Resource allocation graph with a cycle but no deadlock

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1 KL University Department of Computer Science & Engineering Course: Operating System Course code: 13CS203 Course Coordinator: Dr. D.Rajeswara Rao KEY 1. a) If a resource category contains more than one instance, then the presence of a cycle in the resource-allocation graph indicates the possibility of a deadlock, but does not guarantee one. for example Resource allocation graph with a deadlock Resource allocation graph with a cycle but no deadlock b) Safety Algorithm In order to apply the Banker's algorithm, we first need an algorithm for determining whether or not a particular state is safe.

2 This algorithm determines if the current state of a system is safe, according to the following steps: 1. Let Work and Finish be vectors of length m and n respectively. Work is a working copy of the available resources, which will be modified during the analysis. Finish is a vector of booleans indicating whether a particular process can finish. ( or has finished so far in the analysis. ) Initialize Work to Available, and Finish to false for all elements. 2. Find an i such that both (A) Finish[ i ] == false, and (B) Need[ i ] < Work. This process has not finished, but could with the given available working set. If no such i exists, go to step Set Work = Work + Allocation[ i ], and set Finish[ i ] to true. This corresponds to process i finishing up and releasing its resources back into the work pool. Then loop back to step If finish[ i ] == true for all i, then the state is a safe state, because a safe sequence has been found. c) Resource-Allocation Graph In some cases deadlocks can be understood more clearly through the use of Resource-Allocation Graphs, having the following properties: o A set of resource categories, { R1, R2, R3,..., RN }, which appear as square nodes on the graph. Dots inside the resource nodes indicate specific instances of the resource. ( E.g. two dots might represent two laser printers. ) o A set of processes, { P1, P2, P3,..., PN } o Request Edges - A set of directed arcs from Pi to Rj, indicating that process Pi has requested Rj, and is currently waiting for that resource to become available. o Assignment Edges - A set of directed arcs from Rj to Pi indicating that resource Rj has been allocated to process Pi, and that Pi is currently holding resource Rj. o Note that a request edge can be converted into an assignment edge by reversing the direction of the arc when the request is granted. o For example: Resource allocation graph

3 If a resource-allocation graph contains no cycles, then the system is not deadlocked. If a resource-allocation graph does contain cycles AND each resource category contains only a single instance, then a deadlock exists. 1 d The Banker s Algorithm Handles multiple instances for resource types. n = number of processes; m = number of resource types; Data Structures: o Available: vector [1..m], Available[ j ] - the number of instances currently available for resource j. o Max: matrix[1..n, 1..m], Max[ i, j ] - the maximum number of instances of resource j that process i can request at any one time. o Allocation: matrix[1..n, 1..m] - process i currently holds Allocation[ i, j ] instances of resource j. o Need: matrix[1..n, 1..m] - process i may need additional Need[ i, j ] instances of resource j. Need[ i, j ] = Max[ i, j ] - Allocation[ i, j ] Banker s Algorithm - Safety Procedure Local Data Structures: o Work: vector [1..m], initialize Work to Available. o Finish: vector[1..n], initialized to false for each process i. 1. Find process i such that Finish[ i ] = false and Needi? Work if i exists do Work := Work + Allocationi Finish[ i ] := true Go back to ( no such i exists ) if Finish[ i ] = true for all i in 1..n, then the system is in a safe state. Otherwise, the processes whose index is false may potentially be involved in a deadlock in the future.

4 Banker s Algorithm - Resource Request Requesti - request vector - the number of additional instances for each resource type process i requests at this time. 1. If not (Requesti? Needi) raise an error - process i tries to get more resources than what it declared. 2. If not (Requesti? Available) process i must wait - no sufficient resources at this time. 3. Tentatively allocate the requested resources to process i: Available := Available - Requesti Allocationi := Allocationi + Requesti Needi := Needi - Requesti 4. Check safety of state. If safe, the resources are allocated. If not safe then cancel the tentative allocation and process i must wait. 1 e The Difference Between Deadlock Prevention and Deadlock Avoidance Deadlock Prevention: o Preventing deadlocks by constraining how requests for resources can be made in the system and how they are handled (system design). o The goal is to ensure that at least one of the necessary conditions for deadlock can never hold. Deadlock Avoidance: o The system dynamically considers every request and decides whether it is safe to grant it at this point, o The system requires additional apriori information regarding the overall potential use of each resource for each process. o Allows more concurrency. Similar to the difference between a traffic light and a police officer directing traffic. Deadlock Prevention Eliminate one of the four conditions: o Mutual Exclusion: Well, if we need it then we need it. o Hold and Wait: Require a process to request and be allocated all its resources before it begins execution, or allow process to request resources only when the process has none. May lead to low resource utilization. Starvation is a problem - a process may be held for a long time waiting for all its required resources. If it needs additional resources, it releases all of the currently held resources and then requests all of those it needs (the application needs to be aware of all of the required resources). No Preemption:

5 o If a process that is holding some resources requests another resource that cannot be immediately allocated to it, then all resources currently being held are released. o The state of preempted resources has to be saved and later restored. Not practical for many types of resources (e.g. printer). Circular Wait: o Impose a total ordering on all resource types. o Require each process to request resources only in a strict increasing order. o Resources from the same resource type have to be requested together. Deadlock Avoidance Maximum requirements of each resource must be stated in advance by each process. Two approaches: o Do not start a process if its maximum requirement might lead to deadlock. o Do not grant an incremental resource request if this allocation might lead to deadlock. Two algorithms: o One instance per resource type - Resource Allocation Graph algorithm. o Multiple instances per resource type - The Banker s algorithm. 1 f Deadlock Characterization 1. Mutual exclusion: only one process at a time can use a resource. 2. Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes. 3. No preemption: a resource can be released only voluntarily by the process holding it, after that process has Completed its task. 4. Circular wait: there exists a set {P0, P1,, P0} of waiting processes such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2,, Pn 1 is waiting for a resource that is held by Pn, and P0 is waiting for a resource that is held by P0. Deadlock can arise if four conditions hold simultaneously. 2. a) Since segment tables are a collection of base limit registers, segments can be shared when entries in the segment table of two different jobs point to the same physical location. The two segment tables must have identical base pointers, and the shared segment number must be the same in the two processes. b) Number of logical addresses = = = 2 16 => each address is 16 bits. Number of physical addresses = = = 2 15 => each address is 15 bits.

6 c) Segmentation scheme suffers from External fragmentation. d) Segmentation and explain the basic method of segmentation A program is a collection of segments. A segment is a logical unit such as: main program, procedure, function, method, object, local variables, global variables, common block, stack, symbol table and arrays. Logical address consists of a two tuple:<segment-number, offset>, Segment table maps two-dimensional physical addresses; each table entry has: base contains the starting physical address where the segments reside in memory limit specifies the length of the segment Segment-table base register (STBR) points to the segment table s location in memory Segment-table length register (STLR) indicates number of segments used by a program e) Inverted Page table with diagram Rather than each process having a page table and keeping track of all possible logical pages, track all physical pages One entry for each real page of memory Entry consists of the virtual address of the page stored in that real memory location, with information about the process that owns that page Decreases memory needed to store each page table, but increases time needed to search the table when a page reference occurs f)address binding with a diagram Address binding of instructions and data to memory addresses can happen at three different stages Compile time: If memory location known a priori, absolute code can be generated; must recompile code if starting location changes

7 Load time: Must generate relocatable code if memory location is not known at compile time Execution time: Binding delayed until run time if the process can be moved during its execution from one memory segment to another 3 a) To find the page to replace the page in page replacement algorithm, If there is a free frame, use it. If there is no free frame, use a page-replacement algorithm to select any existing frame to be replaced, such frame is known as victim frame. b) The CPU scheduler sees the decreasing CPU utilization and increases the degree of multiprogramming as a result. The new process tries to get started by taking frames from running processes, causing more page faults and a longer queue for the paging device. As a result, CPU utilization drops even further, and the CPU scheduler tries to increase the degree of multiprogramming even more. Thrashing has occurred, and system throughput plunges. The page fault rate increases tremendously As a result, the effective memory-access time increases. No work is getting done, because the processes are spending all their time paging.

8 c) award the marks for any one case (1/2/3/4/5/6). d) Ans: Effective access time = 0.99 * (1sec * (2sec) * (10000sec sec) * (10000sec sec)) = ( ) sec = 34 sec e) f)

9 The basic idea behind demand paging is that when a process is swapped in, its pages are not swapped in all at once. Rather they are swapped in only when the process needs them(on demand). This is termed as lazy swapper, although a pager is a more accurate term. There are cases when no pages are loaded into the memory initially, pages are only loaded when demanded by the process by generating page faults. This is called Pure Demand Paging.