1CSC 4103 - Operating SystemsFall 2009Tevfik Ko!arLouisiana State UniversityDecember 1st , 2009Lecture - XXIVDistributed SystemsDistributed Coordination• Ordering events and achieving synchronization in centralized systems is easier. – We can use common clock and memory• What about distributed systems? – No common clock or memory– happened-before relationship provides partial ordering– How to provide total ordering?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 ProcessesWhich events are concurrent and which ones are ordered?ExerciseWhich of the following event orderings are true?(a) p0 --> p3 : (b) p1 --> q3 :(c) q0 --> p3 :(d) r0 --> p4 :(e) p0 --> r4 :Which of the following statements are true?(a) p2 and q2 are concurrent processes.(b) q1 and r1 are concurrent processes.(c) p0 and q3 are concurrent processes.(d) r0 and p0 are concurrent processes.(e) r0 and p4 are concurrent processes.5Implementation of ! • Associate a timestamp with each system event–Require that for every pair of events A and B, if A ! B, then the timestamp of A is less than the timestamp of B• Within each process Pi, define a logical clock – The logical clock can be implemented as a simple counter that is incremented between any two successive events executed within a process • Logical clock is monotonically increasing• A process advances its logical clock when it receives a message whose timestamp is greater than the current value of its logical clock–Assume A sends a message to B, LC1(A)=200, LC2(B)=195• If the timestamps of two events A and B are the same, then the events are concurrent– We may use the process identity numbers to break ties and to create a total orderingDistributed Mutual Exclusion (DME) • Assumptions–The system consists of n processes; each process Pi resides at a different processor– Each process has a critical section that requires mutual exclusion• Requirement–If Pi is executing in its critical section, then no other process Pj 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– releaseDME: Fully Distributed Approach•When process Pi wants to enter its critical section, it generates a new timestamp, TS, and sends the message request (Pi, TS) to all processes in the system•When process Pj receives a request message, it may reply immediately or it may defer sending a reply back•When process Pi 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 requestsDME: Fully Distributed Approach (Cont.)•The decision whether process Pj replies immediately to a request(Pi, TS) message or defers its reply is based on three factors:–If Pj is in its critical section, then it defers its reply to Pi–If Pj does not want to enter its critical section, then it sends a reply immediately to Pi–If Pj wants to enter its critical section but has not yet entered it, then it compares its own request timestamp with the timestamp TS• If its own request timestamp is greater than TS, then it sends a reply immediately to Pi (Pi asked first)• Otherwise, the reply is deferred– Example: P1 sends a request to P2 and P3 (timestamp=10) P3 sends a request to P1 and P2 (timestamp=4)Undesirable Consequences• The processes need to know the identity of all other processes in the system, which makes the dynamic addition and removal of processes more complex• If one of the processes fails, then the entire scheme collapses– This can be dealt with by continuously monitoring the state of all the processes in the system, and notifying all processes if a process failsToken-Passing Approach• Circulate a token among processes in system– Token is special type of message– Possession of token entitles holder to enter critical section• Processes logically organized in a ring structure• Unidirectional ring guarantees freedom from starvation• Two types of failures– Lost token – election must be called– Failed processes – new logical ring establishedDistributed Deadlock Handling• Prevention: Resource-ordering deadlock-prevention =>define a global ordering among the system resources– Assign a unique number to all system resources– A process may request a resource with unique number i only if it is not holding a resource with a unique number grater than i– Simple to implement; requires little overhead• Prevention: Timestamp-ordering deadlock-prevention– wait-die scheme -- non-reemptive– wound-wait scheme -- preemptive– Unique Timestamp assigned when each process is createdPrevention: Wait-Die Scheme• non-preemptive approach•If Pi requests a resource currently held by Pj, Pi is allowed to wait only if it has a smaller timestamp than does Pj (Pi is older than Pj)–Otherwise, Pi is rolled back (release resources)•Example: Suppose that processes P1, P2, and P3 have timestamps 5, 10, and 15 respectively–if P1 request a resource held by P2, then P1 will wait–If P3 requests a resource held by P2, then P3 will be rolled back• The older the process gets, the more waitsPrevention: Wound-Wait Scheme• Preemptive approach, counterpart to the wait-die system•If Pi requests a resource currently held by Pj, Pi is allowed