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Operating System Design Goals Efficiency Flexibility Consistency Robustness
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Why a Distributed System? Resource sharing
Specialized hardware Data/Database
Computation speedup Partitioning computations allowing concurrent
processing Load balancing
Reliability In event of system resource failure, shift its
processing load to another similar resource.
Operating System Design Goals
Efficiency
Flexibility
Consistency
Robustness
Efficiency Standard measurements
Throughput: Number of process/tasks completed per unit of time.
Turnaround Time: Process/task specific time required to execute from start to finish.
Waiting Time: Time process spent waiting to execute.
Response Time: If in an interactive environment, the time required to provide the first response to a processing request.
Efficiency Distributed system efficiency issues
Data propagation In a centralized system, access to data is for the most
part immediate. Not so the distributed system, data may be found in widely dispersed locations.
Solution, Data migration Computation migration Process migration
Efficiency Communication protocol overhead
Once again, in a centralized system a common basis for communication is present. Not so the distributed system.
Differing character encoding- ASCII vs EBCDIC Differing structuring of binary data.
Big Endian vs Little Endian In a centralized system no need for network protocols
to facilitate interprocess communication.
Solutions/requirements Tightly coded communication constructs at the operating
system level. Handle the ASCII vs EBCDIC or Endian issues with a minimum of fuss.
Effective communication protocols at the network level
Efficiency Load distribution
Issues bottlenecks and congestion Network
WAN LAN
Software components / processes Scalability – also falls in the robustness category
Flexibility Friendliness, what is it?
ease of use of the system interface Ability to relate computation processes to the
users problem space - accountability Freedom, what is it?
The ability to select when, where and how to use the system with restrictions being at a minimum.
Extensibility of the user environment. Ready system features present to provide for the creation of new user tools and services.
Consistency What is it?
Uniformity in using the system. Predictability in system behavior.
Issues: Lack of global information
Timing System state
Component failures Data replication and partitioning Maintaining data integrity in the face of
concurrency over widely dispersed data locations
Robustness A system is said to be robust if it is fault-
tolerant. Or said another way, it has the ability to continue to function, perhaps in a reduced capacity, after the occurrence of some form of system failure.
Common distributed system failures: Communication link failure Processing nodes failure / site outage Client/server processes failure / message loss
Robustness Recovery, step one: detect the failure.
Detect that some system resource has ceased to function.
Recovery, step two: reconfiguration. Isolate the failed system resource from the
system as a whole to prevent attempted use of said failed resource.
Recovery, step three: restoration. Once the failed resource returns to operation the
system must be once again reconfigured to include the formerly failed resource.
Robustness Scalability can also be considered under the
heading of robustness. Scalability is the ability of a system to adapt
to an increased service load. Any system resource who's load is proportional
to the size of the system has the potential of being overwhelmed as the system increases in size.
So, scalability, and therefore robustness, requires that the maximum load on any system resource be determined by some value other than system size.
Robustness Security:
Breach of confidentiality - Access of data Breach of integrity - Modification of data Breach of availability - Destruction of data Theft of service - Use of resources Denial of Serice - Preventing resource use
Protection: Allow multiple users/processes access to shared
system resources in a manner consistent with system protection polices.
i.e. only allow a user/process to access the minimum set of resources needed to accomplish their task.
Questions?
References Randy Chow, Theodore Johnson, “Distributed Operating Systems &
Algorithms”, Addison Wesley, pp.28-29. Abraham Silberschatz, Peter Baer Galvin, Greg Gagne, “Operating
System Concepts”, John Wiley & Sons Inc., pp. 612 - 617
