Time and state are two key notions used in a conventional OS. However, these notions cannot be used in the same manner in a distributed system because it contains several computer systems, each with its own clock and memory, that communicate through messages that incur unpredictable communication delays. This chapter discusses practical alternatives to the traditional notions of time and state. These alternative notions are used in the design of distributed control algorithms and recovery schemes used in a distributed OS. | Chapter 17 Theoretical Issues in Distributed Systems Copyright © 2008 Operating Systems, by Dhananjay Dhamdhere Introduction Notions of Time and State States and Events in a Distributed System Time, Clocks and Event Precedences Recording the State of a Distributed System 17. Operating Systems, by Dhananjay Dhamdhere Copyright © 2008 Operating Systems, by Dhananjay Dhamdhere Notions of Time and State Time indicates when an event occurred State of an entity is the condition/mode of its being Depends on its features Global state of a system comprises the states of all entities in the system at a specific instant of time OS uses the notions of time and state for performing scheduling of resources and the CPU: Find chronological order in which requests occurred Distributed OS uses them for recovery Problem: lack of global clock in distributed systems 17. Operating Systems, by Dhananjay Dhamdhere Copyright © 2008 Operating Systems, by Dhananjay Dhamdhere States and . | Chapter 17 Theoretical Issues in Distributed Systems Copyright © 2008 Operating Systems, by Dhananjay Dhamdhere Introduction Notions of Time and State States and Events in a Distributed System Time, Clocks and Event Precedences Recording the State of a Distributed System 17. Operating Systems, by Dhananjay Dhamdhere Copyright © 2008 Operating Systems, by Dhananjay Dhamdhere Notions of Time and State Time indicates when an event occurred State of an entity is the condition/mode of its being Depends on its features Global state of a system comprises the states of all entities in the system at a specific instant of time OS uses the notions of time and state for performing scheduling of resources and the CPU: Find chronological order in which requests occurred Distributed OS uses them for recovery Problem: lack of global clock in distributed systems 17. Operating Systems, by Dhananjay Dhamdhere Copyright © 2008 Operating Systems, by Dhananjay Dhamdhere States and Events in a Distributed System Local and Global States Events 17. Operating Systems, by Dhananjay Dhamdhere Copyright © 2008 Operating Systems, by Dhananjay Dhamdhere Local and Global States Each entity in a system has its own state State of a memory cell is the value contained in it State of CPU is contents of PSW and GPRs State of process: State of memory allocated to it, CPU state (if running), state of interprocess communication The state of an entity is a local state State of process Pk at time t: Skt Global state: collection of local states of all entities at the same instant of time Global state of system at time t: St={S1t, S2t, , Snt} 17. Operating Systems, by Dhananjay Dhamdhere Copyright © 2008 Operating Systems, by Dhananjay Dhamdhere Events An event can be: sending/receiving a message (over a channel), or other (no messages involved) Channel: an interprocess communication path Process state changes when an event occurs in it We represent an event as follows: .
