Distributed Operating
Systems and Process
Scheduling
Brett O’Neill
CSE 8343 – Group A6
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Overview
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What is a Distributed Operating System (DOS)?
Advantages of a DOS
Process scheduling/allocation in centralized systems
Process scheduling/allocation in distributed systems
DOS Scheduling Algorithms
Questions?
References
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What is a Distributed Operating
System (DOS)?
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A DOS is a collection of
heterogeneous
computers connected via
a network.
The functions of a
conventional operating
system are distributed
throughout the network
To users of a DOS, it is
as if the computer has a
single processor.
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What is a Distributed Operating
System (DOS)? (cont.)
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The multiple processors do not share a common
memory or clock.
Instead, each processor has its own local
memory and communicates with other
processes through communication lines.
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What is a Distributed Operating
System (DOS)? (cont.)
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The goal of a DOS is to provide a common,
consistent view of:
File systems
 Name space
 Time
 Security
 Access to resources
while keeping the details transparent to users.
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What is a Distributed Operating
System (DOS)? (cont.)
Transparency
Description
Access
Hide differences in data representation and how a
resource is accessed
Location
Hide where a resource is located
Migration
Hide that a resource may move to another location
Relocation
Hide that a resource may be moved to another
location while in use
Replication
Hide that a resource may be shared by several
competitive users
Concurrency
Hide that a resource may be shared by several
competitive users
Failure
Hide the failure and recovery of a resource
Persistence
Hide whether a (software) resource is in memory or
on disk
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Advantages of a DOS
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Resource Sharing – a user at one site can use the
resources available at another.
Computation Speedup – if a particular
computation can be partitioned into a number
of subcomputations, the DOS can allow the
distribution of computing among various sites
to run concurrently.
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Advantages of a DOS (cont.)
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Reliability – if one site fails, the remaining sites
can continue operating.
Communication – users at different sites have
the opportunity to exchange information.
Data Migration – data can be transferred
between sites, either by transferring entire files
or portions of files which are needed.
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Advantages of a DOS (cont.)
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Computation Migration – computations can be
transferred across the system. For example, if a
large file resides on a site other than the initiator,
computations on the file can be done where the
file resides using remote procedure calls.
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Advantages of a DOS (cont.)
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Process Migration – it is often advantageous to
execute processes at different sites for the
following reasons:
Load balancing – even the workload among CPU’s
 Computation speedup – total process time can be
reduced
 Hardware preference – a process may be more
suitable for execution on one processor than another
 Software preference – a process my require software
only available at one site
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Process scheduling/allocation in
centralized systems
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In a centralized system, there is never more than one
running process. If there are more processes, the rest
will have to wait until the CPU is free and can be
rescheduled.
As processes enter the system, they are placed in the
job queue, which consists of all processes in the
system.
The processes waiting to execute are kept in the ready
queue.
Processes waiting for a particular I/O device are placed
in the device queue.
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Process scheduling/allocation in
centralized systems (cont.)
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A process migrates through the various queues
throughout its lifetime. The operating system selects
from these queues in some fashion.
The operating system has 2 schedulers for this task:
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Long-term scheduler (job scheduler) – selects processes from
a spool and loads them into memory for execution
Short-term scheduler (CPU scheduler) – selects from among
the processes ready to execute, and allocates the CPU to one
of them
The long-term scheduler must make a careful process
mix of I/O-bound and CPU-bound processes.
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Process scheduling/allocation in
centralized systems (cont.)
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There are several scheduling algorithms to
determine which process in the ready queue
should be allocated to the CPU next:
First-Come, First-Served
 Shortest-Job-First
 Priority Scheduling
 Round-Robin Scheduling
 Multilevel Queue Scheduling
 Multilevel Feedback-Queue Scheduling
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Process scheduling/allocation in
distributed systems
Similar to centralized systems, but other factors to
consider.
There are several factors to take into account when
deciding how to allocate processes in distributed
systems:
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1.
2.
Scheduling levels – local scheduling deals with allocating
processes to a processor, much like in a centralized system.
Global scheduling deals with choosing the location at
which a process will be executed.
Load distribution goals – load balancing strives to
maintain an equal load throughout the distributed system.
Load sharing strives to prevent any particular location
from becoming too busy.
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Process scheduling/allocation in
distributed systems (cont.)
3.
Scheduling efficiency goals –
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Optimal Scheduling Algorithm – the state of all competing
processes and all related information must be available to the
scheduler. These solutions are NP-hard for more than two
processors.
Sub-Optimal solutions –
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Approximation – attempt to find a reasonably efficient solution as
quickly as possible by limiting the search space.
Heuristics – employ “rules of thumb”:
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Dependent processes should be located in close proximity
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Independent processes that change shared files should be located
in close proximity
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Divisible processes with little or no precedence relationships
should be distributed
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If the load is heavy at a particular location, do not schedule other
processes there
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Process scheduling/allocation in
distributed systems (cont.)
4.
5.
Processor binding time – in static binding, the
process is assigned to a processor before its
execution in deterministic fashion. In dynamic
binding, the process is assigned at execution time.
Scheduler responsibility – the scheduler can reside
on a single processor at a centralized server or can
be physically distributed among various processors.
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DOS Scheduling Algorithms
1. Usage Points
A usage table is kept on a centralized server. The goal
is to allocate usage among processors fairly.
When a host takes a process to execute, it receives
credit and one point is reduced from its entry in
the usage table.
When a host requests a resource that is not local, its
usage table entry is increased one point, called
debt.
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DOS Scheduling Algorithms
2. Graph Algorithms
The scheduling problem is represented as a unidirected
graph.
Processes are represented as P nodes, processors are
represented as L nodes, meaning location.
A P node is linked to an L node if P is running on L.
A P node is linked to another P node if the processes
are communicating.
The weight on a link represents a communication cost
or execution cost.
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DOS Scheduling Algorithms
2. Graph Algorithms (cont.)
Minimum-cut – When a vertex is cut, its associated
links are removed. A minimum cut-set represents
the lowest cost possible for cutting a set of
particular vertices. When cut, the different set of
vertices represent a different set of processors. A
minimum cost cut-set represents the assignment
of processes to locations with minimum
communication costs.
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DOS Scheduling Algorithms
2. Graph Algorithms (cont.)
Maximum-flow – If the weight associated with each
edge represents the cost of communication, then
minimum cost of communication can result in
maximum flow of information. Thus finding
maximum-flow is equivalent to finding minimumcut.
Therefore total execution and communication costs
can be minimized and concurrency can increase.
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DOS Scheduling Algorithms
3. Probes
Messages can be sent out to members of the
distributed system to locate an appropriate
processor for each process.
Complete, global information is not necessary in this
case. The scheduler can use localized information
to determine where to execute the process.
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DOS Scheduling Algorithms
4. Scheduling Queues
Quite similar to centralized operating systems.
There are two types of queues:
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Local queue – the standard queue that exists at each
location to maintain a list of processes to run locally
Global queue – a queue that maintains a list of
processes that can run at different locations
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DOS Scheduling Algorithms
4. Scheduling Queues (cont.)
A good example is MACH:
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Scheduling is at the thread level.
Each location has a set of local queues and a set of
global queues with different priorities.
Users can use “scheduling hints” which have 3 different
priority levels.
Scheduling hints allow the user to bypass the scheduling
queue.
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DOS Scheduling Algorithms
5. Stochastic Learning
Heuristics are used to identify the best possible action based on
previous actions, learning from experience:
All possible scheduling actions are associated with a probability.
All probabilities are initialized to certain values when the DOS
is set up.
After a process is sent to a location, the destination sends
information back to the source.
If the destination is overloaded, it sends penalty points back to
the source. If it is underloaded, it sends back bonus points.
These points are used to adjust probabilities accordingly.
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DOS Scheduling Algorithms
6. Automaton Vector
An extension of stochastic learning.
Each entry in the automaton vector represents the workload of
a node.
The contents of the vector are utilized with the probabilities
associated with that node as a weighted load.
This data is used to determine where each process should be
executed.
The automaton vector can collect and calculate data on the fly.
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Questions?
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References
Silberschatz, Galvin, Gagne, Applied Operating
System Concepts
Prashant Shenoy, University of Massachusetts
Mark Sebern, Milwaukee School of Engineering
Jonathon Hodgson, St. Joseph’s University
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