Upload
philip-parks
View
224
Download
4
Embed Size (px)
DESCRIPTION
For concurrent execution of interacting processes:- Communication and Synchronization between processes are the two essential system components Before processes can execute, they need to be:- Scheduled and Allocated with resources.
Citation preview
A System Performance Model
Distributed Process Scheduling
Outline
• Overview
• Process Interaction Models
• A System Performance Model
• Efficiency Loss
• Processor Pool and Workstation Queuing Models
• Comparison of Performance for Workload Sharing
• References
For concurrent execution of interacting processes:-• Communication and
• Synchronization between processes
are the two essential system components
Before processes can execute, they need to be:-
• Scheduled and
• Allocated with resources.
Why scheduling?
1.To enhance overall system performance metrices like:
• Process completion time and
• Processor utilization.
2. To achieve location and performance transparencies by distributed process scheduling.
Why scheduling in distributed systems is of special interestThis is so because of the issues that are different
from those in traditional multiprocessor systems: • The communication overhead is significant.
• The effect of underlying architecture cannot be ignored.
• And the dynamic behaviour of the system must be addressed.
Process Models(in brief)
1. Precedence Process Model
• Processes are represented by a DAG.
• Nodes- sequential processes
• Arcs- eg: i to j requires that process I completes before j can start executing.
Communication Process Model• Processes are created to coexist and
communicate synchronously.
• So we have undirected edges.
Disjoint Process Model
• We assume that processes can be run independently of each other.
• So order in which processes are executed is not important.
System Performance
• Speedup
• -What are the factors on which it depends
• How to calculate speedup when we apply these factors
Speedup depends on three factors• The design of the algorithm
• The efficiency of the scheduling algorithm
• The underlying system architecture.
• So if we take ‘S’ as the speedup factor then the above dependencies can be represented as
• S= F(Algorithm, System, Schedule)
• Where• OSPT= optimal sequential processing time; the
best time that can be achieved on a single processor using the best sequential algorithm.
• CPT= concurrent processing time; actual time achieved with the concurrent algorithm on an ideal n-processor system using an optimal scheduling policy.
• OCPTideal =optimal concurrent processing time on an ideal system;
• Si =ideal speedup obtained by using a multiple processor system over the best sequential time
• Sd = the degradation of the system due to actual implementation compared to an ideal system
Refined formula for speedup…..
• n – number of processors
• RP- Relative Processing requirement,
• RC- Relative Concurrency
…. Refined formula for speedup• Sd- degradation of parallelism due to algorithm
implementation.
Final formula for speedup
- Efficiency Loss, loss of parallelism when
implemented on a real machine. can be decomposed into two terms:
= sched + syst
Efficiency Loss
Efficiency Loss (Cont.)
'
)()()',(
)',(
schedsyst
ideal
idealideal
ideal
ideal
ideal
ideal
OCPTOCPTYCPT
OCPTYCPTYXCPT
OCPTOCPTYXCPT
'
)()(),(
),(
systsched
ideal
ideal
ideal
ideal
ideal
OCPTOCPTXOCPT
OCPTXOCPTZXCPT
OCPTOCPTZXCPT
Workload Distribution• Performance can be further improved by
workload distribution
• Load sharing: static workload distribution• Dispatch process to the idle processors
statically upon arrival• Corresponding to processor pool model
• Load balancing: dynamic workload distribution• Migrate processes dynamically from heavily
loaded processors to lightly loaded processors• Corresponding to migration workstation model
18
Processor-Pool and Workstation Queueing Models
Static Load SharingDynamic Load Balancing
M for Markovian distribution
Comparison of Performance for Workload Sharing
References• “Distributed Operating Systems and Algorithms”
by Randy Chow and Theodore Johnson
• “Operating System Concepts” by Silberschatz, Galvin and Gagne