1CSC 4103 - Operating SystemsSpring 2007Tevfik KoşarLouisiana State UniversityMay 1st , 2007Lecture - XXIVDistributed Systems - IIIDistributed Coordination• Ordering events and achieving synchronization incentralized 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 sequentialprocesses), and A was executed before B, then A → B– If A is the event of sending a message by one process and B isthe 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, thenthese events are executed concurrently.Relative Time for Three Concurrent ProcessesWhich events are concurrent and which ones are ordered?Implementation of →• Associate a timestamp with each system event– Require that for every pair of events A and B, if A → B, then the timestampof 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 isincremented 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 whosetimestamp 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 eventsare concurrent– We may use the process identity numbers to break ties and to create atotal orderingDistributed Mutual Exclusion (DME)• Assumptions– The system consists of n processes; each process Pi resides at adifferent processor– Each process has a critical section that requires mutualexclusion• Requirement– If Pi is executing in its critical section, then no other process Pjis executing in its critical section• We present two algorithms to ensure the mutualexclusion execution of processes in their criticalsectionsDME: Centralized Approach• One of the processes in the system is chosen to coordinate theentry to the critical section• A process that wants to enter its critical section sends arequest message to the coordinator• The coordinator decides which process can enter the criticalsection next, and its sends that process a reply message• When the process receives a reply message from thecoordinator, it enters its critical section• After exiting its critical section, the process sends a releasemessage to the coordinator and proceeds with its execution• This scheme requires three messages per critical-sectionentry:– request– reply– releaseDME: Fully Distributed Approach• When process Pi wants to enter its critical section, itgenerates a new timestamp, TS, and sends the messagerequest (Pi, TS) to all processes in the system• When process Pj receives a request message, it mayreply immediately or it may defer sending a reply back• When process Pi receives a reply message from all otherprocesses in the system, it can enter its critical section• After exiting its critical section, the process sends replymessages to all its deferred requestsDME: Fully Distributed Approach (Cont.)• The decision whether process Pj replies immediately to arequest(Pi, TS) message or defers its reply is based on threefactors:– 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 replyimmediately to Pi– If Pj wants to enter its critical section but has not yet entered it, thenit compares its own request timestamp with the timestamp TS• If its own request timestamp is greater than TS, then itsends 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 otherprocesses in the system, which makes the dynamicaddition and removal of processes more complex• If one of the processes fails, then the entire schemecollapses– This can be dealt with by continuously monitoring the state ofall the processes in the system, and notifying all processes if aprocess 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 establishedDeadlock 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 ifit is not holding a resource with a unique number grater than i– Simple to implement; requires little overhead• Avoidance: Banker’s algorithm => designate one of theprocesses in the system as the process that maintainsthe information necessary to carry out the Banker’salgorithm– Also implemented easily, but may require too much overheadPrevention: Wait-Die Scheme• Based on a nonpreemptive technique• If Pi requests a resource currently held by Pj, Pi isallowed to wait only if it has a smaller timestampthan does Pj (Pi is older than Pj)– Otherwise, Pi is rolled back (dies)• Example: Suppose that processes P1, P2, and P3 havetimestamps 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 rolledbackPrevention: Would-Wait Scheme• Based on a preemptive technique; counterpart to thewait-die system• If Pi requests a resource currently held by Pj, Pi isallowed to wait only if it has a larger timestamp thandoes Pj (Pi is younger than Pj). Otherwise Pj is rolledback (Pj is wounded by Pi)• Example: Suppose that processes P1, P2, and P3 havetimestamps 5, 10, and 15 respectively– If P1 requests a resource held by P2, then the resource will bepreempted from P2 and P2 will be rolled back– If P3 requests a resource held by P2, then P3 will waitDeadlock