conditional variables, Lecture notes of Operating Systems

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Condition Variables

Md Ahsanul Kabir

Another type of sychronization

• Locks allow one type of synchronization between threads – mutual exclusion
• Another common requirement in multi-threaded applications – waiting and signaling - E.g., Thread T1 wants to continue only after T2 has finished some task
• Can accomplish this by busy-waiting on some variable, but inefficient
• Need a new synchronization primitive: condition variables

Example: parent waits for child

Why check condition in while loop?

• In the example code, why do we check condition before calling wait? - In case the child has already run and done is true, then no need to wait
• Why check condition with “while” loop and not “if”?
  • To avoid corner cases of thread being woken up even when condition not true (may be an issue with some implementations)

Example: Producer/Consumer problem

• A common pattern in multi-threaded programs
• Example: in a multi-threaded web server, one thread accepts requests from the network and puts them in a queue. Worker threads get requests from this queue and process them.
• Setup: one or more producer threads, one or more consumer threads, a shared buffer of bounded size

Producer/Consumer with 2 CVs