Synchronization

Synchronization Chapter 5 OS2- Sem1 83-84, R. Jalili

Synchronization • Processes need synchronization. • Access to resources, • Sharing printers • Ordering of events • Sync based on the actual time. • Relative ordering instead of absolute ordering. • Election algorithms for group management. • Distributed Mutual Exclusion OS2- Sem1 83-84, R. Jalili

Clock Synchronization • When each machine has its own clock, an event that occurred after another event may nevertheless be assigned an earlier time. OS2- Sem1 83-84, R. Jalili

Physical Clocks (1) • Counter & Holding register  clock tick • Computation of the mean solar day. OS2- Sem1 83-84, R. Jalili

Physical Clocks (2) • TAI seconds are of constant length, unlike solar seconds. Leap seconds are introduced when necessary to keep in phase with the sun. OS2- Sem1 83-84, R. Jalili

Clock Synchronization Algorithms • The relation between clock time and UTC when clocks tick at different rates. OS2- Sem1 83-84, R. Jalili

Clock Synchronization • The clocks drift at most  per time unit. So, we have 1-  ≤ dC/ dt ≤ 1+  • If two clocks are drifting in opposite directions, at a time t, they may be as much as 2 t apart. • If the OS designers want to guarantee that two clocks ever differ by more than , the clocks must be re-synch at least every /2 seconds. • Various algorithms … OS2- Sem1 83-84, R. Jalili

Cristian's Algorithm • It is assumed that a time server exists. • Getting the current time from a time server periodically, certainly not more than every /2 seconds. • The points • Time must never run backward  gradual decrement or increment • The propagation delay which is ≥ 0. This time is vary and depends on the network load. Cristian’s attempt is to measure the delay WITH the SAME clock. (the round trip of sending a query to the time server and getting reply = (T1 – T0)/2 or a better estimate :: (T1 – T0 – I )/2, I is the interrupt handling time in the time server. OS2- Sem1 83-84, R. Jalili

Cristian's Algorithm OS2- Sem1 83-84, R. Jalili

The Berkeley Algorithm • In the Cristian’s algorithm, the time server was passive. Other machines periodically ask it for the time. • In the Berekley Unix, the time server is polling every machine and checking its time. Based on the average of such times, it advises all to correct their time. • Discarding faulty values from the both extremes. OS2- Sem1 83-84, R. Jalili

The Berkeley Algorithm • The time daemon asks all the other machines for their clock values • The machines answer • The time daemon tells everyone how to adjust their clock OS2- Sem1 83-84, R. Jalili

Additional Clock Synch. Alg. • Most widely used in the Internet is NTP (Network Time Protocol). • It is now possible to have millions of clocks synch. • Use of Synch Clocks: • Enforcing at-most-once message delivery to a server, even in the face of crashes. • Conventional method: seq# and keeping all numbers; if a server crashes, it forgets everything! • If we attach each message a timestamp, and we keep the most recent timestamp seen for each connection, the msg with less timestamp is regarded as duplicate and rejected. OS2- Sem1 83-84, R. Jalili

Use of Synch Clocks • Old timestamps in the table can be removed using a global variable: • G = CurrentTime – MaxLifeTime – MaxClockSkew • If an incoming msg has no correspondent connection and its time is more recent than G, it is accepted; otherwise it is rejected. • In the case of server crash and reboot, G is reloaded from the time stored on disk and increments by the update period and again the same criterion rejects or accepts the incoming messages. OS2- Sem1 83-84, R. Jalili

Logical Clocks • Many cases need agreement on the same time, not exactly the actual time. •  Logical clocks. • Lamport in 1978 introduced the non-absolute clocks. • If two processes do not communicate, it is not necessary to synch their clocks. • All processes do not agree on time, they agree on the order in which events happen. • Base idea is the Happens-Before relation OS2- Sem1 83-84, R. Jalili

Logical Clocks-2 • A  B means A happened before B • Two rules • 1- If A and B belong to the same process and A occurs before B, then A  B is true. • 2- If A is the event of sending m, and B is the event of receipt of m by another process, then A  B is true. • Happens-Before is a transitive relation. • If we do not have either A  B or B  A then A and B are concurrent. • We need a way to measure the time for each event; so we assign C(A) as the time of event A. OS2- Sem1 83-84, R. Jalili

Logical Clocks-3 • A  B implies that C(A) < C(B). • For local events, we have the above relation. • For the case of sending and receiving a message, again we believe t hat we should have the above relation. • The clock value C must go forward. • Correction of a time is achieved through adding a positive number. • Lamport proposed how to assign time to events. OS2- Sem1 83-84, R. Jalili

Lamport Timestamps • Three processes, each with its own clock. The clocks run at different rates. • Lamport's algorithm corrects the clocks. OS2- Sem1 83-84, R. Jalili

Logical Clocks • Rules for assignment of the time values: If A happens before B in the same process, then C(A) < C(B) If A and B represent the sending and receiving of a message, C(A) < C(B) For distinctive events, C(A) is irrelevant to C(B) OS2- Sem1 83-84, R. Jalili

Example: Totally-Ordered Multicasting • Updating a replicated database and leaving it in an inconsistent state. OS2- Sem1 83-84, R. Jalili

Totally-Ordered Multicasting • Assume that msgs are multicast. When a process receives a msg, it broadcast an ack. Eventually all processes have the same queue, including msgs and acks. • A process can deliver a queued msg to its application when it is acked by all processes. • As all processes have the same queue, the order is guaranteed. OS2- Sem1 83-84, R. Jalili

Vector Timestamp • Lamport clock guarantees a total order of events while they the happened-before relation. • A  B  C(A) < C(B) • However, if C(A) < C(B) then either A  B or B A or A || B !!! • What we have is the time values of the events, not their relations • In fact, Lamport timestamps do not capture causality. If I have received an event, I am not sure if I have received its cause. OS2- Sem1 83-84, R. Jalili

Vector Timestamp • Each event is assigned a VT(A) to en event A. • VT(A) < VT (B)  A  B • Example: chained e-mails or news • Each process maintains a V as its vector clock. • Vi[i] is the # of events had occurred at Pi, so each event is happened in Pi, Vi[i] is incremented. • Vi[j] is the # events occurred at Pj, based on Pi knowledge: this property is maintained by piggybacking vectors along messages. • vt (m) says how many events at other processes have taken place before Pi sent m. Thus causality of m to those events can be specified. OS2- Sem1 83-84, R. Jalili

Vector Timestamp • The definition of < • Upgrade: • When Pi receives m, it adjusts its own vector: • Vj[k] = max {Vj[k] , vt(m)[k]} • Vi[i] is incremented by 1 • Msg is deliverable from Pj in Pk, iff • Vt( r)[j] = Vk[j] +1 • Vt (r )[i] ≤ Vk[i] for all i≠ j. OS2- Sem1 83-84, R. Jalili

Global State (1) Relationship of cuts to vt OS2- Sem1 83-84, R. Jalili

Global State (2) • Organization of a process and channels for a distributed snapshot OS2- Sem1 83-84, R. Jalili

Global State (3) • Process Q receives a marker for the first time and records its local state • Q records all incoming message • Q receives a marker for its incoming channel and finishes recording the state of the incoming channel OS2- Sem1 83-84, R. Jalili

Application • The termination detection OS2- Sem1 83-84, R. Jalili

Election Algorithms • Requirement for the coordinator. • Assume that processes can be distinguished somehow; through their MAC address, … • Assume that any process knows the process# of all other processes and if they are up or down currently. • Various algorithms including Bully alg. By Garcia-Molina-1982. • When a process notices that the coordinator no longer responding to requests, it initiates the alg. OS2- Sem1 83-84, R. Jalili

Bully Alg. • P sends an ELECTION to all with higher id# • If no one responds, P wins the election and becomes the coordinator. • If one of the higher id#’s responds, it takes over. • At any moment, a process can get an ELECTION msg from its lower id# colleagues. • The winner announces its victory by sending a msg announcing its leadership. • The biggest guy in town wins!  Bully alg! OS2- Sem1 83-84, R. Jalili

The Bully Algorithm (2) • The bully election algorithm • Process 4 holds an election • Process 5 and 6 respond, telling 4 to stop • Now 5 and 6 each hold an election OS2- Sem1 83-84, R. Jalili

The Bully Algorithm(3) • Process 6 tells 5 to stop • Process 6 wins and tells everyone OS2- Sem1 83-84, R. Jalili

A Ring Algorithm • This alg. is based on ring without token. • Assume that processes are physically or logically ordered. Each process knows its successor. • To start, a process sends its id# inside the ELECTION msg to its successor. If it is down, the next one is selected to send to. • At each intermediate node, the list of id#s is expanded, so the new ring is introduced. • When the initiator received the msg including its id#, it circulated the msg announcing itself as coordinator. • Two concurrent initiation!! OS2- Sem1 83-84, R. Jalili

A Ring Algorithm • Election algorithm using a ring. OS2- Sem1 83-84, R. Jalili

Mutual Exclusion: A Centralized Algorithm • Process 1 asks the coordinator for permission to enter a critical region. Permission is granted • Process 2 then asks permission to enter the same critical region. The coordinator does not reply. • When process 1 exits the critical region, it tells the coordinator, when then replies to 2 OS2- Sem1 83-84, R. Jalili

A Distributed Algorithm • Two processes want to enter the same critical region at the same moment. • Process 0 has the lowest timestamp, so it wins. • When process 0 is done, it sends an OK also, so 2 can now enter the critical region. OS2- Sem1 83-84, R. Jalili

A Token Ring Algorithm • An unordered group of processes on a network. • A logical ring constructed in software. Process 0 has the token. Token lost!! OS2- Sem1 83-84, R. Jalili

Comparison • A comparison of three mutual exclusion algorithms. OS2- Sem1 83-84, R. Jalili

Synchronization