (e.g. file)
or contain memory that has very few hot
spots.
 SVM can operate on the disconnected

Design
1. Oasis is designed to explore adapting a
DSM paradigm for a system of
collaborating mobile applications using
wireless communication.
 Mirage ： X
 Threadmarks ： X
 Quarks ： O

Design
2. Oasis is to support continual operability.
 Can disconnect operate.
 Disconnection of mobile computers in a
distributed system can occur in three
different methods: voluntary, involuntary,
and intermittent.
 Time-out based.

Design
3. Oasis provide referential transparency
for programs that use POSIX shared
memory and POSIX semaphores.
 Oasis takes existing applications that adhere
to the POSIX interface and execute these
programs unchanged in a distributed
system, whereas earlier, they may have
been only functional on an uniprocessor.

System Architecture

Site Architecture

Protocol 1

Protocol 2

Disconnection
 Hoarding
Cache the shared memory pages required
during disconnection
 Lease
Conjunction with hoarding to enforce the time-
based coherence model.
During the fixed period, the mobile machine
may use the resources exclusively.
If exceeds the time, reconciliation
mechanisms

Disconnection
 Three steps to disconnect
1. Applications query the user for information
to be used for disconnection.
2. Checks determine if there are any conflicts
in the pages that are requested.
3. There is a synchronous signal to continue
the disconnection process after the previous
operations are complete.

Disconnection-step 1
 The user is queried for information.
 Estimated disconnection time and the
hoarding policy for the shared memory
pages.
 Hoarding policy
All pages, most recently used pages, least
recently used pages, or most frequently
referenced pages

 Checks the requested pages for conflicts.
 Conflicts arise using leases when the user
requests a lease on a page that is already
leased or owned by another disconnected
machine.
 The user will be given the opportunity to
terminate the disconnected process or to
hoard the desired pages without a lease.

 The disconnection algorithm is initiated by internally or
externally generating a signal which will indirectly
activate the disconnection/reconnection thread.
 The following operations occur during resource
reorganization:
 Acquire global disconnection system lock.
 Flush stored shared memory pages and locks to the
backbone.
 Hoard shared memory pages to the disconnecting site and
optionally establish a lease for those pages.
 Release global disconnection system lock and initiate
independent operation

Disconnected Operation
 Lease that have a valid lease at reconnection
can simply replace pages on the backbone,
which are guaranteed to be unchanged from
pages updated during disconnection.
 Capturing and reconciliation, permits Oasis to
integrate changes that occurred on disconnected
machines with possible conflicting updates that
may have occurred on the backbone.

 Capturing is used to record all write to the
memory while the client operates
disconnected.
Logging
Twinning

 Log

Reconnection Algorithm
 Activate the disconnection/reconnection thread.
1. The backbone global system mutual exclusion
lock is raised.
2. The reconnecting machine must validate the
lease, if one was used.
3. After determining if the lease is valid, two
different situations arise:
 reconnection with a valid lease
 reconnection using reconciliation.

Reconnection With A Valid Lease
 The server holds the lease timer, and this
timer must be cancelled during
reconnection.
 If time out, the reconnecting machine must
use reconciliation.
 If no, the log or all twins can be discarded.

Reconnection Using Reconciliation Methods
 Ignoring changes that occurred when
disconnected. (read-only).
 Merge procedure which is an application-
specific rule-based approach to integrate
disconnected memory changes. (like
Bayou)

Evaluation
 System Performance and Basic Costs
Event Time (ms)
Page fault 17.35
32bytes message round-trip 1.95
1K message round-trip 11.71
Memory-to-memory copy(8k) 0.98

System Performance and Basic Costs

Component Cost of Disconnection Algorithm
 1 Oasis server.
 1 disconnecting site.
 1~6 backbone sites.
 Each site started a test program which
allocated 6 shared memory pages.
 1 shared lock.

 Case 1
 Has all the shared resources stored locally at the time
of disconnection. (worst)
 Case 2
 Has all shared resources located on backbone site.
(best)
 Disconnection consists of 5 components:
 Acquire Global System Lock
 Flush Shared memory Pages
 Acquire and Sync Pages
 Flush Shared locks
 Release Global System Lock

Cost of Disconnection Operation
Methods Time (ms)
logging 0.546/each write
shadowing (twin) 3.25

Cost of Reconciliation Algorithm

Conclusions
 Proof-of-concept.
 Mobile distributed systems.
 Convenient programming methodology

Future Work
 Wireless network.
 Incorporating multiple DSM consistency
protocols.
 Experiments with different hoarding
policies to obtain pages during
disconnection.
 Shared object model instead of a paged
region model.

The Paper 1 End!

Related Work
 Oasis
 Linda
 Java space
 JXTA Space
 TSpaces
 L2imbo

Mobile Grid System Architecture

 Terminals act as resource provider or
consumers.
 Resource providers periodically publish
their capabilities (CPU, storage, memory
etc.) and their updated status (resource
utilization, Mobile Grid availability
connectivity etc.) to the monitoring service.

 The broker, based on the task descriptor
and the resource provider related
information passed by the monitoring
service,
Disseminates the different work units among
the chosen terminals.
Collects the partial results
Sends them back to the resource consumer.

This Paper’s Idea
 Task-duration prediction approach.
 Fault-recovery based on time-out.

DSM vs. Grid
DSM Grid
Need lock? yes no
Programming
paradigm
Acquire() (lock)
Read(x)
Write(x)
Release()
(unlock)
in(x)
out(x)
pop(x)

Thank you!

Architecture of the oasis mobile shared virtual memory system

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