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Semantics of Send and Receive

Semantics of Send and Receive. Can be blocking or nonblocking called “synchronous” and “asynchronous” remember: procedure call is synchronous thread fork is asynchronous send, receive both have synchronous and asynchronous implementations. Picture of Synchronous Send, Blocking Receive.

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Semantics of Send and Receive

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  1. Semantics of Send and Receive • Can be blocking or nonblocking • called “synchronous” and “asynchronous” • remember: • procedure call is synchronous • thread fork is asynchronous • send, receive both have synchronous and asynchronous implementations

  2. Picture of Synchronous Send, Blocking Receive P1 P2 receive Data (blocked) send consume message (blocked) “Go ahead” proceed

  3. Sieve of Eratosthenes:Find primes <= N • Add the number 2 to the set of primes • Create a list of consecutive odd integers • Let p = 3, the first prime number (besides 2) • Count up from p and cross out all multiples of p • Add p to the set of primes • Set p = the smallest number not crossed out • If p <= N, goto step 4; else quit

  4. Sieve of Eratosthenes Using Synchronous Message Passing process Sieve[1] { for j = 3 to N by 2 Sieve[2]!j } process Sieve[i = 2 to Max] { Sieve[i-1]?p print “found a prime”, p while (Sieve[i-1]?num) { if (num mod p != 0) Sieve[i+1]!num } } Uses CSP notation: ? (receive) and ! (send) Terminates in deadlock, but this could be fixed

  5. Picture of Asynchronous Send, Blocking Receive P1 P2 receive Data (blocked) send proceed consume message

  6. Implementation of Asynch. Send/ Blocking Receive • Library must keep track of all channels • one buffer and one semaphore per channel at receiver • On Send(channel, userSpecifiedData) • copy userSpecifiedData into sender-side buffer • (buffer eventually put onto network) • On Receive(channel, userSpecifiedData) • P(thisQueue); copy buffer into userSpecifiedData • On incoming message (specifies channel) • copy message into receiver-side buffer; V(thisQueue)

  7. Tradeoffs in Message Passing • Advantages of blocking send • won’t overwrite message, less buffering • Advantages of nonblocking send • can continue after send (can do other work) • but what if the buffer is full? Block? Fail? • Advantages of blocking receive • know message is received, avoid polling • Advantages of nonblocking receive • can result in fewer copies (buffer posted in advance)

  8. Final Note on Semantics • Other possible semantics exist • Sender also (conceptually) may continue when: • Message is copied out of sender’s address space • Message is put on the network • Message is received at the network device on the receiving end

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