24-Distributed_Systems_II

24-Distributed_Systems_II - CSE 421/521 Operating Systems...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
1 CSE 421/521 - Operating Systems Fall 2011 Tevfik Ko ş ar University at Buffalo November 29 th , 2011 Lecture - XXIV Distributed Systems - II Event Ordering Happened-before relation (denoted by ) If A and B are events in the same process (assuming sequential processes), and A was executed before B , then A B If A is the event of sending a message by one process and B is the event of receiving that message by another process, then A B If A B and B C then A C If two events A and B are not related by the relation, then these events are executed concurrently . Relative Time for Three Concurrent Processes Which events are concurrent and which ones are ordered? Distributed Mutual Exclusion (DME) • Assumptions The system consists of n processes; each process P i resides at a different processor Each process has a critical section that requires mutual exclusion • Requirement If P i is executing in its critical section, then no other process P j is executing in its critical section • We present two algorithms to ensure the mutual exclusion execution of processes in their critical sections DME: Centralized Approach One of the processes in the system is chosen to coordinate the entry to the critical section A process that wants to enter its critical section sends a request message to the coordinator The coordinator decides which process can enter the critical section next, and its sends that process a reply message When the process receives a reply message from the coordinator, it enters its critical section After exiting its critical section, the process sends a release message to the coordinator and proceeds with its execution This scheme requires three messages per critical-section entry: request reply release DME: Fully Distributed Approach • When process P i wants to enter its critical section, it generates a new timestamp, TS , and sends the message request ( P i , TS ) to all processes in the system • When process P j receives a request message, it may reply immediately or it may defer sending a reply back • When process P i receives a reply message from all other processes in the system, it can enter its critical section • After exiting its critical section, the process sends reply messages to all its deferred requests
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
DME: Fully Distributed Approach (Cont.) The decision whether process P j replies immediately to a request ( P i , TS ) message or defers its reply is based on three factors: If P j is in its critical section, then it defers its reply to
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

Page1 / 5

24-Distributed_Systems_II - CSE 421/521 Operating Systems...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online