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Operating Systems: Deadlock 1 Deadlock Examples: Traffic Jam : Dining Philosophers Device allocation –process 1 requests tape drive 1 & gets it –process.

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Presentation on theme: "Operating Systems: Deadlock 1 Deadlock Examples: Traffic Jam : Dining Philosophers Device allocation –process 1 requests tape drive 1 & gets it –process."— Presentation transcript:

1 Operating Systems: Deadlock 1 Deadlock Examples: Traffic Jam : Dining Philosophers Device allocation –process 1 requests tape drive 1 & gets it –process 2 requests tape drive 2 & gets it –process 1 requests tape drive 2 but is blocked –process 2 requests tape drive 1 but is blocked Semaphores : P(s)P(t) P(t)P(s)

2 Operating Systems: Deadlock 2 I/O spooling disc –disc full of spooled input –no room for subsequent output Over-allocation of pages in a virtual memory OS –each process has a allocation of notional pages it must work within –process acquires pages one by one –normally does not use its full allocation –kernel over-allocates total number of notional pages »more efficient uses of memory »like airlines overbooking seats –deadlock may arise »all processes by mischance approach use of their full allocation »kernel cannot provide last pages it promised »partial deadlock also - some processes blocked –recovery ?

3 Operating Systems: Deadlock 3 Resource: –used by a single process at a single point in time –any one of the same type can be allocated Pre-emptible: –can be taken away from a process without ill effect –no deadlocks with pre-emptible resources Non-Pre-emptible: –cannot be taken away without problems –most resources like this –deadlock possible

4 Operating Systems: Deadlock 4 Definition : A set of processes is deadlocked if each process in the set is waiting for an event that only another process in the set can cause Necessary conditions for deadlock : Mutual Exclusion : each resource is either currently assigned to one process or is available to be assigned Hold and wait : processes currently holding resources granted earlier can request new resources Non-Pre-emption : resources previously granted cannot arbitrarily be taken away from a process; they must be explicitly released by the process Circular wait : there must be a circular chain of two or more processes, each of which is waiting for a resource held by the next member of the chain

5 Operating Systems: Deadlock 5 Resource Allocation Modelling using Graphs Nodes :resourceprocess Arcs :resource requested : resource allocated :

6 Operating Systems: Deadlock 6

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8 8 For multiple resources of the same type : Deadlock : A cycle not sufficient to imply a deadlock :

9 Operating Systems: Deadlock 9 Possible Strategies : Ignore - the Ostrich or Head-in-the-Sand algorithm –try to reduce chance of deadlock as far as reasonable –accept that deadlocks will occur occasionally »example: kernel table sizes - max number of pages, open files etc. –MTBF versus deadlock probability ? –cost of any other strategy may be too high »overheads and efficiency

10 Operating Systems: Deadlock 10 Deadlock Prevention –negate one of the necessary conditions negating Mutual Exclusion : –example: shared use of a printer »give exclusive use of the printer to each user in turn wanting to print? »deadlock possibility if exclusive access to another resource also allowed »better to have a spooler process to which print jobs are sent ­complete output file must be generated first –example: file system actions »give a process exclusive access rights to a file directory »example: moving a file from one directory to another ­possible deadlock if allowed exclusive access to two directories simultaneously ­should write code so as only to need to access one directory at a time –solution? »make resources concurrently sharable wherever possible e.g. read-only access »most resources inherently not sharable!

11 Operating Systems: Deadlock 11 Resource Trajectory Graph

12 Operating Systems: Deadlock 12

13 Operating Systems: Deadlock 13 negating Hold and Wait –process could request all the resources it will ever need at once »inefficient - not all resources needed all the time »processes probably will not know in advance what resources they will need »may have to wait excessive time to get all resources at once - starvation ­high priority processes may cause starvation of low priority processes

14 Operating Systems: Deadlock 14 –process could release existing resources it holds if it fails to get a new resource immediately »try again later »a form of two phase locking used in databases –whenever a new resource is needed, the process always releases its existing resources and asks for all of them at once –example: a process which copies a file from tape to disc, sorts the file on disc, and then prints the results on a printer »could request all three resources at the start ­wasteful of printer first, then tape drive later »could initially request tape and disc together, do the read and sort, then release both and finally request disc and printer together to do the printing ­more efficient ­must ensure data stays intact on disc between phases

15 Operating Systems: Deadlock 15 negating Non-Pre-emption –difficult to achieve in practice »cannot take a printer away from a process in the middle of printing »cannot take a semaphore away from a process arbitrarily ­might be in the middle of updating a shared area »cannot take open streams, pipes and sockets away ­process would need to be written very carefully, probably using signals ­very undesirable if possible at all –occasionally possible : »processes resident in main memory »some deadlock occurs such as failure to allocate a page »one or more processes can be swapped out to disc to release their pages and allow remaining processes to continue ­as long as they release any other resources they also hold on the way out »put back on the scheduling queues to be re-admitted to memory later

16 Operating Systems: Deadlock 16 negating Circular Wait –require that a process can only acquire one resource at a time »example: moving a file from one directory to another –require processes to acquire resources in a certain order –example: »1: tape drive »2: disc drive »3: printer »4: plotter »5: typesetter –example: semaphores »semaphores identified by number claimed in numerical order

17 Operating Systems: Deadlock 17 Deadlock Avoidance –deadlock possible but avoided by careful allocation of resources –avoid entering unsafe states –a state is safe if it is not deadlocked and there is a way to satisfy all requests currently pending by running the processes in some order –need to know all future requests of processes

18 Operating Systems: Deadlock 18 Example: can processes run to completion in some order? –with 10 units of resource to allocate : –if A runs first and acquires a further unit :

19 Operating Systems: Deadlock 19 avoidance using resource allocation graphs - for one instance resources –add an extra type of arc - the claim arc to indicate future requests –when the future request is actually made, convert this to an allocation arc –then check for loops

20 Operating Systems: Deadlock 20 Banker’s Algorithm (Dijkstra) Single resource –at each request, consider whether granting will lead to an unsafe state - if so, deny –is state after the notional grant still safe? »are there enough resources to satisfy the demands of some process »if so, process is notionally allowed to proceed »in due course, it is assumed to finish and return all its resources »process closest to its limit is the checked, and the steps repeated »if all processes can eventually run to completion, state is safe

21 Operating Systems: Deadlock 21 Multiple resources –m types of resource, n processes –vector comparison : »A  B if A i  B i for 0  i  m

22 Operating Systems: Deadlock 22 »look for a row in R  A i.e. a process whose requests can be met »if no such row exists, state is unsafe »add this row of R into the same row of C and subtract it from A i.e. notionally allocate the resources to the process »add this row of C back into A and set the row of C to zero i.e. the process notionally completes and returns its resources »repeat these steps until all C is all zero i.e. all processes notionally finished (initial state is safe) or until a suitable row in R cannot be found (unsafe) CR

23 Operating Systems: Deadlock 23 Drawbacks of Banker’s Algorithm –processes rarely know in advance how many resources they will need –the number of processes changes as time progresses –resources once available can disappear –the algorithm assumes processes will return their resources within a reasonable time –processes may only get their resources after an arbitrarily long delay –practical use is therefore rare!

24 Operating Systems: Deadlock 24 Detection and Recovery –let deadlock occur, then detect and recover somehow Methods of Detection - single resources –search for loops in resource allocation graph

25 Operating Systems: Deadlock 25 Depth-first Graph search –use a list of nodes L and progressively mark arcs 1.For each node N in the graph, perform steps 2-6 with N as starting node 2.Initialise L to empty and designate all arcs as unmarked 3.Add current node to L »check if node appears twice in L »if so, graph contains a cycle - algorithm terminates 4.From given node, if there are any unmarked outgoing arcs, goto 5 else goto 6 5.Pick an unmarked outgoing arc and mark it »follow it to new current node and goto 3 6.Have reached a dead end »go back to previous node and goto 4 »if this node is the initial node, graph does not contain cycles and algorithm terminates

26 Operating Systems: Deadlock 26

27 Operating Systems: Deadlock 27 Using an Adjacency Matrix –adjacency matrix represents single hops –two-hops : node(i) -> node(j) : node(i) -> node(1) and node(1) -> node(j) ornode(i) -> node(2) and node(2) -> node(j) ornode(i) -> node(3) and node(3) -> node(j)..... ornode(i) -> node(N) and node(N) -> node(j) –binary (Boolean) matrix multiplication »and replaces multiplication »or replaces addition

28 Operating Systems: Deadlock 28 –2 hops : –3 hops : –4 hops ?> 4 hops ? –identifying disjoint cycles ? –Transitive Closure equivalent to the N matrix multiplications * *

29 Operating Systems: Deadlock 29 Warshall’s Algorithm for computing Transitive Closure : –if there is a way to get from node x to node y and a way to get from node y to node z, then there is a way to get from node x to node z –if there is a way to get from node x to node y using only nodes with indices less than x and a way to get from node y to node z, then there is a way to get from from node x to node z using only nodes with indices less than x+1 for (y=0; y

30 Operating Systems: Deadlock 30 Semaphore loop detection :

31 Operating Systems: Deadlock 31 Multiple resources –apply equivalent of Banker’s algorithm using current resource requests –any processes unsatisfied are deadlocked When to check for deadlock? –every time a resource request is made –regularly at fixed time intervals –when CPU utilisation drops below some threshold

32 Operating Systems: Deadlock 32 Recovery from Deadlock Pre-emption –take resources from a process and give to others –how to select a victim? »order of precedence for pre-empting »number of resources already held »how many more will it need to complete? »amount of CPU time already used –swapping process out of memory may be sufficient »but may still hold resources involved in deadlock –may need to roll pre-empted process back »back to some safe restart point or go back to beginning »may need to checkpoint processes »not convenient for user interaction! –need to avoid starvation of a low priority process always being pre-empted »include number of previous pre-emptions as a choice factor

33 Operating Systems: Deadlock 33 Process Termination –drastic ultimate solution –abort all processes involved in the deadlock »all resources returned for re-use –abort processes one by one until deadlock resolved »how to choose order of precedence? »will the process need to be rerun? –aborting a process may cause severe difficulties »may be in the process of updating a file which will be left inconsistent –a process gets into an infinite program loop while holding resources »common situation in practice

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