Chapter pages1 Distributed Process Management Chapter 14

Chapter pages2 Process Migration Move an active process from one machine to another The process migrates to a target machine Transferring a sufficient amount of the state of a process from one machine to another

Chapter pages3 Why Migrate? Load sharing move processes from heavily loaded to lightly load systems load can be balanced to improve overall performance Communications performance processes that interact intensively can be moved to the same node to reduce communications cost may be better to move process to where the data reside when the data is large

Chapter pages4 Why Migrate? Availability long-running process may need to move because the machine it is running on will be down Utilizing special capabilities process can take advantage of unique hardware or software capabilities

Chapter pages5 Initiation of Migration Operating system when goal is load balancing Process when goal is to reach a particular resource

Chapter pages6 What is Migrated? Must destroy the process on the source system Process control block and any links must be moved

Chapter pages7 What is Migrated? Eager (all):Transfer entire address space no trace of process is left behind if address space is large and if the process does not need most of it, then this approach my be unnecessarily expensive

Chapter pages8 What is Migrated Precopy: Process continues to execute on the source node while the address space is copied pages modified on the source during precopy operation have to be copied a second time reduces the time that a process is frozen and cannot execute during migration

Chapter pages9 What is Migrated? Eager (dirty): Transfer only that portion of the address space that is in main memory any additional blocks of the virtual address space are transferred on demand the source machine is involved throughout the life of the process good if process is temporarily going to another machine good for a thread since the threads left behind need the same address space

Chapter pages10 What is Migrated? Copy-on-reference: Pages are only brought over on reference variation of eager (dirty) has lowest initial cost of process migration

Chapter pages11 What is Migrated? Flushing: Pages are cleared from main memory by flushing dirty pages to disk later use copy-on-reference strategy relieves the source of holding any pages of the migrated process in main memory

Chapter pages12 Negotiation of Process Migration Starter 1: Will you take P? 2: Yes, migrate to machine 3 3: MigrateOut P 4: Offer P 7: Accept offer 5: Offer P 6: MigrateIn P A P B KJ SD

Chapter pages13 Distributed Global States Operating system cannot know the current state of all process in the distributed system A process can only know the current state of all processes on the local system Remote processes only know state information that is received by messages these messages represent the state in the past

Chapter pages14 Example Bank account is distributed over two branches The total amount in the account is the sum at each branch At 3 PM the account balance is determined Messages are sent to request the information

Chapter pages15 Example Branch A Branch B 3:00 Total = $ S A = $ S B = $0.00

Chapter pages16 Example If at the time of balance determination, the balance from branch A is in transit to branch B The result is a false reading

Chapter pages17 Example Branch A Branch B 3:00 Total = $0.00 S A = $0.00 S B = $0.00 msg = “Transfer $100 to Branch B” 2:59 3:01

Chapter pages18 Example All messages in transit must be examined at time of observation Total consists of balance at both branches and amount in message

Chapter pages19 Example If clocks at the two branches are not perfectly synchronized Transfer amount at 3:01 from branch A Amount arrives at branch B at 2:59 At 3:00 the amount is counted twice

Chapter pages20 Example Branch A Branch B Total = $ S A = $ S B = $ :003:01 2:59 3:00 msg = “Transfer $100 to Branch B”

Chapter pages21 Example of a Snapshot Process 1 Outgoing channels 2 sent 1,2,3,4,5,6 3 sent 1,2,3,4,5,6 Incoming channels Process 2 Outgoing channels 3 sent 1,2,3,4 4 sent 1,2,3,4 Incoming channels 1 received 1,2,3,4 stored 5,6 3 received 1,2,3,4,5,6,7,8 Process 3 Outgoing channels 2 sent 1,2,3,4,5,6,7,8 Incoming channels 1 received 1,2,3 stored 4,5,6 2 received 1,2,3 stored 4 4 received 1,2,3 Process 4 Outgoing channels 3 sent 1,2,3 Incoming channels 2 received 1,2 stored 3,4

Chapter pages22 Ordering of Events Events must be order to ensure mutual exclusion and avoid deadlock Clocks are not synchronized Communication delays State information for a process is not up to date

Chapter pages23 Ordering of Events Need to consistently say that one event occurs before another event Messages are sent when want to enter critical section and when leaving critical section Time-stamping orders events on a distributed system system clock is not used

Chapter pages24 Time-Stamping Each system on the network maintains a counter which functions as a clock Each site has a numerical identifier When a message is received, the receiving system sets is counter to one more than the maximum of its current value and the incoming time-stamp (counter)

Chapter pages25 Time-Stamping If two messages have the same time- stamp, they are ordered by the number of their sites For this method to work, each message is sent from one process to all other processes ensures all sites have same ordering of messages for mutual exclusion and deadlock all processes must be aware of the situation

Chapter pages26 Token-Passing Algorithm if not token_present then begin clock := clock + 1;Prelude broadcast(Request, clock I); wait(access, token); token_present := True; end endif; token_held := True: token(i) := clock;Postlude token_head := False; for j := i + 1 to n, 1 to i - 1 do if (request(j) > token(J)) [Symbol]^token_present then begin token_present := False; send(access, token(j)) end endif; when received (Request, k, j) doNotation: request(j) := max(request(j), k); send(j, access, token)send message of type access, with token, by process j if token_present[Symbol]Ynot token_held then broadcast(request, clock, i) send message from process i of type request, with timestamp clock, to all other processes endif received(request, t, j)receive message from process j of type request, enddo; with timestamp t

Chapter pages27 Distributed Deadlock Detection Algorithm {Date object Dj receiving a lock_request(Ti)} begin if Locked_by(Dj) = nil then send granted else begin send not granted to Ti; send Locked)by(Dj) to Ti end end. {Transaction Ti makes a lock request for data object Dj} begin send lock_request(Ti) to Dj; wait for granted/not granted; if granted then begin Locked_by(Dj) := Ti; Held_by(Ti) :=  end else {suppose Dj is being used by transaction Tj} begin Held_by(Ti) := Tj; Enqueue(Ti, Request_Q(Tj)); if Wait_for(Tj) = nil then Wait_for(Ti) := Tj else Wait_for(Ti) := Wait_for(Tj); update(Wait)for(Ti), Request_Q(Ti)) end end. {Transaction Tj receiving update message} begin if Wait_for(Tj)  Wait_for(Ti) then Wait_for(Tj) := Wait_for(Ti); if Wait_for(Tj)  Request_Q(Tj) = nil then update(Wait_for(Ti), Request_Q(Tj)) else begin DECLARE DEADLOCK; {initiate deadlock resolution as follows} {Tj is chosen as the transaction to be aborted} {Tj releases all the data objects it holds} send clear(Tj, Held_by(Tj)); allocate each data object Di held by Tj to the first requester Tk in Request_Q(Tj); for every transaction Tn in Request_Q(Tj) requesting data object Di held by Tj do Enqueue(Tn, Request_Q(Tk)); end end. {Transaction Tk receiving a clear(Tj, Tk) message begin purge the tuple having Tj as the requesting transaction from Request_Q(Tk) end.

Chapter pages28 Example of Distributed Deadlock Algorithm T5T5 T1T1 T2T2 T3T3 T4T4 T6T6 T0T0 T5T5 T1T1 T2T2 T3T3 T4T4 T6T6 T0T0