COS 318: Operating SystemsMessage Passing2Today’s Topics!Message passing"Semantics"How to use!Implementation issues"Synchronous vs. asynchronous"Buffering"Indirection"Exceptions3Big PictureProcessSenderProcessReceiver4Send and Receive Primitivessend(dest, type, msg)Senderrecv(src, type,msg)Receiverpid, file, port,… pid, file, port, any,…buffer, n-bytesbuffer, ?-bytesmsg typeexpected msg typeMany ways to design the message passing API5Synchronous Message Passing!Move data between processes"Sender: when data is ready, send it to the receiver process"Receiver: when the data has arrived and when the receive process is ready to take the data, move the data to the destination data structure!Synchronization"Sender: signal the receiver process that a particular event happens"Receiver: block until the event has happened…………SRSend( R, buf, n );Recv( S, &buf, &n );6Example: Producer-Consumer!Questions"Does this work?"Would it work with multiple producers and 1 consumer?"Would it work with 1 producer and multiple consumers?"What about multiple producers and multiple consumers?Producer(){ ... while (1) { produce item; recv(Consumer, &credit); send(Consumer, item); }}Consumer(){ ... for (i=0; i<N; i++) send(Producer, credit); while (1) { recv(Producer, &item); send(Producer, credit); consume item; }}7Implementation Issues!Buffering messages!Direct vs. indirect!Unidirectional vs. bidirectional!Asynchronous vs. synchronous!Event handler vs. receive!How to handle exceptions?……8Buffering Messages!No buffering"Sender must wait until the receiver receives the message"Rendezvous on each message!Bounded buffer"Finite size"Sender blocks on buffer full"Use monitor to solve the problem!Unbounded buffer"“Infinite” size"Sender never blocksbuffer9Direct Communication!A single buffer at the receiver"More than one process may send messages to the receiver"To receive from a specific sender, it requires searching through the whole buffer!A buffer at each sender"A sender may send messages to multiple receivers"To get a message, it also requires searching through the whole buffer……10Indirect Communication!Use mailbox as the abstraction"Allow many-to-many communication"Require open/close a mailbox!Buffering"A buffer, its mutex and condition variables should be at the mailbox!Message size"Not necessarily. One can break a large message into packets!Mailbox vs. pipe"A mailbox allows many to many communication"A pipe implies one sender and one receivermboxpipe11Synchronous vs. Asynchronous: Send!Synchronous"Block if resource is busy"Initiate data transfer"Block until data is out of its source memory"Rendezvous: block until receiver has done recv and sent acknowledgment!Asynchronous"Block if resource is busy"Initiate data transfer and return"Completion• Require applications to check status• Notify or signal the applicationsend( dest, type, msg)msg transfer resourcestatus = async_send( dest, type, msg )…if !send_complete( status ) wait for completion;…use msg data structure;…12Synchronous vs. Asynchronous: Receive!Synchronous"Return data if there is a message!Asynchronous"Return data if there is a message"Return status if there is no message (probe)recv( src, type, msg )msg transfer resourcestatus = async_recv( src, type, msg );if ( status == SUCCESS ) consume msg;while ( probe(src) != HaveMSG ) wait for msg arrivalrecv( src, type, msg );consume msg;13Event Handler vs. Receive!hrecv( src, type, msg, func )"msg is an arg of func"Execute “func” on a message arrival!Which one is more powerful?"Recv with a thread can emulate a Handler"Handler can be used to emulate recv by using Monitor!Pros and Consvoid func( char * msg ) { …}…hrecv( src, type, msg, func)…while(1) { recv(src,type, msg); func(msg);}programCreate a thread…14Example: Keyboard Input!How do you implement keyboard input?"Need an interrupt handler"Generate a mbox message from the interrupt handler!Suppose a keyboard device thread converts input characters into an mbox message"How would you synchronize between the keyboard interrupt handler and device thread?"How can a device thread convert input into mbox messages?mboxV(s);…while (1) { P(s); Acquire(m); convert … Release(m);};InterrupthandlerDevicethreadProcess15Exception: Process Termination!R waits for a message from S, but S has terminated"Problem: R may be blocked forever!S sends a message to R, but R has terminated"Problem: S has no buffer and will be blocked foreverS RS R16Exception: Message Loss!Use ack and timeout to detect and retransmit a lost message"Require the receiver to send an ack message for each message "Sender blocks until an ack message is back or timeout status = send( dest, msg, timeout );"If timeout happens and no ack, then retransmit the message!Issues"Duplicates"Losing ack messagesS Rsendack17Exception: Message Loss, cont’d!Retransmission must handle "Duplicate messages on receiver side"Out-of-sequence ack messages on sender side!Retransmission"Use sequence number for each message to identify duplicates"Remove duplicates on receiver side"Sender retransmits on an out-of-sequence ack!Reduce ack messages"Bundle ack messages"Receiver sends noack messages: can be complex"Piggy-back acks in send messagesS Rsend1ack1send2ack218Exception: Message Corruption!Detection"Compute a checksum over the entire message and send the checksum (e.g. CRC code) as part of the message"Recompute a checksum on receive and compare with the checksum in the message!Correction"Trigger retransmission"Use correction codes to recoverDataCompute checksum19Example: Sockets API!Abstraction for TCP and UDP"Guest lecture by Prof. Rexford on 11/29!Addressing"IP address and port number (216 ports available for users)!Create and close a socket"sockid = socket(af, type, protocol);"Sockerr = close(sockid);!Bind a socket to a local address"sockerr = bind(sockid, localaddr, addrlength);!Negotiate the connection"listen(sockid, length);"accept(sockid, addr, length);!Connect a socket to destimation"connect(sockid, destaddr, addrlength);socket socketbindlistenacceptreadconnectwritewrite readServer Client20Summary!Message passing"Move data between processes"Implicit synchronization"API design is important!Implementation issues"Synchronous method is most common"Asynchronous method provides overlapping but requires careful design considerations"Indirection makes implementation flexible"Exception needs to be carefully