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Critical Section Problem ● Synchronization Hardware ● Semaphores ● Classical Problems of Synchronization ● Monitors

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Critical Section Problem ● Synchronization Hardware ● Semaphores ● Classical Problems of Synchronization ● Monitors Principles of Operating Systems Lecture 4 - Process Synchronization Ardalan Amiri Sani ([email protected]) [lecture slides contains some content adapted from previous slides by Prof. Nalini Venkatasubramanian, and course text slides © Silberschatz] Outline ● The Critical Section Problem ● Synchronization Hardware ● Semaphores ● Classical Problems of Synchronization ● Monitors 2 Producer-Consumer Problem ● Paradigm for cooperating processes; ● producer process produces information that is consumed by a consumer process. ● We need a buffer of items that can be filled by producer and emptied by consumer. ● Unbounded-buffer places no practical limit on the size of the buffer. Consumer may wait, producer never waits. ● Bounded-buffer assumes that there is a fixed buffer size. Consumer waits for new item, producer waits if buffer is full. ● Producer and Consumer must synchronize. 3 Bounded Buffer - message passing ● Producer repeat message next_produced; … while (true) { /* produce an item in next produced */ Producer produce an item in nextp; … send(next_produced); send(consumer, nextp); } until false; ● Consumer message next_consumed; repeat Consumer while (true) { receive(producer, nextc); receive(next_consumed); … consume item from nextc; /* consume the item in next consumed */ … } until false; 4 Bounded Buffer - Shared Memory Solution ● Shared data #define BUFFER_SIZE 10 ● Shared data typedef struct { var n; . type item = ….; } item; var buffer: array[0..n-1] of item; in, out: 0..n-1; item buffer[BUFFER_SIZE]; in :=0; out:= 0; /* shared buffer = circular array */ int in = 0; /* Buffer empty if in == out */ int out = 0; /* Buffer full if (in+1) mod n == out */ /* noop means ‘do nothing’ */ 5 Bounded Buffer - Shared Memory Solution: producer item next_produced; ● Producer process - creates filled buffers while (true) { repeat /* produce an item in next produced */ … while (((in + 1) % BUFFER_SIZE) == out) produce an item in nextp ; /* do nothing */ … buffer[in] = next_produced; while in+1 mod n = out do noop; in = (in + 1) % BUFFER_SIZE; buffer[in] := nextp; } in := in+1 mod n; until false; 6 Bounded Buffer - Shared Memory Solution: consumer item next_consumed; while (true) { ● Consumer process - Empties filled buffers while (in == out) repeat ; /* do nothing */ while in = out do noop; next_consumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; nextc := buffer[out] ; out:= out+1 mod n; … /* consume the item in next consumed */ consume the next item in nextc } … until false 7 Problem with this solution ● Shared memory solution to the bounded-buffer problem allows at most (n-1) items in the buffer at the same time. 8 Solution ● A solution that uses all N buffers is not that simple. ● Modify producer-consumer code by adding a variable counter, initialized to 0, incremented each time a new item is added to the buffer ● Shared data #define BUFFER_SIZE 10 typedef struct { . } item; item buffer[BUFFER_SIZE]; int in = 0; int out = 0; int counter = 0; 9 Bounded Buffer - Shared Memory Solution: producer item next_produced; while (true) { ● Producer process - creates filled buffers /* produce an item in next produced */ repeat … while (counter == BUFFER_SIZE) ; produce an item in nextp /* do nothing */ … buffer[in] = next_produced; while counter = n do noop; in = (in + 1) % BUFFER_SIZE; buffer[in] := nextp; counter++; in := in+1 mod n; } counter := counter+1; until false; 10 Bounded Buffer - Shared Memory Solution: consumer item next_consumed; while (true) { ● Consumer process - Empties filled buffers repeat while (counter == 0) while counter = 0 do noop; ; /* do nothing */ nextc := buffer[out] ; out:= out+1 mod n; next_consumed = buffer[out]; counter := counter - 1; … out = (out + 1) % BUFFER_SIZE; consume the next item in nextc … counter--; until false; /* consume the item in next consumed */ } 11 Problem with this solution ● Concurrent access to shared data may result in data inconsistency. ● Maintaining data consistency requires mechanisms to ensure the orderly execution of cooperating processes. 12 Access to shared data ● The statements counter++; counter--; must be executed atomically. ● Atomic operations ● An operation that runs to completion or not at all. 13 Race Condition ● counter++ could be implemented with the following instructions in CPU: register1 = counter register1 = register1 + 1 counter = register1 ● counter-- could be implemented with the following instructions in CPU: register2 = counter register2 = register2 - 1 counter = register2 ● Consider this execution interleaving with “count = 5” initially (we expect count = 5 in the end too): S0: producer execute register1 = counter {register1 = 5} S1: producer execute register1 = register1 + 1 {register1 = 6} S2: consumer execute register2 = counter {register2 = 5} S3: consumer execute register2 = register2 – 1 {register2 = 4} S4: producer execute counter = register1 {counter = 6 } S5: consumer execute counter = register2 {counter = 4 !!} 14 Race Condition ● If threads are working on separate data, we don’t need to worry about race conditions: Thread A Thread B x = 1; y = 2; ● However, there can race conditions when we have shared data. Consider the following (Initially, y = 12): Thread A Thread B x = 1; y = 2; x = y+1; y = y*2; ● What are the possible values of x? 15 Race Condition ● If threads are working on separate data, we don’t need to worry about race conditions: Thread A Thread B x = 1; y = 2; ● However, there can race conditions when we have shared data. Consider the following (Initially, y = 12): Thread A Thread B x = 1; y = 2; x = y+1; y = y*2; ● What are the possible values of x? Answer = (13, 3, 5) ● Or, what are the possible values of x below? Thread A Thread B x = 1; x = 2; ● What are the possible values of x? 16 Race Condition ● If threads are working on separate data, we don’t need to worry about race conditions: Thread A Thread B x = 1; y = 2; ● However, there can race conditions when we have shared data. Consider the following (Initially, y = 12): Thread A Thread B x = 1; y = 2; x = y+1; y = y*2; ● What are the possible values of x? Answer = (13, 3, 5) ● Or, what are the possible values of x below? Thread A Thread B x = 1; x = 2; ● What are the possible values of x? Answer = (1, 2) ● X’s value is non-deterministic in the past two examples 17 Critical Section Problem ● Consider system of n processes {p0, p1, … pn-1} competing to access shared data ● Each process has critical section segment of code ● Process may be changing common variables, updating table, writing file, etc. ● When one process in critical section, no other may be in its critical section ● Critical section problem is to design protocol to achieve/solve this ● Each process must ask permission to enter critical section in entry section, may follow critical section with exit section, then remainder section 18 Critical Section ● General structure of process Pi 19 Critical Section Problem - Requirements ● Mutual Exclusion • If process Pi is executing in its critical section, then no other processes can be executing in their critical sections. ● Progress • If no process is executing in its critical section and there exists some processes that wish to enter their critical section, then the selection of the processes that will enter the critical section next cannot be postponed indefinitely. ● Bounded Waiting • A bound must exist on the number of times that other processes are allowed to enter their critical sections after a process has made a request to enter its critical section and before that request is granted. 20 Critical Section Problem - Assumptions ● Assume that each process executes at a nonzero speed in the critical section. That is, assume that each process finishes executing the critical section once entered ● No assumption concerning relative speed of the n processes. ● Assume that a process can get stuck in its remainder section indefinitely, e.g., in a non-terminating while loop. 21 Solution: Critical Section Problem -- Initial Attempt ● Only 2 processes, Pi and Pj ● General structure of process Pi (Pj) ● Processes may share some common variables to synchronize their actions. 22 Algorithm 1 ● Shared Variable: ● Shared Variables: ● int turn = i; ● var turn: (0..1); ● (turn == i) means that Pi can enter its critical section and (turn == j) means initially turn = 0; that Pj can enter its critical section ● turn = i Pi can enter its critical section ● Process Pi Process Pj ● Process Pi do { do { repeat while turn <> i do no-op; while (turn == j); while (turn == i); critical section critical section critical section turn := j; turn = j; remainder section turn = i; until false remainder section remainder section Satisfies mutual exclusion, but not progress. } while (true); } while (true); 23 Algorithm 1 ● Satisfies mutual exclusion ● The turn is equal to either i or j and hence one of Pi and Pj ● Shared Variables: can enter the critical section ● var turn: (0..1); ● Does not satisfy progress initially turn = 0; ● turn = i Pi can enter its critical section ● Example: Pi finishes the critical section and then gets stuck indefinitely in its remainder section. Then Pj enters the ● Process Pi repeat critical section, finishes, and then finishes its remainder while turn <> i do no-op; section. Pj then tries to enter the critical section again, but it critical section cannot since turn was set to i by Pj in the previous iteration. turn := j; Since Pi is stuck in the remainder section, turn will be equal remainder section until false to i indefinitely and Pj can’t enter although it wants
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