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L Cdr Manish Tandon, CAT
CONCURRENT PROCESS
L Cdr Manish Tandon, CAT
CONCURRENT PROCESS
What are the consequences and ramification of the co-existence of processes within one system?
What are the effects of processes competing for resources?
How are the need of processes which want to communicate with each other met?
L Cdr Manish Tandon, CAT
TYPES OF PROCESS
Fully independent processes; Users working on applications within the system such as compiling and running their own programmes.
Independent but related processes;Users contributing to one application such as several clerks in an order processing department.
L Cdr Manish Tandon, CAT
TYPES OF PROCESS
Concurrent Programming applications, Applications constituted as a set of co-operating processes.
L Cdr Manish Tandon, CAT
RESOURCES
Obvious Resources: Processor, Main Memory, I/O devices, Secondary Storage, etc.
Logical Resources: Data Items in Main Memory or on secondary storage employed for system such as message queues, shared data structures etc.
L Cdr Manish Tandon, CAT
RESOURCES
Resources can be classified as reusable or consumable.
Reusable resource is not destroyed by being used , eg.. Physical Resource – Processor, etc. if resource can be used by one process at a time it is called serially reusable , eg. Printer.
L Cdr Manish Tandon, CAT
RESOURCES
Consumable Resource is a transient data item, a signal which is created by one process and ceases to exist when received by another.
L Cdr Manish Tandon, CAT
MUTUAL EXCLUSION & DEADLOCK
Principal problem arising from sharing of serially reusable resources is to ensure that they are only used by one process at one time ie… ensure mutual exclusion. Eg. A printer must remain allocated to the process until the printout is completed.
L Cdr Manish Tandon, CAT
MUTUAL EXCLUSION & DEADLOCK
Mutual Exclusion creates the potential danger of Deadlock.
A process is said to be deadlocked if it is waiting for a resource held by the other.
L Cdr Manish Tandon, CAT
ACHIVING MUTUAL EXCLUSIONOperating System maintains the centralised control of resources , hence Mutual Exclusion is readily achieved.
Software Solutions – Flags, etc.
Hardware Solutions - Machine Instructions (Test And Set), etc.
Semaphores – Development of Software Solution, Low Level software techniques such as GOTO, etc.
L Cdr Manish Tandon, CAT
ACHIVING MUTUAL EXCLUSION
Monitor – programming constructs used to control access to shared data.
File and Record Locking.
L Cdr Manish Tandon, CAT
DEADLOCK
Deadlock can arise in many different and unanticipated situations. They generally occur in normal running of a system and can not be attributed to faulty programming only.
L Cdr Manish Tandon, CAT
CONDITIONS FOR DEADLOCKMutual Exclusion : Only one process can use a resource at a time.
Resource Holding : Processes hold some resources while requesting more.
No Preemption : Resources can not be forcibly removed from a process.
Circular Wait : A closed circle of processes exists, where each process requires a resource held by next.
L Cdr Manish Tandon, CAT
DEALING WITH DEADLOCKDeadlock Prevention : Deadlock not occur due to adoption of sound resource allocation policy. Eliminates all four conditions and does not let them occur. Inefficient way of handling deadlock.
L Cdr Manish Tandon, CAT
DEALING WITH DEADLOCKDeadlock Avoidance : Although deadlock is still possible, the system will avoid it when one threatens, ie.. when a deadlock would arise from a particular resource allocation - this allocation is not done. Various techniques used – Bankers Algorithm, etc.
L Cdr Manish Tandon, CAT
DEALING WITH DEADLOCKDeadlock Detection : A deadlock detection strategy accepts the risk of a deadlock occurring and periodically executes a procedure to detect any in place. The usual principle applied is to detect circular wait situation in resource allocation and request information maintained by the system… while this is quite feasible , it can consume considerable processing time. The frequency of checking routine would need to reflect the probability of deadlock occurrence.
L Cdr Manish Tandon, CAT
INTERPROCESS COMMUNICATIONModern Operating System provide variety of facilities to processes which require to them communicate with on another. The nature of communication ranges from the simple signaling of an event, to the passing of messages via a queuing system . Techniques available for these include :
Signals Shared Files ie.. Pipes Message Passing Shared Memory
L Cdr Manish Tandon, CAT
INTERPROCESS COMMUNICATIONSignals : A fairly primitive form of communication used to alert a process to occurrence of some event usually abnormal condition something similar to Interrupt. Eg. Ctrl ‘C’ Key Combination, Kernel sends signal SIGINT to all processes which would terminate by default due to receipt of this signal except SHELL which ignores it.
L Cdr Manish Tandon, CAT
INTERPROCESS COMMUNICATIONPipes : Pipe is a mechanism whereby the output of one process is directed into the input of another. Implicit in this operation is the creation of an temporary intermediate file into which the first process writes its output and from which the second process reads its input. Widely used in UNIX.
L Cdr Manish Tandon, CAT
INTERPROCESS COMMUNICATIONMessage Passing : A sequence of arbitrary characters can be sent from one process to other. Similar to Pipe, but Message Passing is more flexible and transparent. Communication takes place via message queues.
L Cdr Manish Tandon, CAT
INTERPROCESS COMMUNICATIONShared Memory : Concept of shared memory segments , which can be shared by two or more processes. Unix provides this facility, by providing four specific system calls for this technique.
