What kind of problem arises with sleep and wakeup mechanism of achieving mutual exclusion? Explain with suitable code snippet.

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Suresh Chand · Accepted Answer

When a process wants to enter its critical section, it checks to see if then entry is allowed. If it is not, the process goes into tight loop and waits (i.e. start busy waiting) until it is allowed to enter. This approach waste CPU-time. Now, look at some inter-process communication primitives is the pair of sleep-wake up. Sleep: It is a system call that causes the caller to block, that is, be suspended until some other process wakes it up. Wakeup: It is a system call that wakes up the process. Example of sleep()/wake() usage Producer-consumer problem: Let two processes share a buffer with limited capacity. Process called producer will put pieces of imformation in the buffer. Process called consumer will take pieces of information from that buffer. Let us assume: If Producer is trying to put a message into full buffer, Producer goes to sleep If Comsumer is trying ti take message from empty buffer, Consumer goes to sleep. Producer-Consumer with Race conditions: #define N 100 int count = 0; void producer(void){ int item; while(TRUE){ item = produce_item(); if( count == N ){ sleep(); } insert_item(item); count++; if( count == 1 ){ wake_up(consumer); } } } void consumer(void){ int item; while(TRUE){ if( count == 0 ){ sleep(); } item = remove_item(); count--; if( count == N-1 ){ wake_up(producer); } consume_item(item); } } The producer produces the item and inserts it into buffer. The value of the global variable count got increased at each insertion. If the buffer is filled completely and no slot is available then the producer will sleep, otherwise it keep inserting. On the consumer's end, the value of count got decreaed at every iteration. If buffer is empty at any point the consumer will sleep otherwise it keeps consuming the items and decreased the value of count by 1. The consumer will be waked up by the consumer if there is at least one slot available in the buffer so that the producer can write that. Problem: Consumer about to go to sleep but got preemted, buffer >= 1 and producer will send the wake_up signal but the consumer is not sleeping yet and the signal will get wasted. And then consumer got scheduled and went to sleep. And the producer is thinking that consumer is not sleeping. When count is N the producer will got to sleep. Now, if you see that both producer and consumer are sleeping. And that is deadlock. Solution: Whenever someone is about to sleep the a bit is maintained to tell do not sleep. But when there are N producer and consumer then we have to record N wake up calls. So, Dijkstra introduced something called as Semaphores which tell how many wake up calls happend.

What kind of problem arises with sleep and wakeup mechanism of achieving mutual exclusion? Explain with suitable code snippet.

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What kind of problem arises with sleep and wakeup mechanism of achieving mutual exclusion? Explain with suitable code snippet.

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